Convert.ChangeType 方法

定義

傳回指定之類型的物件,此物件的值與指定的物件相等。

多載

ChangeType(Object, Type)

傳回指定之類型的物件,此物件的值與指定的物件相等。

ChangeType(Object, TypeCode)

傳回物件,該物件屬於指定的類型,且其值等於指定的物件。

ChangeType(Object, Type, IFormatProvider)

傳回物件,該物件屬於指定的類型,且其值等於指定的物件。 參數提供特定文化特性格式資訊。

ChangeType(Object, TypeCode, IFormatProvider)

傳回物件,該物件屬於指定的類型,且其值等於指定的物件。 參數提供特定文化特性格式資訊。

ChangeType(Object, Type)

傳回指定之類型的物件,此物件的值與指定的物件相等。

public:
 static System::Object ^ ChangeType(System::Object ^ value, Type ^ conversionType);
public static object ChangeType (object value, Type conversionType);
public static object? ChangeType (object? value, Type conversionType);
static member ChangeType : obj * Type -> obj
Public Shared Function ChangeType (value As Object, conversionType As Type) As Object

參數

value
Object

實作 IConvertible 介面的物件。

conversionType
Type

要傳回的物件類型。

傳回

Object

類型為 conversionType 且值與 value 相等的物件。

-或-

如果 Nothingvaluenull 不是實值類型,則為 null 參考 (在 Visual Basic 中為 conversionType)。

例外狀況

不支援此轉換。

-或-

valuenull,且 conversionType 是實值類型。

-或-

value 未實作 IConvertible 介面。

value 不是 conversionType 可辨識的格式。

value 代表超出 conversionType範圍的數字。

conversionTypenull

範例

在下列程式碼中,說明了如何使用 ChangeType 方法。

using namespace System;

int main()
{
   Double d = -2.345;
   int i =  *safe_cast<Int32^>(Convert::ChangeType( d, int::typeid ));
   Console::WriteLine( "The double value {0} when converted to an int becomes {1}", d, i );
   String^ s = "12/12/98";
   DateTime dt =  *safe_cast<DateTime^>(Convert::ChangeType( s, DateTime::typeid ));
   Console::WriteLine( "The string value {0} when converted to a Date becomes {1}", s, dt );
}
using System;

public class ChangeTypeTest {
    public static void Main() {

        Double d = -2.345;
        int i = (int)Convert.ChangeType(d, typeof(int));

        Console.WriteLine("The double value {0} when converted to an int becomes {1}", d, i);

        string s = "12/12/98";
        DateTime dt = (DateTime)Convert.ChangeType(s, typeof(DateTime));

        Console.WriteLine("The string value {0} when converted to a Date becomes {1}", s, dt);
    }
}
open System

let d = -2.345
let i = Convert.ChangeType(d, typeof<int>) :?> int

printfn $"The double value {d} when converted to an int becomes {i}"

let s = "12/12/98"
let dt = Convert.ChangeType(s, typeof<DateTime>) :?> DateTime

printfn $"The string value {s} when converted to a Date becomes {dt}"
Public Class ChangeTypeTest
    
    Public Shared Sub Main()
        Dim d As [Double] = - 2.345
        Dim i As Integer = CInt(Convert.ChangeType(d, GetType(Integer)))
        
        Console.WriteLine("The double value {0} when converted to an int becomes {1}", d, i)
        Dim s As String = "12/12/98"
        Dim dt As DateTime = CType(Convert.ChangeType(s, GetType(DateTime)), DateTime)
        
        Console.WriteLine("The string value {0} when converted to a Date becomes {1}", s, dt)
    End Sub
End Class

備註

ChangeType 是一種一般用途的轉換方法,可將 指定的 value conversionType 物件轉換為 。 參數 value 可以是任何類型的物件, conversionType 也可以是 Type 代表任何基底或自訂類型的 物件。 若要讓轉換成功, value 必須實 IConvertible 作 介面,因為 方法只會包裝對適當 IConvertible 方法的呼叫。 方法需要支援的 轉換 value conversionType

這個方法會使用目前線程的文化特性進行轉換。

給呼叫者的注意事項

方法 ChangeType(Object, Type) 可以將列舉值轉換成另一種型別。 不過,即使來源類型是列舉的基礎型別,它也無法將另一個型別轉換成列舉值。 若要將類型轉換成列舉值,請使用 C#) 中的轉換運算子 (,或在 Visual Basic) 中使用轉換函數 (。 下列範例說明從 大陸 列舉值的轉換。

:::code language=「csharp」 source=「~/snippets/csharp/System/Convert/ChangeType/changetype_enum2.cs」 interactive=「try-dotnet」 id=「Snippet5」::::code language=「fsharp」 source=「~/snippets/fsharp/System/Convert/ChangeType/changetype_enum2.fs」 id=「Snippet5」::: :::code language=「vb」 source=「~/snippets/visualbasic/VS_Snippets_CLR_System/system.convert.changetype/vb/changetype_enum2.vb」 id=「Snippet5」::

方法 ChangeType(Object, Type) 可以將可為 Null 的類型轉換成另一個型別。 不過,即使 是 的基礎型別, conversionType 它也無法將另一個型別轉換成可為 Null 類型的 Nullable<T> 值。若要執行轉換,您可以在 C#) 中使用轉換運算子 (,或在 Visual Basic) 中使用轉換函式 (。 下列範例說明從可為 Null 的型別轉換。

:::code language=「csharp」 source=「~/snippets/csharp/System/Convert/ChangeType/changetype_nullable.cs」 interactive=「try-dotnet」 id=「Snippet7」::: :::code language=「fsharp」 source=「~/snippets/fsharp/System/Convert/ChangeType/changetype_nullable.fs」 id=「Snippet7」::: :::code language=「vb」 source=「~/snippets/visualbasic/VS_Snippets_CLR_System/system.convert.changetype/vb/changetype_nullable.vb」 id=「Snippet7」::

適用於

ChangeType(Object, TypeCode)

傳回物件,該物件屬於指定的類型,且其值等於指定的物件。

public:
 static System::Object ^ ChangeType(System::Object ^ value, TypeCode typeCode);
public static object? ChangeType (object? value, TypeCode typeCode);
public static object ChangeType (object value, TypeCode typeCode);
static member ChangeType : obj * TypeCode -> obj
Public Shared Function ChangeType (value As Object, typeCode As TypeCode) As Object

參數

value
Object

實作 IConvertible 介面的物件。

typeCode
TypeCode

要傳回的物件類型。

傳回

Object

基礎類型為 typeCode 且值與 value 相等的物件。

-或-

如果 Nothingvalue,而且 nulltypeCodeEmptyString,則為 null 參考 (在 Visual Basic 中為 Object)。

例外狀況

不支援此轉換。

-或-

valuenull,而且 typeCode 會指定實值型別。

-或-

value 未實作 IConvertible 介面。

value 不是 typeCode 類型可辨識的格式。

value 代表超出 typeCode 類型範圍的數字。

typeCode 無效。

範例

下列範例說明如何盡可能使用 ChangeType(Object, TypeCode) 方法,將 變更 Object 為 參數所 TypeCode 指定的型別。

using namespace System;

void main()
{
   Double d = -2.345;
   int i = (int) Convert::ChangeType(d, TypeCode::Int32);

   Console::WriteLine("The Double {0} when converted to an Int32 is {1}", d, i);

   String^ s = "12/12/2009";
   DateTime dt = (DateTime)Convert::ChangeType(s, DateTime::typeid);

   Console::WriteLine("The String {0} when converted to a Date is {1}", s, dt);        
}
// The example displays the following output:
//    The Double -2.345 when converted to an Int32 is -2
//    The String 12/12/2009 when converted to a Date is 12/12/2009 12:00:00 AM
using System;

public class ChangeTypeTest {
    public static void Main() {

        Double d = -2.345;
        int i = (int)Convert.ChangeType(d, TypeCode.Int32);

        Console.WriteLine("The Double {0} when converted to an Int32 is {1}", d, i);

        string s = "12/12/2009";
        DateTime dt = (DateTime)Convert.ChangeType(s, typeof(DateTime));

        Console.WriteLine("The String {0} when converted to a Date is {1}", s, dt);
    }
}
// The example displays the following output:
//    The Double -2.345 when converted to an Int32 is -2
//    The String 12/12/2009 when converted to a Date is 12/12/2009 12:00:00 AM
open System

let d = -2.345
let i = Convert.ChangeType(d, TypeCode.Int32) :?> int

printfn $"The Double {d} when converted to an Int32 is {i}"

let s = "12/12/2009"
let dt = Convert.ChangeType(s, typeof<DateTime>) :?> DateTime

printfn $"The String {s} when converted to a Date is {dt}"
// The example displays the following output:
//    The Double -2.345 when converted to an Int32 is -2
//    The String 12/12/2009 when converted to a Date is 12/12/2009 12:00:00 AM
Public Class ChangeTypeTest
    
    Public Shared Sub Main()
        Dim d As [Double] = - 2.345
        Dim i As Integer = CInt(Convert.ChangeType(d, TypeCode.Int32))
        
        Console.WriteLine("The Double {0} when converted to an Int32 is {1}", d, i)
        Dim s As String = "12/12/2009"
        Dim dt As DateTime = CDate(Convert.ChangeType(s, TypeCode.DateTime))
        
        Console.WriteLine("The String {0} when converted to a Date is {1}", s, dt)
    End Sub 
End Class 
' The example displays the following output:
'    The Double -2.345 when converted to an Int32 is -2
'    The String 12/12/2009 when converted to a Date is 12/12/2009 12:00:00 AM

備註

ChangeType(Object, TypeCode) 是一種一般用途的轉換方法,可將 指定的 value 物件轉換成 所 typeCode 指定的預先定義型別。 參數 value 可以是任何類型的物件。 若要讓轉換成功, value 必須實 IConvertible 作 介面,因為 方法只會包裝對適當 IConvertible 方法的呼叫。 方法也需要支援的 轉換 value typeCode

方法 ChangeType(Object, TypeCode) 不支援轉換成 value 自訂類型。 若要執行這類轉換,請呼叫 ChangeType(Object, Type) 方法。

適用於

ChangeType(Object, Type, IFormatProvider)

傳回物件,該物件屬於指定的類型,且其值等於指定的物件。 參數提供特定文化特性格式資訊。

public:
 static System::Object ^ ChangeType(System::Object ^ value, Type ^ conversionType, IFormatProvider ^ provider);
public static object ChangeType (object value, Type conversionType, IFormatProvider provider);
public static object? ChangeType (object? value, Type conversionType, IFormatProvider? provider);
static member ChangeType : obj * Type * IFormatProvider -> obj
Public Shared Function ChangeType (value As Object, conversionType As Type, provider As IFormatProvider) As Object

參數

value
Object

實作 IConvertible 介面的物件。

conversionType
Type

要傳回的物件類型。

provider
IFormatProvider

物件,提供特定文化特性格式資訊。

傳回

Object

類型為 conversionType 且值與 value 相等的物件。

-或-

如果 valueTypevalue 相等,則為 conversionType

-或-

如果 Nothingvaluenull 不是實值類型,則為 null 參考 (在 Visual Basic 中為 conversionType)。

例外狀況

不支援此轉換。

-或-

valuenull,且 conversionType 是實值類型。

-或-

value 未實作 IConvertible 介面。

value 格式不是 provider 所辨識的 conversionType

value 代表超出 conversionType範圍的數字。

conversionTypenull

範例

下列範例定義一個實作 Temperature 介面的 IConvertible 類別。

using namespace System;
using namespace System::Globalization;

public ref class Temperature : IConvertible
{
private:
   Decimal m_Temp;

public:
   Temperature(Decimal temperature)
   {
      m_Temp = temperature;
   }
   
   property Decimal Celsius {
      Decimal get() { return m_Temp; }
   }
   
   property Decimal Kelvin {
      Decimal get() { return m_Temp + (Decimal) 273.15; }
   }
   
   property Decimal Fahrenheit {
      Decimal get() { return Math::Round((Decimal) (m_Temp * 9 / 5 + 32), 2); }
   }
   
   virtual String^ ToString()
   override {
      return m_Temp.ToString("N2") + "�C";
   }

   // IConvertible implementations.
   virtual TypeCode GetTypeCode()
   {
      return TypeCode::Object;
   }
   
   virtual bool ToBoolean(IFormatProvider^ provider) 
   {
      if (m_Temp == 0)
         return false;
      else
         return true;
   } 
   
   virtual Byte ToByte(IFormatProvider^ provider)
   {
      if (m_Temp < Byte::MinValue || m_Temp > Byte::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the Byte type.", 
                                                   m_Temp));
      else
         return Decimal::ToByte(m_Temp);
   }
   
   virtual Char ToChar(IFormatProvider^ provider)
   {
      throw gcnew InvalidCastException("Temperature to Char conversion is not supported.");
   } 
   
   virtual DateTime ToDateTime(IFormatProvider^ provider)
   {
      throw gcnew InvalidCastException("Temperature to DateTime conversion is not supported.");
   }
   
   virtual Decimal ToDecimal(IFormatProvider^ provider)
   {
      return m_Temp;
   }
   
   virtual Double ToDouble(IFormatProvider^ provider)
   {
      return Decimal::ToDouble(m_Temp);
   }   
   
   virtual Int16 ToInt16(IFormatProvider^ provider)
   {
      if (m_Temp < Int16::MinValue || m_Temp > Int16::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the Int16 type.",
                                                   m_Temp));
      else
         return Decimal::ToInt16(m_Temp);
   }
   
   virtual Int32 ToInt32(IFormatProvider^ provider)
      {
      if (m_Temp < Int32::MinValue || m_Temp > Int32::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the Int32 type.",
                                                   m_Temp));
      else
         return Decimal::ToInt32(m_Temp);
   }
   
   virtual Int64 ToInt64(IFormatProvider^ provider)
   {
      if (m_Temp < Int64::MinValue || m_Temp > Int64::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the Int64 type.",
                                                   m_Temp));
      else
         return Decimal::ToInt64(m_Temp);
   }
   
   virtual SByte ToSByte(IFormatProvider^ provider)
   {
      if (m_Temp < SByte::MinValue || m_Temp > SByte::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the SByte type.",
                                                   m_Temp));
      else
         return Decimal::ToSByte(m_Temp);
   }

   virtual Single ToSingle(IFormatProvider^ provider)
   {
      return Decimal::ToSingle(m_Temp);
   }

   virtual String^ ToString(IFormatProvider^ provider)
   {
      return m_Temp.ToString("N2", provider) + "�C";
   }
   
   virtual Object^ ToType(Type^ conversionType, IFormatProvider^ provider)
   {
      switch (Type::GetTypeCode(conversionType))
      {
      case TypeCode::Boolean: 
            return ToBoolean(nullptr);
      case TypeCode::Byte:
            return ToByte(nullptr);
      case TypeCode::Char:
            return ToChar(nullptr);
      case TypeCode::DateTime:
            return ToDateTime(nullptr);
      case TypeCode::Decimal:
            return ToDecimal(nullptr);
      case TypeCode::Double:
            return ToDouble(nullptr);
      case TypeCode::Int16:
            return ToInt16(nullptr);
      case TypeCode::Int32:
            return ToInt32(nullptr);
      case TypeCode::Int64:
            return ToInt64(nullptr);
      case TypeCode::Object:
            if (Temperature::typeid->Equals(conversionType))
               return this;
            else
               throw gcnew InvalidCastException(String::Format("Conversion to a {0} is not supported.",
                                                            conversionType->Name));
      case TypeCode::SByte:
            return ToSByte(nullptr);
      case TypeCode::Single:
            return ToSingle(nullptr);
      case TypeCode::String:
            return ToString(provider);
      case TypeCode::UInt16:
            return ToUInt16(nullptr);
      case TypeCode::UInt32:
            return ToUInt32(nullptr);
      case TypeCode::UInt64:
            return ToUInt64(nullptr);   
         default:
            throw gcnew InvalidCastException(String::Format("Conversion to {0} is not supported.", conversionType->Name));   
      }
   }
   
   virtual UInt16 ToUInt16(IFormatProvider^ provider)
   {
      if (m_Temp < UInt16::MinValue || m_Temp > UInt16::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the UInt16 type.",
                                                   m_Temp));
      else
         return Decimal::ToUInt16(m_Temp);
   }

   virtual UInt32 ToUInt32(IFormatProvider^ provider)
   {
      if (m_Temp < UInt32::MinValue || m_Temp > UInt32::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the UInt32 type.",
                                                   m_Temp));
      else
         return Decimal::ToUInt32(m_Temp);
   }
   
   virtual UInt64 ToUInt64(IFormatProvider^ provider)
   {
      if (m_Temp < UInt64::MinValue || m_Temp > UInt64::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the UInt64 type.",
                                                   m_Temp));
      else
         return Decimal::ToUInt64(m_Temp);
   }
};
using System;
using System.Globalization;

public class Temperature : IConvertible
{
   private decimal m_Temp;

   public Temperature(decimal temperature)
   {
      this.m_Temp = temperature;
   }

   public decimal Celsius
   {
      get { return this.m_Temp; }
   }

   public decimal Kelvin
   {
      get { return this.m_Temp + 273.15m; }
   }

   public decimal Fahrenheit
   {
      get { return Math.Round((decimal) (this.m_Temp * 9 / 5 + 32), 2); }
   }

   public override string ToString()
   {
      return m_Temp.ToString("N2") + "°C";
   }

   // IConvertible implementations.
   public TypeCode GetTypeCode()
   {
      return TypeCode.Object;
   }

   public bool ToBoolean(IFormatProvider provider)
   {
      if (m_Temp == 0)
         return false;
      else
         return true;
   }

   public byte ToByte(IFormatProvider provider)
   {
      if (m_Temp < Byte.MinValue || m_Temp > Byte.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Byte type.",
                                                   this.m_Temp));
      else
         return Decimal.ToByte(this.m_Temp);
   }

   public char ToChar(IFormatProvider provider)
   {
      throw new InvalidCastException("Temperature to Char conversion is not supported.");
   }

   public DateTime ToDateTime(IFormatProvider provider)
   {
      throw new InvalidCastException("Temperature to DateTime conversion is not supported.");
   }

   public decimal ToDecimal(IFormatProvider provider)
   {
      return this.m_Temp;
   }

   public double ToDouble(IFormatProvider provider)
   {
      return Decimal.ToDouble(this.m_Temp);
   }

   public short ToInt16(IFormatProvider provider)
   {
      if (this.m_Temp < Int16.MinValue || this.m_Temp > Int16.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Int16 type.",
                                                   this.m_Temp));
      else
         return Decimal.ToInt16(this.m_Temp);
   }

   public int ToInt32(IFormatProvider provider)
      {
      if (this.m_Temp < Int32.MinValue || this.m_Temp > Int32.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Int32 type.",
                                                   this.m_Temp));
      else
         return Decimal.ToInt32(this.m_Temp);
   }

   public long ToInt64(IFormatProvider provider)
   {
      if (this.m_Temp < Int64.MinValue || this.m_Temp > Int64.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Int64 type.",
                                                   this.m_Temp));
      else
         return Decimal.ToInt64(this.m_Temp);
   }

   public sbyte ToSByte(IFormatProvider provider)
   {
      if (this.m_Temp < SByte.MinValue || this.m_Temp > SByte.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the SByte type.",
                                                   this.m_Temp));
      else
         return Decimal.ToSByte(this.m_Temp);
   }

   public float ToSingle(IFormatProvider provider)
   {
      return Decimal.ToSingle(this.m_Temp);
   }

   public string ToString(IFormatProvider provider)
   {
      return m_Temp.ToString("N2", provider) + "°C";
   }

   public object ToType(Type conversionType, IFormatProvider provider)
   {
      switch (Type.GetTypeCode(conversionType))
      {
         case TypeCode.Boolean:
            return this.ToBoolean(null);
         case TypeCode.Byte:
            return this.ToByte(null);
         case TypeCode.Char:
            return this.ToChar(null);
         case TypeCode.DateTime:
            return this.ToDateTime(null);
         case TypeCode.Decimal:
            return this.ToDecimal(null);
         case TypeCode.Double:
            return this.ToDouble(null);
         case TypeCode.Int16:
            return this.ToInt16(null);
         case TypeCode.Int32:
            return this.ToInt32(null);
         case TypeCode.Int64:
            return this.ToInt64(null);
         case TypeCode.Object:
            if (typeof(Temperature).Equals(conversionType))
               return this;
            else
               throw new InvalidCastException(String.Format("Conversion to a {0} is not supported.",
                                                            conversionType.Name));
         case TypeCode.SByte:
            return this.ToSByte(null);
         case TypeCode.Single:
            return this.ToSingle(null);
         case TypeCode.String:
            return this.ToString(provider);
         case TypeCode.UInt16:
            return this.ToUInt16(null);
         case TypeCode.UInt32:
            return this.ToUInt32(null);
         case TypeCode.UInt64:
            return this.ToUInt64(null);
         default:
            throw new InvalidCastException(String.Format("Conversion to {0} is not supported.", conversionType.Name));
      }
   }

   public ushort ToUInt16(IFormatProvider provider)
   {
      if (this.m_Temp < UInt16.MinValue || this.m_Temp > UInt16.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the UInt16 type.",
                                                   this.m_Temp));
      else
         return Decimal.ToUInt16(this.m_Temp);
   }

   public uint ToUInt32(IFormatProvider provider)
   {
      if (this.m_Temp < UInt32.MinValue || this.m_Temp > UInt32.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the UInt32 type.",
                                                   this.m_Temp));
      else
         return Decimal.ToUInt32(this.m_Temp);
   }

   public ulong ToUInt64(IFormatProvider provider)
   {
      if (this.m_Temp < UInt64.MinValue || this.m_Temp > UInt64.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the UInt64 type.",
                                                   this.m_Temp));
      else
         return Decimal.ToUInt64(this.m_Temp);
   }
}
open System
open System.Globalization

type Temperature(temperature: decimal) = 

    member _.Celsius = temperature

    member _.Kelvin =
        temperature + 273.15m

    member _.Fahrenheit =
        Math.Round(decimal (temperature * 9m / 5m + 32m), 2)

    override _.ToString() =
        temperature.ToString "N2" + "°C"

    // IConvertible implementations.
    interface IConvertible with
        member _.GetTypeCode() =
            TypeCode.Object

        member _.ToBoolean(provider: IFormatProvider) =
            temperature <> 0M

        member _.ToByte(provider: IFormatProvider) =
            if temperature < decimal Byte.MinValue || temperature > decimal Byte.MaxValue then
                raise (OverflowException $"{temperature} is out of range of the Byte type.")
            else
                Decimal.ToByte temperature

        member _.ToChar(provider: IFormatProvider) =
            raise (InvalidCastException "Temperature to Char conversion is not supported.")

        member _.ToDateTime(provider: IFormatProvider) =
            raise (InvalidCastException "Temperature to DateTime conversion is not supported.")

        member _.ToDecimal(provider: IFormatProvider) =
            temperature

        member _.ToDouble(provider: IFormatProvider) =
            Decimal.ToDouble temperature

        member _.ToInt16(provider: IFormatProvider) =
            if temperature < decimal Int16.MinValue || temperature > decimal Int16.MaxValue then
                raise (OverflowException $"{temperature} is out of range of the Int16 type.")
            else
                Decimal.ToInt16 temperature

        member _.ToInt32(provider: IFormatProvider) =
            if temperature < decimal Int32.MinValue || temperature > decimal Int32.MaxValue then
                raise (OverflowException $"{temperature} is out of range of the Int32 type.")
            else
                Decimal.ToInt32 temperature

        member _.ToInt64(provider: IFormatProvider) =
            if temperature < decimal Int64.MinValue || temperature > decimal Int64.MaxValue then
                raise (OverflowException $"{temperature} is out of range of the Int64 type.")
            else
                Decimal.ToInt64 temperature

        member _.ToSByte(provider: IFormatProvider) =
            if temperature < decimal SByte.MinValue || temperature > decimal SByte.MaxValue then
                raise (OverflowException $"{temperature} is out of range of the SByte type.")
            else
                Decimal.ToSByte temperature

        member _.ToSingle(provider: IFormatProvider) =
            Decimal.ToSingle temperature

        member _.ToString(provider: IFormatProvider) =
            temperature.ToString("N2", provider) + "°C"

        member this.ToType(conversionType: Type, provider: IFormatProvider) =
            let this = this :> IConvertible
            match Type.GetTypeCode conversionType with
            | TypeCode.Boolean->
                this.ToBoolean null
            | TypeCode.Byte ->
                this.ToByte null
            | TypeCode.Char ->
                this.ToChar null
            | TypeCode.DateTime ->
                this.ToDateTime null
            | TypeCode.Decimal ->
                this.ToDecimal null
            | TypeCode.Double ->
                this.ToDouble null
            | TypeCode.Int16 ->
                this.ToInt16 null
            | TypeCode.Int32 ->
                this.ToInt32 null
            | TypeCode.Int64 ->
                this.ToInt64 null
            | TypeCode.Object ->
                if typeof<Temperature>.Equals conversionType then
                    this
                else
                    raise (InvalidCastException $"Conversion to a {conversionType.Name} is not supported.")
            | TypeCode.SByte ->
                this.ToSByte null
            | TypeCode.Single ->
                this.ToSingle null
            | TypeCode.String ->
                this.ToString provider
            | TypeCode.UInt16 ->
                this.ToUInt16 null
            | TypeCode.UInt32->
                this.ToUInt32 null
            | TypeCode.UInt64->
                this.ToUInt64 null
            | _ ->
                raise (InvalidCastException $"Conversion to {conversionType.Name} is not supported.")

        member _.ToUInt16(provider: IFormatProvider) =
            if temperature < decimal UInt16.MinValue || temperature > decimal UInt16.MaxValue then
                raise (OverflowException $"{temperature} is out of range of the UInt16 type.")
            else
                Decimal.ToUInt16 temperature

        member _.ToUInt32(provider: IFormatProvider) =
            if temperature < decimal UInt32.MinValue || temperature > decimal UInt32.MaxValue then
                raise (OverflowException $"{temperature} is out of range of the UInt32 type.")
            else
                Decimal.ToUInt32 temperature

        member _.ToUInt64(provider: IFormatProvider) =
            if temperature < decimal UInt64.MinValue || temperature > decimal UInt64.MaxValue then
                raise (OverflowException $"{temperature} is out of range of the UInt64 type.")
            else
                Decimal.ToUInt64 temperature
Imports System.Globalization

Public Class Temperature : Implements IConvertible
   Private m_Temp As Decimal

   Public Sub New(temperature As Decimal)
      Me.m_Temp = temperature
   End Sub
   
   Public ReadOnly Property Celsius() As Decimal
      Get
         Return Me.m_Temp
      End Get   
   End Property
   
   Public ReadOnly Property Kelvin() As Decimal
      Get
         Return Me.m_Temp + 273.15d   
      End Get
   End Property
   
   Public ReadOnly Property Fahrenheit() As Decimal
      Get
         Return Math.Round(CDec(Me.m_Temp * 9 / 5 + 32), 2)
      End Get      
   End Property
   
   Public Overrides Function ToString() As String
      Return m_Temp.ToString("N2") & "°C"
   End Function

   ' IConvertible implementations.
   Public Function GetTypeCode() As TypeCode _
                   Implements IConvertible.GetTypeCode
      Return TypeCode.Object
   End Function
   
   Public Function ToBoolean(provider As IFormatProvider) As Boolean _
                   Implements IConvertible.ToBoolean
      If m_Temp = 0 Then
         Return False
      Else
         Return True
      End If
   End Function 
   
   Public Function ToByte(provider As IFormatProvider) As Byte _
                   Implements IConvertible.ToByte
      If m_Temp < Byte.MinValue Or m_Temp > Byte.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Byte type.", _ 
                                                   Me.m_Temp)) 
      Else
         Return Decimal.ToByte(Me.m_Temp)
      End If       
   End Function
   
   Public Function ToChar(provider As IFormatProvider) As Char _
                   Implements IConvertible.ToChar
      Throw New InvalidCastException("Temperature to Char conversion is not supported.")
   End Function 
   
   Public Function ToDateTime(provider As IFormatProvider) As Date _
                   Implements IConvertible.ToDateTime
      Throw New InvalidCastException("Temperature to DateTime conversion is not supported.")
   End Function
   
   Public Function ToDecimal(provider As IFormatProvider) As Decimal _
                   Implements IConvertible.ToDecimal
      Return Me.m_Temp
   End Function
   
   Public Function ToDouble(provider As IFormatProvider) As Double _
                   Implements IConvertible.ToDouble
      Return Decimal.ToDouble(Me.m_Temp)
   End Function   
   
   Public Function ToInt16(provider As IFormatProvider) As Int16 _
                   Implements IConvertible.ToInt16
      If Me.m_Temp < Int16.MinValue Or Me.m_Temp > Int16.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int16 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToInt16(Me.m_Temp)   
      End If
   End Function
   
   Public Function ToInt32(provider As IFormatProvider) As Int32 _
                   Implements IConvertible.ToInt32
      If Me.m_Temp < Int32.MinValue Or Me.m_Temp > Int32.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int32 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToInt32(Me.m_Temp)
      End If      
   End Function
   
   Public Function ToInt64(provider As IFormatProvider) As Int64 _
                   Implements IConvertible.ToInt64
      If Me.m_Temp < Int64.MinValue Or Me.m_Temp > Int64.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int64 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToInt64(Me.m_Temp)
      End If      
   End Function
   
   Public Function ToSByte(provider As IFormatProvider) As SByte _
                   Implements IConvertible.ToSByte
      If Me.m_Temp < SByte.MinValue Or Me.m_Temp > SByte.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the SByte type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToSByte(Me.m_Temp)
      End If      
   End Function

   Public Function ToSingle(provider As IFormatProvider) As Single _
                   Implements IConvertible.ToSingle
      Return Decimal.ToSingle(Me.m_Temp)
   End Function

   Public Overloads Function ToString(provider As IFormatProvider) As String _
                   Implements IConvertible.ToString
      Return m_Temp.ToString("N2", provider) & "°C"
   End Function
   
   Public Function ToType(conversionType As Type, provider As IFormatProvider) As Object _
                   Implements IConvertible.ToType
      Select Case Type.GetTypeCode(conversionType)
         Case TypeCode.Boolean 
            Return Me.ToBoolean(Nothing)
         Case TypeCode.Byte
            Return Me.ToByte(Nothing)
         Case TypeCode.Char
            Return Me.ToChar(Nothing)
         Case TypeCode.DateTime
            Return Me.ToDateTime(Nothing)
         Case TypeCode.Decimal
            Return Me.ToDecimal(Nothing)
         Case TypeCode.Double
            Return Me.ToDouble(Nothing)
         Case TypeCode.Int16
            Return Me.ToInt16(Nothing)
         Case TypeCode.Int32
            Return Me.ToInt32(Nothing)
         Case TypeCode.Int64
            Return Me.ToInt64(Nothing)
         Case TypeCode.Object
            If GetType(Temperature).Equals(conversionType) Then
               Return Me
            Else
               Throw New InvalidCastException(String.Format("Conversion to a {0} is not supported.", _
                                                            conversionType.Name))
            End If 
         Case TypeCode.SByte
            Return Me.ToSByte(Nothing)
         Case TypeCode.Single
            Return Me.ToSingle(Nothing)
         Case TypeCode.String
            Return Me.ToString(provider)
         Case TypeCode.UInt16
            Return Me.ToUInt16(Nothing)
         Case TypeCode.UInt32
            Return Me.ToUInt32(Nothing)
         Case TypeCode.UInt64
            Return Me.ToUInt64(Nothing)   
         Case Else
            Throw New InvalidCastException(String.Format("Conversion to {0} is not supported.", conversionType.Name))   
      End Select
   End Function
   
   Public Function ToUInt16(provider As IFormatProvider) As UInt16 _
                   Implements IConvertible.ToUInt16
      If Me.m_Temp < UInt16.MinValue Or Me.m_Temp > UInt16.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt16 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToUInt16(Me.m_Temp)
      End If   
   End Function

   Public Function ToUInt32(provider As IFormatProvider) As UInt32 _
                   Implements IConvertible.ToUInt32
      If Me.m_Temp < UInt32.MinValue Or Me.m_Temp > UInt32.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt32 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToUInt32(Me.m_Temp)
      End If   
   End Function
   
   Public Function ToUInt64(provider As IFormatProvider) As UInt64 _
                   Implements IConvertible.ToUInt64
      If Me.m_Temp < UInt64.MinValue Or Me.m_Temp > UInt64.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt64 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToUInt64(Me.m_temp)
      End If   
   End Function
End Class

下列範例會建立 類別的 Temperature 實例,並呼叫 ChangeType(Object, Type, IFormatProvider) 方法,將它轉換成 .NET 所支援的基本數數值型別,以及 轉換為 String 。 它說明方法會 ChangeType 包裝對來源類型實作的 IConvertible 呼叫。

void main()
{     
   Temperature^ cool = gcnew Temperature(5);
   array<Type^>^ targetTypes = gcnew array<Type^> { SByte::typeid, Int16::typeid, Int32::typeid,
                                                    Int64::typeid, Byte::typeid, UInt16::typeid,
                                                    UInt32::typeid, UInt64::typeid, Decimal::typeid,
                                                    Single::typeid, Double::typeid, String::typeid };
   CultureInfo^ provider = gcnew CultureInfo("fr-FR");
      
   for each (Type^ targetType in targetTypes)
   {
      try {
         Object^ value = Convert::ChangeType(cool, targetType, provider);
         Console::WriteLine("Converted {0} {1} to {2} {3}.",
                           cool->GetType()->Name, cool->ToString(),
                           targetType->Name, value);
      }
      catch (InvalidCastException^) {
         Console::WriteLine("Unsupported {0} --> {1} conversion.",
                           cool->GetType()->Name, targetType->Name);
      }                     
      catch (OverflowException^) {
         Console::WriteLine("{0} is out of range of the {1} type.",
                           cool, targetType->Name);
      }
   }
}
// The example dosplays the following output:
//       Converted Temperature 5.00�C to SByte 5.
//       Converted Temperature 5.00�C to Int16 5.
//       Converted Temperature 5.00�C to Int32 5.
//       Converted Temperature 5.00�C to Int64 5.
//       Converted Temperature 5.00�C to Byte 5.
//       Converted Temperature 5.00�C to UInt16 5.
//       Converted Temperature 5.00�C to UInt32 5.
//       Converted Temperature 5.00�C to UInt64 5.
//       Converted Temperature 5.00�C to Decimal 5.
//       Converted Temperature 5.00�C to Single 5.
//       Converted Temperature 5.00�C to Double 5.
//       Converted Temperature 5.00�C to String 5,00�C.
public class Example
{
   public static void Main()
   {
      Temperature cool = new Temperature(5);
      Type[] targetTypes = { typeof(SByte), typeof(Int16), typeof(Int32),
                             typeof(Int64), typeof(Byte), typeof(UInt16),
                             typeof(UInt32), typeof(UInt64), typeof(Decimal),
                             typeof(Single), typeof(Double), typeof(String) };
      CultureInfo provider = new CultureInfo("fr-FR");

      foreach (Type targetType in targetTypes)
      {
         try {
            object value = Convert.ChangeType(cool, targetType, provider);
            Console.WriteLine("Converted {0} {1} to {2} {3}.",
                              cool.GetType().Name, cool.ToString(),
                              targetType.Name, value);
         }
         catch (InvalidCastException) {
            Console.WriteLine("Unsupported {0} --> {1} conversion.",
                              cool.GetType().Name, targetType.Name);
         }
         catch (OverflowException) {
            Console.WriteLine("{0} is out of range of the {1} type.",
                              cool, targetType.Name);
         }
      }
   }
}
// The example dosplays the following output:
//       Converted Temperature 5.00°C to SByte 5.
//       Converted Temperature 5.00°C to Int16 5.
//       Converted Temperature 5.00°C to Int32 5.
//       Converted Temperature 5.00°C to Int64 5.
//       Converted Temperature 5.00°C to Byte 5.
//       Converted Temperature 5.00°C to UInt16 5.
//       Converted Temperature 5.00°C to UInt32 5.
//       Converted Temperature 5.00°C to UInt64 5.
//       Converted Temperature 5.00°C to Decimal 5.
//       Converted Temperature 5.00°C to Single 5.
//       Converted Temperature 5.00°C to Double 5.
//       Converted Temperature 5.00°C to String 5,00°C.
let cool = Temperature 5
let targetTypes = 
    [ typeof<SByte>; typeof<Int16>; typeof<Int32>
      typeof<Int64>; typeof<Byte>; typeof<UInt16>
      typeof<UInt32>; typeof<UInt64>; typeof<Decimal>;
      typeof<Single>; typeof<Double>; typeof<String>; ]

let provider = CultureInfo "fr-FR"

for targetType in targetTypes do
    try
        let value = Convert.ChangeType(cool, targetType, provider)
        printfn $"Converted {cool.GetType().Name} {cool} to {targetType.Name} {value}."
    with
    | :? InvalidCastException ->
        printfn $"Unsupported {cool.GetType().Name} --> {targetType.Name} conversion."
    | :? OverflowException ->
        printfn $"{cool} is out of range of the {targetType.Name} type."

// The example dosplays the following output:
//       Converted Temperature 5.00°C to SByte 5.
//       Converted Temperature 5.00°C to Int16 5.
//       Converted Temperature 5.00°C to Int32 5.
//       Converted Temperature 5.00°C to Int64 5.
//       Converted Temperature 5.00°C to Byte 5.
//       Converted Temperature 5.00°C to UInt16 5.
//       Converted Temperature 5.00°C to UInt32 5.
//       Converted Temperature 5.00°C to UInt64 5.
//       Converted Temperature 5.00°C to Decimal 5.
//       Converted Temperature 5.00°C to Single 5.
//       Converted Temperature 5.00°C to Double 5.
//       Converted Temperature 5.00°C to String 5,00°C.
Module Example
   Public Sub Main()
      Dim cool As New Temperature(5)
      Dim targetTypes() As Type = { GetType(SByte), GetType(Int16), GetType(Int32), _
                                    GetType(Int64), GetType(Byte), GetType(UInt16), _
                                    GetType(UInt32), GetType(UInt64), GetType(Decimal), _
                                    GetType(Single), GetType(Double), GetType(String) }
      Dim provider As New CultureInfo("fr-FR")
      
      For Each targetType As Type In targetTypes
         Try
            Dim value As Object = Convert.ChangeType(cool, targetType, provider)
            Console.WriteLine("Converted {0} {1} to {2} {3}.", _
                              cool.GetType().Name, cool.ToString(), _
                              targetType.Name, value)
         Catch e As InvalidCastException
            Console.WriteLine("Unsupported {0} --> {1} conversion.", _
                              cool.GetType().Name, targetType.Name)
         Catch e As OverflowException
            Console.WriteLine("{0} is out of range of the {1} type.", _
                              cool, targetType.Name)
         End Try                     
      Next
   End Sub
End Module
' The example displays the following output:
'       Converted Temperature 5.00°C to SByte 5.
'       Converted Temperature 5.00°C to Int16 5.
'       Converted Temperature 5.00°C to Int32 5.
'       Converted Temperature 5.00°C to Int64 5.
'       Converted Temperature 5.00°C to Byte 5.
'       Converted Temperature 5.00°C to UInt16 5.
'       Converted Temperature 5.00°C to UInt32 5.
'       Converted Temperature 5.00°C to UInt64 5.
'       Converted Temperature 5.00°C to Decimal 5.
'       Converted Temperature 5.00°C to Single 5.
'       Converted Temperature 5.00°C to Double 5.
'       Converted Temperature 5.00°C to String 5,00°C.

備註

ChangeType 是一種一般用途的轉換方法,可將 指定的 value conversionType 物件轉換為 。 參數 value 可以是任何類型的物件, conversionType 也可以是 Type 代表任何基底或自訂類型的 物件。 若要讓轉換成功, value 必須實 IConvertible 作 介面,因為 方法只會包裝對適當 IConvertible 方法的呼叫。 方法需要支援的 轉換 value conversionType

參數 provider 是提供 IFormatProvider 轉換格式資訊的實作。 此參數的使用方式和方式取決於基礎 IConvertible 實作。 如果 value 是基底資料類型, provider 則僅用於下列轉換:

如果 value 是應用程式定義的型別,其 IConvertible 實作可能會使用 provider 參數。

給呼叫者的注意事項

方法 ChangeType(Object, Type, IFormatProvider) 可以將列舉值轉換成另一種型別。 不過,即使來源類型是列舉的基礎型別,它也無法將另一個型別轉換成列舉值。 若要將類型轉換成列舉值,請使用 C#) 中的轉換運算子 (,或在 Visual Basic) 中使用轉換函數 (。 下列範例說明從 大陸 列舉值的轉換。

:::code language=「csharp」 source=「~/snippets/csharp/System/Convert/ChangeType/changetype_enum2.cs」 interactive=「try-dotnet」 id=「Snippet5」::::code language=「fsharp」 source=「~/snippets/fsharp/System/Convert/ChangeType/changetype_enum2.fs」 id=「Snippet5」::: :::code language=「vb」 source=「~/snippets/visualbasic/VS_Snippets_CLR_System/system.convert.changetype/vb/changetype_enum2.vb」 id=「Snippet5」::

方法 ChangeType(Object, Type, IFormatProvider) 可以將可為 Null 的類型轉換成另一個型別。 不過,即使 是 的基礎型別, conversionType 它也無法將另一個型別轉換成可為 Null 類型的 Nullable<T> 值。 若要執行轉換,您可以在 C#) 中使用轉換運算子 (,或在 Visual Basic) 中使用轉換函式 (。 下列範例說明從可為 Null 的型別轉換。

:::code language=「csharp」 source=「~/snippets/csharp/System/Convert/ChangeType/changetype_nullable_1.cs」 interactive=「try-dotnet」 id=「Snippet8」:::::code language=「fsharp」 source=「~/snippets/fsharp/System/Convert/ChangeType/changetype_nullable_1.fs」 id=「Snippet8」::: :::code language=「vb」 source=「~/snippets/visualbasic/VS_Snippets_CLR_System/system.convert.changetype/vb/changetype_nullable_1.vb」 id=「Snippet8」::

適用於

ChangeType(Object, TypeCode, IFormatProvider)

傳回物件,該物件屬於指定的類型,且其值等於指定的物件。 參數提供特定文化特性格式資訊。

public:
 static System::Object ^ ChangeType(System::Object ^ value, TypeCode typeCode, IFormatProvider ^ provider);
public static object ChangeType (object value, TypeCode typeCode, IFormatProvider provider);
public static object? ChangeType (object? value, TypeCode typeCode, IFormatProvider? provider);
static member ChangeType : obj * TypeCode * IFormatProvider -> obj
Public Shared Function ChangeType (value As Object, typeCode As TypeCode, provider As IFormatProvider) As Object

參數

value
Object

實作 IConvertible 介面的物件。

typeCode
TypeCode

要傳回的物件類型。

provider
IFormatProvider

物件,提供特定文化特性格式資訊。

傳回

Object

基礎類型為 typeCode 且值與 value 相等的物件。

-或-

如果 Nothingvalue,而且 nulltypeCodeEmptyString,則為 null 參考 (在 Visual Basic 中為 Object)。

例外狀況

不支援此轉換。

-或-

valuenull,而且 typeCode 會指定實值型別。

-或-

value 未實作 IConvertible 介面。

value 格式不是 provider 所辨識的 typeCode 類型。

value 代表超出 typeCode 類型範圍的數字。

typeCode 無效。

範例

下列範例會定義名為 InterceptProvider 的自訂格式提供者,指出何時呼叫其 GetFormat 方法,並傳回 NumberFormatInfo fr-FR 文化特性的 ,以及 DateTimeFormatInfo en-US 文化特性的 物件。 這個格式提供者會用於方法的所有呼叫 ChangeType(Object, TypeCode, IFormatProvider) 中。 此範例接著會建立具有 DoubleDateTime 值的陣列,並使用每個值和列舉的每個成員 TypeCode 重複呼叫 ChangeType(Object, TypeCode, IFormatProvider) 。 此範例說明方法何時使用 IFormatProvider 參數,並說明如何使用 provider 參數來執行區分文化特性的格式。

using namespace System;
using namespace System::Globalization;

ref class InterceptProvider : IFormatProvider
{
public: 
   virtual Object^ GetFormat(Type^ formatType) 
   {
      CultureInfo^ culture;
      if (formatType == NumberFormatInfo::typeid) {
         Console::WriteLine("   Returning a fr-FR numeric format provider.");
         
         culture = gcnew CultureInfo("fr-FR");
         return culture->NumberFormat;
      }  
      else if (formatType == DateTimeFormatInfo::typeid) {
         Console::WriteLine("   Returning an en-US date/time format provider.");
         culture = gcnew CultureInfo("en-US");
         return culture->DateTimeFormat;
      }
      else {
         Console::WriteLine("   Requesting a format provider of {0}.", formatType->Name);
         return nullptr;
      }
   }
};

void main()
{
   array<Object^>^ values = gcnew array<Object^> { 103.5, gcnew DateTime(2010, 12, 26, 14, 34, 0) };
   IFormatProvider^ provider = gcnew InterceptProvider();
      
   // Convert value to each of the types represented in TypeCode enum.
   for each (Object^ value in values)
   {
      // Iterate types in TypeCode enum.
      for each (TypeCode enumType in (array<TypeCode>^) Enum::GetValues(TypeCode::typeid))
      {         
         if (enumType == TypeCode::DBNull || enumType == TypeCode::Empty) continue;
            
         try {
            Console::WriteLine("{0} ({1}) --> {2} ({3}).", 
                              value, value->GetType()->Name,
                              Convert::ChangeType(value, enumType, provider),
                              enumType.ToString());
         }
         catch (InvalidCastException^ e) {
            Console::WriteLine("Cannot convert a {0} to a {1}",
                              value->GetType()->Name, enumType.ToString());
         }                     
         catch (OverflowException^ e) {
            Console::WriteLine("Overflow: {0} is out of the range of a {1}",
                              value, enumType.ToString());
         }
      }
      Console::WriteLine();
   }
}
// The example displays the following output:
//    103.5 (Double) --> 103.5 (Object).
//    103.5 (Double) --> True (Boolean).
//    Cannot convert a Double to a Char
//    103.5 (Double) --> 104 (SByte).
//    103.5 (Double) --> 104 (Byte).
//    103.5 (Double) --> 104 (Int16).
//    103.5 (Double) --> 104 (UInt16).
//    103.5 (Double) --> 104 (Int32).
//    103.5 (Double) --> 104 (UInt32).
//    103.5 (Double) --> 104 (Int64).
//    103.5 (Double) --> 104 (UInt64).
//    103.5 (Double) --> 103.5 (Single).
//    103.5 (Double) --> 103.5 (Double).
//    103.5 (Double) --> 103.5 (Decimal).
//    Cannot convert a Double to a DateTime
//       Returning a fr-FR numeric format provider.
//    103.5 (Double) --> 103,5 (String).
//    
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (Object).
//    Cannot convert a DateTime to a Boolean
//    Cannot convert a DateTime to a Char
//    Cannot convert a DateTime to a SByte
//    Cannot convert a DateTime to a Byte
//    Cannot convert a DateTime to a Int16
//    Cannot convert a DateTime to a UInt16
//    Cannot convert a DateTime to a Int32
//    Cannot convert a DateTime to a UInt32
//    Cannot convert a DateTime to a Int64
//    Cannot convert a DateTime to a UInt64
//    Cannot convert a DateTime to a Single
//    Cannot convert a DateTime to a Double
//    Cannot convert a DateTime to a Decimal
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (DateTime).
//       Returning an en-US date/time format provider.
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (String).
using System;
using System.Globalization;

public class InterceptProvider : IFormatProvider
{
   public object GetFormat(Type formatType)
   {
      if (formatType == typeof(NumberFormatInfo)) {
         Console.WriteLine("   Returning a fr-FR numeric format provider.");
         return new System.Globalization.CultureInfo("fr-FR").NumberFormat;
      }
      else if (formatType == typeof(DateTimeFormatInfo)) {
         Console.WriteLine("   Returning an en-US date/time format provider.");
         return new System.Globalization.CultureInfo("en-US").DateTimeFormat;
      }
      else {
         Console.WriteLine("   Requesting a format provider of {0}.", formatType.Name);
         return null;
      }
   }
}

public class Example
{
   public static void Main()
   {
      object[] values = { 103.5d, new DateTime(2010, 12, 26, 14, 34, 0) };
      IFormatProvider provider = new InterceptProvider();

      // Convert value to each of the types represented in TypeCode enum.
      foreach (object value in values)
      {
         // Iterate types in TypeCode enum.
         foreach (TypeCode enumType in ((TypeCode[]) Enum.GetValues(typeof(TypeCode))))
         {
            if (enumType == TypeCode.DBNull || enumType == TypeCode.Empty) continue;

            try {
               Console.WriteLine("{0} ({1}) --> {2} ({3}).",
                                 value, value.GetType().Name,
                                 Convert.ChangeType(value, enumType, provider),
                                 enumType.ToString());
            }
            catch (InvalidCastException) {
               Console.WriteLine("Cannot convert a {0} to a {1}",
                                 value.GetType().Name, enumType.ToString());
            }
            catch (OverflowException) {
               Console.WriteLine("Overflow: {0} is out of the range of a {1}",
                                 value, enumType.ToString());
            }
         }
         Console.WriteLine();
      }
   }
}
// The example displays the following output:
//    103.5 (Double) --> 103.5 (Object).
//    103.5 (Double) --> True (Boolean).
//    Cannot convert a Double to a Char
//    103.5 (Double) --> 104 (SByte).
//    103.5 (Double) --> 104 (Byte).
//    103.5 (Double) --> 104 (Int16).
//    103.5 (Double) --> 104 (UInt16).
//    103.5 (Double) --> 104 (Int32).
//    103.5 (Double) --> 104 (UInt32).
//    103.5 (Double) --> 104 (Int64).
//    103.5 (Double) --> 104 (UInt64).
//    103.5 (Double) --> 103.5 (Single).
//    103.5 (Double) --> 103.5 (Double).
//    103.5 (Double) --> 103.5 (Decimal).
//    Cannot convert a Double to a DateTime
//       Returning a fr-FR numeric format provider.
//    103.5 (Double) --> 103,5 (String).
//
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (Object).
//    Cannot convert a DateTime to a Boolean
//    Cannot convert a DateTime to a Char
//    Cannot convert a DateTime to a SByte
//    Cannot convert a DateTime to a Byte
//    Cannot convert a DateTime to a Int16
//    Cannot convert a DateTime to a UInt16
//    Cannot convert a DateTime to a Int32
//    Cannot convert a DateTime to a UInt32
//    Cannot convert a DateTime to a Int64
//    Cannot convert a DateTime to a UInt64
//    Cannot convert a DateTime to a Single
//    Cannot convert a DateTime to a Double
//    Cannot convert a DateTime to a Decimal
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (DateTime).
//       Returning an en-US date/time format provider.
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (String).
open System
open System.Globalization

type InterceptProvider() =
    interface IFormatProvider with
        member _.GetFormat(formatType: Type) =
            if formatType = typeof<NumberFormatInfo> then
                printfn "   Returning a fr-FR numeric format provider."
                CultureInfo("fr-FR").NumberFormat
            elif formatType = typeof<DateTimeFormatInfo> then
                printfn "   Returning an en-US date/time format provider."
                CultureInfo("en-US").DateTimeFormat
            else
                printfn $"   Requesting a format provider of {formatType.Name}."
                null

let values: obj[] = [| 103.5; DateTime(2010, 12, 26, 14, 34, 0)|]
let provider = InterceptProvider()

// Convert value to each of the types represented in TypeCode enum.
for value in values do
    // Iterate types in TypeCode enum.
    for enumType in Enum.GetValues typeof<TypeCode> :?> TypeCode[] do
        match enumType with
        | TypeCode.DBNull | TypeCode.Empty -> ()
        | _ ->
            try
                printfn $"{value} ({value.GetType().Name}) --> {Convert.ChangeType(value, enumType, provider)} ({enumType})."
            with
            | :? InvalidCastException ->
                printfn $"Cannot convert a {value.GetType().Name} to a {enumType}"
            | :? OverflowException ->
                printfn $"Overflow: {value} is out of the range of a {enumType}"
    printfn ""

// The example displays the following output:
//    103.5 (Double) --> 103.5 (Object).
//    103.5 (Double) --> True (Boolean).
//    Cannot convert a Double to a Char
//    103.5 (Double) --> 104 (SByte).
//    103.5 (Double) --> 104 (Byte).
//    103.5 (Double) --> 104 (Int16).
//    103.5 (Double) --> 104 (UInt16).
//    103.5 (Double) --> 104 (Int32).
//    103.5 (Double) --> 104 (UInt32).
//    103.5 (Double) --> 104 (Int64).
//    103.5 (Double) --> 104 (UInt64).
//    103.5 (Double) --> 103.5 (Single).
//    103.5 (Double) --> 103.5 (Double).
//    103.5 (Double) --> 103.5 (Decimal).
//    Cannot convert a Double to a DateTime
//       Returning a fr-FR numeric format provider.
//    103.5 (Double) --> 103,5 (String).
//
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (Object).
//    Cannot convert a DateTime to a Boolean
//    Cannot convert a DateTime to a Char
//    Cannot convert a DateTime to a SByte
//    Cannot convert a DateTime to a Byte
//    Cannot convert a DateTime to a Int16
//    Cannot convert a DateTime to a UInt16
//    Cannot convert a DateTime to a Int32
//    Cannot convert a DateTime to a UInt32
//    Cannot convert a DateTime to a Int64
//    Cannot convert a DateTime to a UInt64
//    Cannot convert a DateTime to a Single
//    Cannot convert a DateTime to a Double
//    Cannot convert a DateTime to a Decimal
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (DateTime).
//       Returning an en-US date/time format provider.
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (String).
Imports System.Globalization

Public Class InterceptProvider : Implements IFormatProvider
   Public Function GetFormat(formatType As Type) As Object _
          Implements IFormatProvider.GetFormat
      If formatType.Equals(GetType(NumberFormatInfo)) Then
         Console.WriteLine("   Returning a fr-FR numeric format provider.")
         Return New CultureInfo("fr-FR").NumberFormat
      ElseIf formatType.Equals(GetType(DateTimeFormatInfo)) Then
         Console.WriteLine("   Returning an en-US date/time format provider.")
         Return New CultureInfo("en-US").DateTimeFormat
      Else
         Console.WriteLine("   Requesting a format provider of {0}.", formatType.Name)
         Return Nothing
      End If
   End Function
End Class

Module Example
   Public Sub Main()
      Dim values() As Object = { 103.5r, #12/26/2010 2:34PM# }
      Dim provider As New InterceptProvider()
      
      ' Convert value to each of the types represented in TypeCode enum.
      For Each value As Object In values
         ' Iterate types in TypeCode enum.
         For Each enumType As TypeCode In DirectCast([Enum].GetValues(GetType(TypeCode)), TypeCode())         
            If enumType = TypeCode.DbNull Or enumType = TypeCode.Empty Then Continue For
            
            Try
               Console.WriteLine("{0} ({1}) --> {2} ({3}).", _
                                 value, value.GetType().Name, _
                                 Convert.ChangeType(value, enumType, provider), _
                                 enumType.ToString())
            Catch e As InvalidCastException
               Console.WriteLine("Cannot convert a {0} to a {1}", _
                                 value.GetType().Name, enumType.ToString())
            Catch e As OverflowException
               Console.WriteLine("Overflow: {0} is out of the range of a {1}", _
                                 value, enumType.ToString())
            End Try
         Next
         Console.WriteLine()
      Next
   End Sub
End Module
' The example displays the following output:
'    103.5 (Double) --> 103.5 (Object).
'    103.5 (Double) --> True (Boolean).
'    Cannot convert a Double to a Char
'    103.5 (Double) --> 104 (SByte).
'    103.5 (Double) --> 104 (Byte).
'    103.5 (Double) --> 104 (Int16).
'    103.5 (Double) --> 104 (UInt16).
'    103.5 (Double) --> 104 (Int32).
'    103.5 (Double) --> 104 (UInt32).
'    103.5 (Double) --> 104 (Int64).
'    103.5 (Double) --> 104 (UInt64).
'    103.5 (Double) --> 103.5 (Single).
'    103.5 (Double) --> 103.5 (Double).
'    103.5 (Double) --> 103.5 (Decimal).
'    Cannot convert a Double to a DateTime
'       Returning a fr-FR numeric format provider.
'    103.5 (Double) --> 103,5 (String).
'    
'    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (Object).
'    Cannot convert a DateTime to a Boolean
'    Cannot convert a DateTime to a Char
'    Cannot convert a DateTime to a SByte
'    Cannot convert a DateTime to a Byte
'    Cannot convert a DateTime to a Int16
'    Cannot convert a DateTime to a UInt16
'    Cannot convert a DateTime to a Int32
'    Cannot convert a DateTime to a UInt32
'    Cannot convert a DateTime to a Int64
'    Cannot convert a DateTime to a UInt64
'    Cannot convert a DateTime to a Single
'    Cannot convert a DateTime to a Double
'    Cannot convert a DateTime to a Decimal
'    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (DateTime).
'       Returning an en-US date/time format provider.
'    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (String).

備註

ChangeType(Object, TypeCode, IFormatProvider) 是一種一般用途的轉換方法,可將 指定的 value 物件轉換成 所 typeCode 指定的預先定義型別。 參數 value 可以是任何類型的物件。 若要讓轉換成功, value 必須實 IConvertible 作 介面,因為 方法只會包裝對適當 IConvertible 方法的呼叫。 方法也需要支援的 轉換 value typeCode

方法 ChangeType(Object, TypeCode, IFormatProvider) 不支援轉換成 value 自訂類型。 若要執行這類轉換,請呼叫 ChangeType(Object, Type, IFormatProvider) 方法。

參數 provider 是提供 IFormatProvider 轉換格式資訊的實作。 此參數的使用方式和方式取決於基礎 IConvertible 實作。 如果 value 是基底資料類型, provider 則僅用於下列轉換。 null IFormatProvider 如果引數傳遞至這些方法,則會 CultureInfo 使用代表目前文化特性的物件。

如果 value 是應用程式定義的型別,則其 IConvertible 實作可能會使用 provider 參數。

適用於