共用方式為


.NET 中的類型轉換

每個值都有相關聯的類型,該類型定義屬性,例如配置給值的空間量、能夠擁有的可能值範圍,以及提供的成員。 許多值都可以表示成多種類型。 例如,數值 4 就可以表示成整數值或浮點 (Floating-Point) 值。 類型轉換會建立新類型的值,與舊類型的值相等,但是不一定會保留原始物件的識別 (或實際的值)。

.NET 自動支援下列轉換︰

  • 從衍生類別轉換成基底類別。 舉例來說,這表示任何類別或結構的執行個體都可以轉換成 Object 執行個體。 這種轉換不需要轉型或轉換運算子。

  • 從基底類別轉換回原始的衍生類別。 在 C# 中,這種轉換需要轉型運算子。 在 Visual Basic 中,如果已 Option Strict 開啟則需要 CType 運算子。

  • 從實作介面的類型轉換成代表該介面的介面物件。 這種轉換不需要轉型或轉換運算子。

  • 從介面物件轉換回實作該介面的原始類型。 在 C# 中,這種轉換需要轉型運算子。 在 Visual Basic 中,如果已 Option Strict 開啟則需要 CType 運算子。

除了這些自動轉換以外,.NET 還提供多項支援自訂類型轉換的功能。 其中包括下列各項:

使用隱含運算子的隱含轉換

擴展轉換包含從現有類型的值來建立新的值,這個類型具有比目標類型更嚴格的範圍或更受限制的成員清單。 擴展轉換不可能導致資料遺失 (雖然可能導致精確度降低)。 因為不可能遺失資料,編譯器可以隱含地 (或直接地) 處理轉換,而不需要使用明確的轉換方法或轉型運算子。

注意

雖然執行隱含轉換的程式碼可以呼叫轉換方法或使用轉型運算子,但支援隱含轉換的編譯器並不需要這些動作。

例如,Decimal 類型支援從 ByteCharInt16Int32Int64SByteUInt16UInt32UInt64 值的隱含轉換。 下列範例說明其中一部分隱含轉換在指派值給 Decimal 變數時的情形。

  byte byteValue = 16;
  short shortValue = -1024;
  int intValue = -1034000;
  long longValue = 1152921504606846976;
  ulong ulongValue = UInt64.MaxValue;

  decimal decimalValue;

  decimalValue = byteValue;
  Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                    byteValue.GetType().Name, decimalValue);

  decimalValue = shortValue;
  Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                    shortValue.GetType().Name, decimalValue);

  decimalValue = intValue;
  Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                    intValue.GetType().Name, decimalValue);

  decimalValue = longValue;
  Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                    longValue.GetType().Name, decimalValue);

decimalValue = ulongValue;
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  ulongValue.GetType().Name, decimalValue);
// The example displays the following output:
//    After assigning a Byte value, the Decimal value is 16.
//    After assigning a Int16 value, the Decimal value is -1024.
//    After assigning a Int32 value, the Decimal value is -1034000.
//    After assigning a Int64 value, the Decimal value is 1152921504606846976.
//    After assigning a UInt64 value, the Decimal value is 18446744073709551615.
Dim byteValue As Byte = 16
Dim shortValue As Short = -1024
Dim intValue As Integer = -1034000
Dim longValue As Long = CLng(1024 ^ 6)
Dim ulongValue As ULong = ULong.MaxValue

Dim decimalValue As Decimal

decimalValue = byteValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  byteValue.GetType().Name, decimalValue)

decimalValue = shortValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  shortValue.GetType().Name, decimalValue)

decimalValue = intValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  intValue.GetType().Name, decimalValue)

decimalValue = longValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  longValue.GetType().Name, decimalValue)

decimalValue = ulongValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  ulongValue.GetType().Name, decimalValue)
' The example displays the following output:
'    After assigning a Byte value, the Decimal value is 16.
'    After assigning a Int16 value, the Decimal value is -1024.
'    After assigning a Int32 value, the Decimal value is -1034000.
'    After assigning a Int64 value, the Decimal value is 1152921504606846976.
'    After assigning a UInt64 value, the Decimal value is 18446744073709551615.

如果特定的語言編譯器支援自訂運算子,您也可以在自己的自訂類型中定義隱含轉換。 下列範例針對使用正負號和大小表示之名為 ByteWithSign 的帶正負號位元組資料類型,提供部分實作。 它支援將 ByteSByte 值隱含轉換為 ByteWithSign 值。

public struct ByteWithSign
{
    private SByte signValue;
    private Byte value;

    public static implicit operator ByteWithSign(SByte value)
    {
        ByteWithSign newValue;
        newValue.signValue = (SByte)Math.Sign(value);
        newValue.value = (byte)Math.Abs(value);
        return newValue;
    }

    public static implicit operator ByteWithSign(Byte value)
    {
        ByteWithSign newValue;
        newValue.signValue = 1;
        newValue.value = value;
        return newValue;
    }

    public override string ToString()
    {
        return (signValue * value).ToString();
    }
}
Public Structure ImplicitByteWithSign
    Private signValue As SByte
    Private value As Byte

    Public Overloads Shared Widening Operator CType(value As SByte) As ImplicitByteWithSign
        Dim newValue As ImplicitByteWithSign
        newValue.signValue = CSByte(Math.Sign(value))
        newValue.value = CByte(Math.Abs(value))
        Return newValue
    End Operator

    Public Overloads Shared Widening Operator CType(value As Byte) As ImplicitByteWithSign
        Dim NewValue As ImplicitByteWithSign
        newValue.signValue = 1
        newValue.value = value
        Return newValue
    End Operator

    Public Overrides Function ToString() As String
        Return (signValue * value).ToString()
    End Function
End Structure

然後,用戶端程式碼可以宣告 ByteWithSign 變數並指派 ByteSByte 值給它,而不需要執行任何明確的轉換,也不需要使用任何轉型運算子,如下列範例所示。

SByte sbyteValue = -120;
ByteWithSign value = sbyteValue;
Console.WriteLine(value);
value = Byte.MaxValue;
Console.WriteLine(value);
// The example displays the following output:
//       -120
//       255
Dim sbyteValue As SByte = -120
Dim value As ImplicitByteWithSign = sbyteValue
Console.WriteLine(value.ToString())
value = Byte.MaxValue
Console.WriteLine(value.ToString())
' The example displays the following output:
'       -120
'       255

使用明確運算子的明確轉換

縮小轉換包含從現有類型的值來建立新的值,這個類型具有比目標類型更廣的範圍或更大的成員清單。 因為縮小轉換可能導致資料遺失,編譯器通常會要求必須透過呼叫轉換方法或轉型運算子來明確執行轉換。 也就是說,開發人員程式碼中必須明確處理轉換。

注意

縮小轉換需要轉換方法或轉型運算子的主要目的是讓開發人員知道可能遺失資料,或可能發生 OverflowException,以便在程式碼中處理。 不過,某些編譯器可能會放寬這項需求。 例如,在 Visual Basic 中,如果 Option Strict 已關閉 (此為預設),則 Visual Basic 編譯器會嘗試隱含地執行縮小轉換。

例如,UInt32Int64UInt64 資料類型的範圍超過 Int32 資料類型的範圍,如下表所示。

類型 與 Int32 的範圍比較
Int64 Int64.MaxValue 大於 Int32.MaxValue,而 Int64.MinValue 小於 (負值範圍大於) Int32.MinValue
UInt32 UInt32.MaxValue 大於 Int32.MaxValue
UInt64 UInt64.MaxValue 大於 Int32.MaxValue

為了處理縮小轉換,.NET 允許類型定義 Explicit 運算子。 然後,個別語言編譯器就可以使用自己的語法來實作這個運算子,也可以呼叫 Convert 類別的成員來執行轉換。 (如需 Convert 類別的詳細資訊,請參閱本主題稍後的 Convert 類別。)下列範例說明如何使用語言功能,處理從這些可能超出範圍之整數值至 Int32 值的明確轉換。

long number1 = int.MaxValue + 20L;
uint number2 = int.MaxValue - 1000;
ulong number3 = int.MaxValue;

int intNumber;

try
{
    intNumber = checked((int)number1);
    Console.WriteLine("After assigning a {0} value, the Integer value is {1}.",
                      number1.GetType().Name, intNumber);
}
catch (OverflowException)
{
    if (number1 > int.MaxValue)
        Console.WriteLine("Conversion failed: {0} exceeds {1}.",
                          number1, int.MaxValue);
    else
        Console.WriteLine("Conversion failed: {0} is less than {1}.",
                          number1, int.MinValue);
}

try
{
    intNumber = checked((int)number2);
    Console.WriteLine("After assigning a {0} value, the Integer value is {1}.",
                      number2.GetType().Name, intNumber);
}
catch (OverflowException)
{
    Console.WriteLine("Conversion failed: {0} exceeds {1}.",
                      number2, int.MaxValue);
}

try
{
    intNumber = checked((int)number3);
    Console.WriteLine("After assigning a {0} value, the Integer value is {1}.",
                      number3.GetType().Name, intNumber);
}
catch (OverflowException)
{
    Console.WriteLine("Conversion failed: {0} exceeds {1}.",
                      number1, int.MaxValue);
}

// The example displays the following output:
//    Conversion failed: 2147483667 exceeds 2147483647.
//    After assigning a UInt32 value, the Integer value is 2147482647.
//    After assigning a UInt64 value, the Integer value is 2147483647.
Dim number1 As Long = Integer.MaxValue + 20L
Dim number2 As UInteger = Integer.MaxValue - 1000
Dim number3 As ULong = Integer.MaxValue

Dim intNumber As Integer

Try
    intNumber = CInt(number1)
    Console.WriteLine("After assigning a {0} value, the Integer value is {1}.",
                        number1.GetType().Name, intNumber)
Catch e As OverflowException
    If number1 > Integer.MaxValue Then
        Console.WriteLine("Conversion failed: {0} exceeds {1}.",
                                          number1, Integer.MaxValue)
    Else
        Console.WriteLine("Conversion failed: {0} is less than {1}.\n",
                                          number1, Integer.MinValue)
    End If
End Try

Try
    intNumber = CInt(number2)
    Console.WriteLine("After assigning a {0} value, the Integer value is {1}.",
                        number2.GetType().Name, intNumber)
Catch e As OverflowException
    Console.WriteLine("Conversion failed: {0} exceeds {1}.",
                                      number2, Integer.MaxValue)
End Try

Try
    intNumber = CInt(number3)
    Console.WriteLine("After assigning a {0} value, the Integer value is {1}.",
                        number3.GetType().Name, intNumber)
Catch e As OverflowException
    Console.WriteLine("Conversion failed: {0} exceeds {1}.",
                                      number1, Integer.MaxValue)
End Try
' The example displays the following output:
'    Conversion failed: 2147483667 exceeds 2147483647.
'    After assigning a UInt32 value, the Integer value is 2147482647.
'    After assigning a UInt64 value, the Integer value is 2147483647.

明確轉換在不同的語言中可能會產生不同的結果,這些結果可能與對應的 Convert 方法所傳回的值不同。 例如,如果 Double 值 12.63251 轉換成 Int32,則 Visual Basic CInt 方法與 .NET Convert.ToInt32(Double) 方法都會四捨五入 Double 而傳回 13,但 C# (int) 運算子會截斷 Double 而傳回 12。 同樣地,C# (int) 運算子不支援布林值對整數的轉換,但 Visual Basic CInt 方法會將 true 的值轉換為 -1。 另一方面,Convert.ToInt32(Boolean) 方法會將 true 的值轉換為 1。

大多數的編譯器都允許明確轉換以已檢查或未檢查的方式執行。 如果執行已檢查的轉換,當要轉換之類型的值不在目標類型的範圍內時,則會擲回 OverflowException。 在同樣的狀況下執行未檢查的轉換時,轉換可能不會擲回例外狀況,但實際行為會變得不明確,且可能產生不正確的值。

注意

在 C# 中,已檢查的轉換可以透過使用 checked 關鍵字搭配轉型運算子來執行,也可以指定 /checked+ 編譯器選項來執行。 相反地,未檢查的轉換可使用 unchecked 關鍵字搭配轉型運算子來執行,或可以指定 /checked- 編譯器選項來執行。 根據預設,明確轉換是未檢查的。 在 Visual Basic 中,已檢查的轉換可藉由清除專案的 [進階編譯器設定] 對話方塊中的 [移除整數的溢位檢查] 核取方塊來執行,或可以指定 /removeintchecks- 編譯器選項來執行。 相反地,未檢查的轉換可以藉由選取專案的 [進階編譯器設定] 對話方塊中的 [移除整數的溢位檢查] 核取方塊來執行,也可以指定 /removeintchecks+ 編譯器選項來執行。 根據預設,明確轉換是檢查的。

下列 C# 範例使用 checkedunchecked 關鍵字,說明將超出 Byte 範圍的值轉換為 Byte 時的行為差異。 已檢查的轉換會擲回例外狀況,但未檢查的轉換會指派 Byte.MaxValueByte 變數。

int largeValue = Int32.MaxValue;
byte newValue;

try
{
    newValue = unchecked((byte)largeValue);
    Console.WriteLine("Converted the {0} value {1} to the {2} value {3}.",
                      largeValue.GetType().Name, largeValue,
                      newValue.GetType().Name, newValue);
}
catch (OverflowException)
{
    Console.WriteLine("{0} is outside the range of the Byte data type.",
                      largeValue);
}

try
{
    newValue = checked((byte)largeValue);
    Console.WriteLine("Converted the {0} value {1} to the {2} value {3}.",
                      largeValue.GetType().Name, largeValue,
                      newValue.GetType().Name, newValue);
}
catch (OverflowException)
{
    Console.WriteLine("{0} is outside the range of the Byte data type.",
                      largeValue);
}
// The example displays the following output:
//    Converted the Int32 value 2147483647 to the Byte value 255.
//    2147483647 is outside the range of the Byte data type.

如果特定的語言編譯器支援自訂多載運算子,您也可以在自己的自訂類型中定義明確轉換。 下列範例針對使用正負號和大小表示之名為 ByteWithSign 的帶正負號位元組資料類型,提供部分實作。 它支援將 Int32UInt32 值明確轉換為 ByteWithSign 值。

public struct ByteWithSignE
{
    private SByte signValue;
    private Byte value;

    private const byte MaxValue = byte.MaxValue;
    private const int MinValue = -1 * byte.MaxValue;

    public static explicit operator ByteWithSignE(int value)
    {
        // Check for overflow.
        if (value > ByteWithSignE.MaxValue || value < ByteWithSignE.MinValue)
            throw new OverflowException(String.Format("'{0}' is out of range of the ByteWithSignE data type.",
                                                      value));

        ByteWithSignE newValue;
        newValue.signValue = (SByte)Math.Sign(value);
        newValue.value = (byte)Math.Abs(value);
        return newValue;
    }

    public static explicit operator ByteWithSignE(uint value)
    {
        if (value > ByteWithSignE.MaxValue)
            throw new OverflowException(String.Format("'{0}' is out of range of the ByteWithSignE data type.",
                                                      value));

        ByteWithSignE newValue;
        newValue.signValue = 1;
        newValue.value = (byte)value;
        return newValue;
    }

    public override string ToString()
    {
        return (signValue * value).ToString();
    }
}
Public Structure ByteWithSign
    Private signValue As SByte
    Private value As Byte

    Private Const MaxValue As Byte = Byte.MaxValue
    Private Const MinValue As Integer = -1 * Byte.MaxValue

    Public Overloads Shared Narrowing Operator CType(value As Integer) As ByteWithSign
        ' Check for overflow.
        If value > ByteWithSign.MaxValue Or value < ByteWithSign.MinValue Then
            Throw New OverflowException(String.Format("'{0}' is out of range of the ByteWithSign data type.", value))
        End If

        Dim newValue As ByteWithSign

        newValue.signValue = CSByte(Math.Sign(value))
        newValue.value = CByte(Math.Abs(value))
        Return newValue
    End Operator

    Public Overloads Shared Narrowing Operator CType(value As UInteger) As ByteWithSign
        If value > ByteWithSign.MaxValue Then
            Throw New OverflowException(String.Format("'{0}' is out of range of the ByteWithSign data type.", value))
        End If

        Dim NewValue As ByteWithSign

        newValue.signValue = 1
        newValue.value = CByte(value)
        Return newValue
    End Operator

    Public Overrides Function ToString() As String
        Return (signValue * value).ToString()
    End Function
End Structure

然後,用戶端程式碼可以宣告 ByteWithSign 變數並指派 Int32UInt32 值給它 (如果指派包含轉型運算子或轉換方法的話),如下列範例所示。

ByteWithSignE value;

try
{
    int intValue = -120;
    value = (ByteWithSignE)intValue;
    Console.WriteLine(value);
}
catch (OverflowException e)
{
    Console.WriteLine(e.Message);
}

try
{
    uint uintValue = 1024;
    value = (ByteWithSignE)uintValue;
    Console.WriteLine(value);
}
catch (OverflowException e)
{
    Console.WriteLine(e.Message);
}
// The example displays the following output:
//       -120
//       '1024' is out of range of the ByteWithSignE data type.
Dim value As ByteWithSign

Try
    Dim intValue As Integer = -120
    value = CType(intValue, ByteWithSign)
    Console.WriteLine(value)
Catch e As OverflowException
    Console.WriteLine(e.Message)
End Try

Try
    Dim uintValue As UInteger = 1024
    value = CType(uintValue, ByteWithSign)
    Console.WriteLine(value)
Catch e As OverflowException
    Console.WriteLine(e.Message)
End Try
' The example displays the following output:
'       -120
'       '1024' is out of range of the ByteWithSign data type.

IConvertible 介面

為了支援從任何類型轉換至通用語言執行平台基底類型的作業,.NET 提供了 IConvertible 介面。 實作類型需要提供下列項目:

  • 傳回實作類型之 TypeCode 的方法。

  • 將實作類型轉換為每一個通用語言執行平台基底類型 (BooleanByteDateTimeDecimalDouble 等) 的方法。

  • 將實作類型的執行個體轉換為另一個指定之類型的通用轉換方法。 不支援的轉換應該擲回 InvalidCastException

每一個通用語言執行平台基底類型 (也就是 BooleanByteCharDateTimeDecimalDoubleInt16Int32Int64SByteSingleStringUInt16UInt32UInt64),以及 DBNullEnum 類型,都實作 IConvertible 介面。 不過,這些都是明確的介面實作,且只能透過 IConvertible 介面變數來呼叫轉換方法,如下列範例所示。 這個範例將 Int32 值轉換為其對等的 Char 值。

int codePoint = 1067;
IConvertible iConv = codePoint;
char ch = iConv.ToChar(null);
Console.WriteLine("Converted {0} to {1}.", codePoint, ch);
Dim codePoint As Integer = 1067
Dim iConv As IConvertible = codePoint
Dim ch As Char = iConv.ToChar(Nothing)
Console.WriteLine("Converted {0} to {1}.", codePoint, ch)

由於必須呼叫其介面上的轉換方法,而不是呼叫實作類型上的轉換方法,這種需求使得明確介面實作相當耗費資源。 相反地,在通用語言執行平台基底類型之間轉換時,我們建議您呼叫 Convert 類別的適當成員。 如需詳細資訊,請參閱下一節 Convert 類別

注意

除了 .NET 提供的 IConvertible 介面和 Convert 類別以外,個別語言可能還會提供執行轉換的方式。 例如,C# 使用轉型 (Casting) 運算子、Visual Basic 使用編譯器實作的轉換函式,例如 CTypeCIntDirectCast

在大多數情況下,IConvertible 介面的設計主要是支援在 .NET 中的各基底類型之間轉換。 不過,也可以使用自訂類型來實作介面,以支援從該類型轉換至其他自訂類型。 如需詳細資訊,請參閱本主題稍後的使用 ChangeType 方法的自訂轉換一節。

Convert 類別

雖然可以呼叫每一個基底類別的 IConvertible 介面實作來執行類型轉換,但建議的語言中立方式是呼叫 System.Convert 類別的方法,在不同的基底類型之間轉換。 此外,也可以使用 Convert.ChangeType(Object, Type, IFormatProvider) 方法將指定的自訂類型轉換為另一種類型。

基底類型之間的轉換

Convert 類別提供在基底類型之間轉換的語言中立方式,且所有以通用語言執行平台為目標的語言都可以使用。 它對於擴展和縮小轉換提供一組完整的方法,且對於不支援的轉換會擲回 InvalidCastException (例如,將 DateTime 值轉換為整數值)。 縮小轉換是在已檢查的情況下執行,如果轉換失敗,則會擲回 OverflowException

重要

因為 Convert 類別包含在每一個基底類型之間來回轉換的方法,所以不需要呼叫每一個基底類型的 IConvertible 明確介面實作。

下列範例說明如何使用 System.Convert 類別,在 .NET 基底類型之間執行數個擴展和縮小轉換。

// Convert an Int32 value to a Decimal (a widening conversion).
int integralValue = 12534;
decimal decimalValue = Convert.ToDecimal(integralValue);
Console.WriteLine("Converted the {0} value {1} to " +
                                  "the {2} value {3:N2}.",
                                  integralValue.GetType().Name,
                                  integralValue,
                                  decimalValue.GetType().Name,
                                  decimalValue);
// Convert a Byte value to an Int32 value (a widening conversion).
byte byteValue = Byte.MaxValue;
int integralValue2 = Convert.ToInt32(byteValue);
Console.WriteLine("Converted the {0} value {1} to " +
                                  "the {2} value {3:G}.",
                                  byteValue.GetType().Name,
                                  byteValue,
                                  integralValue2.GetType().Name,
                                  integralValue2);

// Convert a Double value to an Int32 value (a narrowing conversion).
double doubleValue = 16.32513e12;
try
{
    long longValue = Convert.ToInt64(doubleValue);
    Console.WriteLine("Converted the {0} value {1:E} to " +
                                      "the {2} value {3:N0}.",
                                      doubleValue.GetType().Name,
                                      doubleValue,
                                      longValue.GetType().Name,
                                      longValue);
}
catch (OverflowException)
{
    Console.WriteLine("Unable to convert the {0:E} value {1}.",
                                      doubleValue.GetType().Name, doubleValue);
}

// Convert a signed byte to a byte (a narrowing conversion).
sbyte sbyteValue = -16;
try
{
    byte byteValue2 = Convert.ToByte(sbyteValue);
    Console.WriteLine("Converted the {0} value {1} to " +
                                      "the {2} value {3:G}.",
                                      sbyteValue.GetType().Name,
                                      sbyteValue,
                                      byteValue2.GetType().Name,
                                      byteValue2);
}
catch (OverflowException)
{
    Console.WriteLine("Unable to convert the {0} value {1}.",
                                      sbyteValue.GetType().Name, sbyteValue);
}
// The example displays the following output:
//       Converted the Int32 value 12534 to the Decimal value 12,534.00.
//       Converted the Byte value 255 to the Int32 value 255.
//       Converted the Double value 1.632513E+013 to the Int64 value 16,325,130,000,000.
//       Unable to convert the SByte value -16.
' Convert an Int32 value to a Decimal (a widening conversion).
Dim integralValue As Integer = 12534
Dim decimalValue As Decimal = Convert.ToDecimal(integralValue)
Console.WriteLine("Converted the {0} value {1} to the {2} value {3:N2}.",
                  integralValue.GetType().Name,
                  integralValue,
                  decimalValue.GetType().Name,
                  decimalValue)

' Convert a Byte value to an Int32 value (a widening conversion).
Dim byteValue As Byte = Byte.MaxValue
Dim integralValue2 As Integer = Convert.ToInt32(byteValue)
Console.WriteLine("Converted the {0} value {1} to " +
                                  "the {2} value {3:G}.",
                                  byteValue.GetType().Name,
                                  byteValue,
                                  integralValue2.GetType().Name,
                                  integralValue2)

' Convert a Double value to an Int32 value (a narrowing conversion).
Dim doubleValue As Double = 16.32513e12
Try
    Dim longValue As Long = Convert.ToInt64(doubleValue)
    Console.WriteLine("Converted the {0} value {1:E} to " +
                                      "the {2} value {3:N0}.",
                                      doubleValue.GetType().Name,
                                      doubleValue,
                                      longValue.GetType().Name,
                                      longValue)
Catch e As OverflowException
    Console.WriteLine("Unable to convert the {0:E} value {1}.",
                                      doubleValue.GetType().Name, doubleValue)
End Try

' Convert a signed byte to a byte (a narrowing conversion).     
Dim sbyteValue As SByte = -16
Try
    Dim byteValue2 As Byte = Convert.ToByte(sbyteValue)
    Console.WriteLine("Converted the {0} value {1} to " +
                                      "the {2} value {3:G}.",
                                      sbyteValue.GetType().Name,
                                      sbyteValue,
                                      byteValue2.GetType().Name,
                                      byteValue2)
Catch e As OverflowException
    Console.WriteLine("Unable to convert the {0} value {1}.",
                                      sbyteValue.GetType().Name, sbyteValue)
End Try
' The example displays the following output:
'       Converted the Int32 value 12534 to the Decimal value 12,534.00.
'       Converted the Byte value 255 to the Int32 value 255.
'       Converted the Double value 1.632513E+013 to the Int64 value 16,325,130,000,000.
'       Unable to convert the SByte value -16.

在某些情況下,尤其是在浮點值之間來回轉換時,即使沒有擲回 OverflowException,轉換也可能會降低精確度。 下列範例說明這種精確度降低情況。 在第一種情況下,Decimal 值在轉換為 Double 時會降低精確度 (有效位數較少)。 在第二種情況下,Double 值會從 42.72 四捨五入為 43,以完成轉換。

double doubleValue;

// Convert a Double to a Decimal.
decimal decimalValue = 13956810.96702888123451471211m;
doubleValue = Convert.ToDouble(decimalValue);
Console.WriteLine("{0} converted to {1}.", decimalValue, doubleValue);

doubleValue = 42.72;
try
{
    int integerValue = Convert.ToInt32(doubleValue);
    Console.WriteLine("{0} converted to {1}.",
                                      doubleValue, integerValue);
}
catch (OverflowException)
{
    Console.WriteLine("Unable to convert {0} to an integer.",
                                      doubleValue);
}
// The example displays the following output:
//       13956810.96702888123451471211 converted to 13956810.9670289.
//       42.72 converted to 43.
Dim doubleValue As Double

' Convert a Double to a Decimal.
Dim decimalValue As Decimal = 13956810.96702888123451471211d
doubleValue = Convert.ToDouble(decimalValue)
Console.WriteLine("{0} converted to {1}.", decimalValue, doubleValue)

doubleValue = 42.72
Try
    Dim integerValue As Integer = Convert.ToInt32(doubleValue)
    Console.WriteLine("{0} converted to {1}.",
                                      doubleValue, integerValue)
Catch e As OverflowException
    Console.WriteLine("Unable to convert {0} to an integer.",
                                      doubleValue)
End Try
' The example displays the following output:
'       13956810.96702888123451471211 converted to 13956810.9670289.
'       42.72 converted to 43.

如需同時列出 Convert 類別所支援之擴展和縮小轉換的表格,請參閱類型轉換表

使用 ChangeType 方法進行自訂轉換

Convert 類別除了支援轉換為每一個基底類型,也可以用於將自訂類型轉換為一個或多個預先定義的類型。 這種轉換由 Convert.ChangeType(Object, Type, IFormatProvider) 方法執行,而這個方法會進一步包裝對於 IConvertible.ToType 參數之 value 方法的呼叫。 這表示由 value 參數所表示的物件必須提供 IConvertible 介面的實作。

注意

由於 Convert.ChangeType(Object, Type)Convert.ChangeType(Object, Type, IFormatProvider) 方法會使用 Type 物件指定轉換 value 的目標類型,因此這兩種方法可以在編譯時期用來動態轉換不明類型的物件。 但請注意,IConvertiblevalue 實作仍然必須支援這項轉換。

下列範例說明 IConvertible 介面可能的實作,這種實作允許 TemperatureCelsius 物件轉換為 TemperatureFahrenheit 物件,反之亦然。 範例中定義一個基底類別 Temperature,這個類別實作 IConvertible 介面並覆寫 Object.ToString 方法。 衍生的 TemperatureCelsiusTemperatureFahrenheit 類別分別覆寫基底類別的 ToTypeToString 方法。

using System;

public abstract class Temperature : IConvertible
{
    protected decimal temp;

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

    public decimal Value
    {
        get { return this.temp; }
        set { this.temp = value; }
    }

    public override string ToString()
    {
        return temp.ToString(null as IFormatProvider) + "º";
    }

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

    public bool ToBoolean(IFormatProvider provider)
    {
        throw new InvalidCastException(String.Format("Temperature-to-Boolean conversion is not supported."));
    }

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

    public double ToDouble(IFormatProvider provider)
    {
        return (double)temp;
    }

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

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

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

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

    public float ToSingle(IFormatProvider provider)
    {
        return (float)temp;
    }

    public virtual string ToString(IFormatProvider provider)
    {
        return temp.ToString(provider) + "°";
    }

    // If conversionType is implemented by another IConvertible method, call it.
    public virtual object ToType(Type conversionType, IFormatProvider provider)
    {
        switch (Type.GetTypeCode(conversionType))
        {
            case TypeCode.Boolean:
                return this.ToBoolean(provider);
            case TypeCode.Byte:
                return this.ToByte(provider);
            case TypeCode.Char:
                return this.ToChar(provider);
            case TypeCode.DateTime:
                return this.ToDateTime(provider);
            case TypeCode.Decimal:
                return this.ToDecimal(provider);
            case TypeCode.Double:
                return this.ToDouble(provider);
            case TypeCode.Empty:
                throw new NullReferenceException("The target type is null.");
            case TypeCode.Int16:
                return this.ToInt16(provider);
            case TypeCode.Int32:
                return this.ToInt32(provider);
            case TypeCode.Int64:
                return this.ToInt64(provider);
            case TypeCode.Object:
                // Leave conversion of non-base types to derived classes.
                throw new InvalidCastException(String.Format("Cannot convert from Temperature to {0}.",
                                               conversionType.Name));
            case TypeCode.SByte:
                return this.ToSByte(provider);
            case TypeCode.Single:
                return this.ToSingle(provider);
            case TypeCode.String:
                IConvertible iconv = this;
                return iconv.ToString(provider);
            case TypeCode.UInt16:
                return this.ToUInt16(provider);
            case TypeCode.UInt32:
                return this.ToUInt32(provider);
            case TypeCode.UInt64:
                return this.ToUInt64(provider);
            default:
                throw new InvalidCastException("Conversion not supported.");
        }
    }

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

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

    public ulong ToUInt64(IFormatProvider provider)
    {
        if (temp < UInt64.MinValue || temp > UInt64.MaxValue)
            throw new OverflowException(String.Format("{0} is out of range of the UInt64 data type.", temp));
        else
            return (ulong)Math.Round(temp);
    }
}

public class TemperatureCelsius : Temperature, IConvertible
{
    public TemperatureCelsius(decimal value) : base(value)
    {
    }

    // Override ToString methods.
    public override string ToString()
    {
        return this.ToString(null);
    }

    public override string ToString(IFormatProvider provider)
    {
        return temp.ToString(provider) + "°C";
    }

    // If conversionType is a implemented by another IConvertible method, call it.
    public override object ToType(Type conversionType, IFormatProvider provider)
    {
        // For non-objects, call base method.
        if (Type.GetTypeCode(conversionType) != TypeCode.Object)
        {
            return base.ToType(conversionType, provider);
        }
        else
        {
            if (conversionType.Equals(typeof(TemperatureCelsius)))
                return this;
            else if (conversionType.Equals(typeof(TemperatureFahrenheit)))
                return new TemperatureFahrenheit((decimal)this.temp * 9 / 5 + 32);
            else
                throw new InvalidCastException(String.Format("Cannot convert from Temperature to {0}.",
                                               conversionType.Name));
        }
    }
}

public class TemperatureFahrenheit : Temperature, IConvertible
{
    public TemperatureFahrenheit(decimal value) : base(value)
    {
    }

    // Override ToString methods.
    public override string ToString()
    {
        return this.ToString(null);
    }

    public override string ToString(IFormatProvider provider)
    {
        return temp.ToString(provider) + "°F";
    }

    public override object ToType(Type conversionType, IFormatProvider provider)
    {
        // For non-objects, call base method.
        if (Type.GetTypeCode(conversionType) != TypeCode.Object)
        {
            return base.ToType(conversionType, provider);
        }
        else
        {
            // Handle conversion between derived classes.
            if (conversionType.Equals(typeof(TemperatureFahrenheit)))
                return this;
            else if (conversionType.Equals(typeof(TemperatureCelsius)))
                return new TemperatureCelsius((decimal)(this.temp - 32) * 5 / 9);
            // Unspecified object type: throw an InvalidCastException.
            else
                throw new InvalidCastException(String.Format("Cannot convert from Temperature to {0}.",
                                               conversionType.Name));
        }
    }
}
Public MustInherit Class Temperature
    Implements IConvertible

    Protected temp As Decimal

    Public Sub New(temperature As Decimal)
        Me.temp = temperature
    End Sub

    Public Property Value As Decimal
        Get
            Return Me.temp
        End Get
        Set
            Me.temp = Value
        End Set
    End Property

    Public Overrides Function ToString() As String
        Return temp.ToString() & "º"
    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
        Throw New InvalidCastException(String.Format("Temperature-to-Boolean conversion is not supported."))
    End Function

    Public Function ToByte(provider As IFormatProvider) As Byte Implements IConvertible.ToByte
        If temp < Byte.MinValue Or temp > Byte.MaxValue Then
            Throw New OverflowException(String.Format("{0} is out of range of the Byte data type.", temp))
        Else
            Return CByte(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 DateTime 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 temp
    End Function

    Public Function ToDouble(provider As IFormatProvider) As Double Implements IConvertible.ToDouble
        Return CDbl(temp)
    End Function

    Public Function ToInt16(provider As IFormatProvider) As Int16 Implements IConvertible.ToInt16
        If temp < Int16.MinValue Or temp > Int16.MaxValue Then
            Throw New OverflowException(String.Format("{0} is out of range of the Int16 data type.", temp))
        End If
        Return CShort(Math.Round(temp))
    End Function

    Public Function ToInt32(provider As IFormatProvider) As Int32 Implements IConvertible.ToInt32
        If temp < Int32.MinValue Or temp > Int32.MaxValue Then
            Throw New OverflowException(String.Format("{0} is out of range of the Int32 data type.", temp))
        End If
        Return CInt(Math.Round(temp))
    End Function

    Public Function ToInt64(provider As IFormatProvider) As Int64 Implements IConvertible.ToInt64
        If temp < Int64.MinValue Or temp > Int64.MaxValue Then
            Throw New OverflowException(String.Format("{0} is out of range of the Int64 data type.", temp))
        End If
        Return CLng(Math.Round(temp))
    End Function

    Public Function ToSByte(provider As IFormatProvider) As SByte Implements IConvertible.ToSByte
        If temp < SByte.MinValue Or temp > SByte.MaxValue Then
            Throw New OverflowException(String.Format("{0} is out of range of the SByte data type.", temp))
        Else
            Return CSByte(temp)
        End If
    End Function

    Public Function ToSingle(provider As IFormatProvider) As Single Implements IConvertible.ToSingle
        Return CSng(temp)
    End Function

    Public Overridable Overloads Function ToString(provider As IFormatProvider) As String Implements IConvertible.ToString
        Return temp.ToString(provider) & " °C"
    End Function

    ' If conversionType is a implemented by another IConvertible method, call it.
    Public Overridable Function ToType(conversionType As Type, provider As IFormatProvider) As Object Implements IConvertible.ToType
        Select Case Type.GetTypeCode(conversionType)
            Case TypeCode.Boolean
                Return Me.ToBoolean(provider)
            Case TypeCode.Byte
                Return Me.ToByte(provider)
            Case TypeCode.Char
                Return Me.ToChar(provider)
            Case TypeCode.DateTime
                Return Me.ToDateTime(provider)
            Case TypeCode.Decimal
                Return Me.ToDecimal(provider)
            Case TypeCode.Double
                Return Me.ToDouble(provider)
            Case TypeCode.Empty
                Throw New NullReferenceException("The target type is null.")
            Case TypeCode.Int16
                Return Me.ToInt16(provider)
            Case TypeCode.Int32
                Return Me.ToInt32(provider)
            Case TypeCode.Int64
                Return Me.ToInt64(provider)
            Case TypeCode.Object
                ' Leave conversion of non-base types to derived classes.
                Throw New InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", _
                                               conversionType.Name))
            Case TypeCode.SByte
                Return Me.ToSByte(provider)
            Case TypeCode.Single
                Return Me.ToSingle(provider)
            Case TypeCode.String
                Return Me.ToString(provider)
            Case TypeCode.UInt16
                Return Me.ToUInt16(provider)
            Case TypeCode.UInt32
                Return Me.ToUInt32(provider)
            Case TypeCode.UInt64
                Return Me.ToUInt64(provider)
            Case Else
                Throw New InvalidCastException("Conversion not supported.")
        End Select
    End Function

    Public Function ToUInt16(provider As IFormatProvider) As UInt16 Implements IConvertible.ToUInt16
        If temp < UInt16.MinValue Or temp > UInt16.MaxValue Then
            Throw New OverflowException(String.Format("{0} is out of range of the UInt16 data type.", temp))
        End If
        Return CUShort(Math.Round(temp))
    End Function

    Public Function ToUInt32(provider As IFormatProvider) As UInt32 Implements IConvertible.ToUInt32
        If temp < UInt32.MinValue Or temp > UInt32.MaxValue Then
            Throw New OverflowException(String.Format("{0} is out of range of the UInt32 data type.", temp))
        End If
        Return CUInt(Math.Round(temp))
    End Function

    Public Function ToUInt64(provider As IFormatProvider) As UInt64 Implements IConvertible.ToUInt64
        If temp < UInt64.MinValue Or temp > UInt64.MaxValue Then
            Throw New OverflowException(String.Format("{0} is out of range of the UInt64 data type.", temp))
        End If
        Return CULng(Math.Round(temp))
    End Function
End Class

Public Class TemperatureCelsius : Inherits Temperature : Implements IConvertible
    Public Sub New(value As Decimal)
        MyBase.New(value)
    End Sub

    ' Override ToString methods.
    Public Overrides Function ToString() As String
        Return Me.ToString(Nothing)
    End Function

    Public Overrides Function ToString(provider As IFormatProvider) As String
        Return temp.ToString(provider) + "°C"
    End Function

    ' If conversionType is a implemented by another IConvertible method, call it.
    Public Overrides Function ToType(conversionType As Type, provider As IFormatProvider) As Object
        ' For non-objects, call base method.
        If Type.GetTypeCode(conversionType) <> TypeCode.Object Then
            Return MyBase.ToType(conversionType, provider)
        Else
            If conversionType.Equals(GetType(TemperatureCelsius)) Then
                Return Me
            ElseIf conversionType.Equals(GetType(TemperatureFahrenheit))
                Return New TemperatureFahrenheit(CDec(Me.temp * 9 / 5 + 32))
                ' Unspecified object type: throw an InvalidCastException.
            Else
                Throw New InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", _
                                               conversionType.Name))
            End If
        End If
    End Function
End Class

Public Class TemperatureFahrenheit : Inherits Temperature : Implements IConvertible
    Public Sub New(value As Decimal)
        MyBase.New(value)
    End Sub

    ' Override ToString methods.
    Public Overrides Function ToString() As String
        Return Me.ToString(Nothing)
    End Function

    Public Overrides Function ToString(provider As IFormatProvider) As String
        Return temp.ToString(provider) + "°F"
    End Function

    Public Overrides Function ToType(conversionType As Type, provider As IFormatProvider) As Object
        ' For non-objects, call base method.
        If Type.GetTypeCode(conversionType) <> TypeCode.Object Then
            Return MyBase.ToType(conversionType, provider)
        Else
            ' Handle conversion between derived classes.
            If conversionType.Equals(GetType(TemperatureFahrenheit)) Then
                Return Me
            ElseIf conversionType.Equals(GetType(TemperatureCelsius))
                Return New TemperatureCelsius(CDec((MyBase.temp - 32) * 5 / 9))
                ' Unspecified object type: throw an InvalidCastException.
            Else
                Throw New InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", _
                                               conversionType.Name))
            End If
        End If
    End Function
End Class

下列範例說明對這些 IConvertible 實作的多個呼叫,以將 TemperatureCelsius 物件轉換為 TemperatureFahrenheit 物件,反之亦然。

TemperatureCelsius tempC1 = new TemperatureCelsius(0);
TemperatureFahrenheit tempF1 = (TemperatureFahrenheit)Convert.ChangeType(tempC1, typeof(TemperatureFahrenheit), null);
Console.WriteLine("{0} equals {1}.", tempC1, tempF1);
TemperatureCelsius tempC2 = (TemperatureCelsius)Convert.ChangeType(tempC1, typeof(TemperatureCelsius), null);
Console.WriteLine("{0} equals {1}.", tempC1, tempC2);
TemperatureFahrenheit tempF2 = new TemperatureFahrenheit(212);
TemperatureCelsius tempC3 = (TemperatureCelsius)Convert.ChangeType(tempF2, typeof(TemperatureCelsius), null);
Console.WriteLine("{0} equals {1}.", tempF2, tempC3);
TemperatureFahrenheit tempF3 = (TemperatureFahrenheit)Convert.ChangeType(tempF2, typeof(TemperatureFahrenheit), null);
Console.WriteLine("{0} equals {1}.", tempF2, tempF3);
// The example displays the following output:
//       0°C equals 32°F.
//       0°C equals 0°C.
//       212°F equals 100°C.
//       212°F equals 212°F.
Dim tempC1 As New TemperatureCelsius(0)
Dim tempF1 As TemperatureFahrenheit = CType(Convert.ChangeType(tempC1, GetType(TemperatureFahrenheit), Nothing), TemperatureFahrenheit)
Console.WriteLine("{0} equals {1}.", tempC1, tempF1)
Dim tempC2 As TemperatureCelsius = CType(Convert.ChangeType(tempC1, GetType(TemperatureCelsius), Nothing), TemperatureCelsius)
Console.WriteLine("{0} equals {1}.", tempC1, tempC2)
Dim tempF2 As New TemperatureFahrenheit(212)
Dim tempC3 As TEmperatureCelsius = CType(Convert.ChangeType(tempF2, GEtType(TemperatureCelsius), Nothing), TemperatureCelsius)
Console.WriteLine("{0} equals {1}.", tempF2, tempC3)
Dim tempF3 As TemperatureFahrenheit = CType(Convert.ChangeType(tempF2, GetType(TemperatureFahrenheit), Nothing), TemperatureFahrenheit)
Console.WriteLine("{0} equals {1}.", tempF2, tempF3)
' The example displays the following output:
'       0°C equals 32°F.
'       0°C equals 0°C.
'       212°F equals 100°C.
'       212°F equals 212°F.

TypeConverter 類別

.NET 也可讓您延伸 System.ComponentModel.TypeConverter 類別,並透過 System.ComponentModel.TypeConverterAttribute 屬性來建立類型轉換器與類型之間的關聯,以定義自訂類型的類型轉換器。 下表特別強調這種作法與實作自訂類型之 IConvertible 介面的差異。

注意

只要自訂類型有為其定義的類型轉換子,即會提供設計階段支援給自訂類型。

使用 TypeConverter 的轉換 使用 IConvertible 的轉換
是從 TypeConverter 衍生另外一個類別,以針對自訂類型來實作。 透過套用 TypeConverterAttribute 屬性,將這個衍生類別與自訂類型建立關聯。 是由自訂類型所實作來執行轉換。 該類型的使用者在該類型上叫用 IConvertible 轉換方法。
可以在設計階段和在執行階段兩處使用。 只能在執行階段使用。
使用反映,因此,速度比 IConvertible 所啟用的轉換還要慢。 不使用反映。
允許在自訂類型與其他資料類型之間的雙向類型轉換。 例如,針對 TypeConverter 所定義的 MyType 允許從 MyType 轉換為 String,也允許從 String 轉換為 MyType 允許從自訂類型轉換為其他資料類型,但不允許從其他資料類型轉換為自訂類型。

如需使用類型轉換子執行轉換的詳細資訊,請參閱 System.ComponentModel.TypeConverter

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