操作说明：使用反射检查和实例化泛型类型

2025-06-18

关于泛型类型的信息是通过与其他类型信息相同的方法获得的：通过检查一个表示泛型类型的Type对象。主要区别在于泛型类型具有表示其泛型类型参数的对象列表 Type 。本节中的第一个过程将检查泛型类型。

可以通过将类型参数绑定到泛型类型定义的类型参数来创建表示 Type 构造类型的对象。第二个过程演示了这一点。

检查泛型类型及其类型参数

获取表示泛型类型的实例 Type 。在以下代码中，使用 C# typeof 运算符（GetType 在 Visual Basic 中）获取类型。有关获取 Type 对象的其他方法，请参阅 Type。在余下的过程中，类型包含在名为 t 的方法参数中。
```
Type d1 = typeof(Dictionary<,>);
```
```
Dim d1 As Type = GetType(Dictionary(Of ,))
```

使用该 IsGenericType 属性确定类型是否为泛型，并使用 IsGenericTypeDefinition 该属性确定该类型是否为泛型类型定义。

Console.WriteLine($"   Is this a generic type? {t.IsGenericType}");
Console.WriteLine($"   Is this a generic type definition? {t.IsGenericTypeDefinition}");

Console.WriteLine("   Is this a generic type? " _
    & t.IsGenericType)
Console.WriteLine("   Is this a generic type definition? " _
    & t.IsGenericTypeDefinition)

使用该方法获取包含泛型类型参数的 GetGenericArguments 数组。

Type[] typeParameters = t.GetGenericArguments();

Dim typeParameters() As Type = t.GetGenericArguments()

对于每种类型参数，请使用IsGenericParameter属性来确定它是类型参数（例如，在泛型类型定义中），还是为类型参数指定的类型（例如，在构造类型中）。

Console.WriteLine($"   List {typeParameters.Length} type arguments:");
foreach (Type tParam in typeParameters)
{
    if (tParam.IsGenericParameter)
    {
        DisplayGenericParameter(tParam);
    }
    else
    {
        Console.WriteLine($"      Type argument: {tParam}");
    }
}

Console.WriteLine("   List {0} type arguments:", _
    typeParameters.Length)
For Each tParam As Type In typeParameters
    If tParam.IsGenericParameter Then
        DisplayGenericParameter(tParam)
    Else
        Console.WriteLine("      Type argument: {0}", _
            tParam)
    End If
Next

在类型系统中，泛型类型参数由普通类型的实例 Type表示，就像普通类型一样。以下代码显示表示泛型类型参数的对象的名称和参数位置 Type 。此处，参数位置无足轻重；它在用户检查已用作其他泛型类型的类型实参的类型形参时更有价值。
```
Console.WriteLine($"      Type parameter: {tp.Name} position {tp.GenericParameterPosition}");
```
```
Private Shared Sub DisplayGenericParameter(ByVal tp As Type)
    Console.WriteLine("      Type parameter: {0} position {1}", _
        tp.Name, tp.GenericParameterPosition)
```

通过使用 GetGenericParameterConstraints 该方法获取单个数组中的所有约束，确定泛型类型参数的基类型约束和接口约束。不保证约束按任何特定顺序排列。

foreach (Type iConstraint in tp.GetGenericParameterConstraints())
{
    if (iConstraint.IsInterface)
    {
        Console.WriteLine($"         Interface constraint: {iConstraint}");
    }
}

Console.WriteLine($"         Base type constraint: {tp.BaseType ?? tp.BaseType: None}");

Dim classConstraint As Type = Nothing

For Each iConstraint As Type In tp.GetGenericParameterConstraints()
    If iConstraint.IsInterface Then
        Console.WriteLine("         Interface constraint: {0}", _
            iConstraint)
    End If
Next

If classConstraint IsNot Nothing Then
    Console.WriteLine("         Base type constraint: {0}", _
        tp.BaseType)
Else
    Console.WriteLine("         Base type constraint: None")
End If

使用该 GenericParameterAttributes 属性可发现类型参数的特殊约束，例如要求它是引用类型。该属性还包括表示方差的值，可以屏蔽这些差异，如以下代码所示。

GenericParameterAttributes sConstraints =
    tp.GenericParameterAttributes &
    GenericParameterAttributes.SpecialConstraintMask;

Dim sConstraints As GenericParameterAttributes = _
    tp.GenericParameterAttributes And _
    GenericParameterAttributes.SpecialConstraintMask

特殊约束属性是标志，表示无特殊约束的同一标志（GenericParameterAttributes.None）也表示无协变或逆变。因此，若要测试上述任一条件，必须使用适当的掩码。在这种情况下，使用 GenericParameterAttributes.SpecialConstraintMask 来隔离特殊约束标志。

if (sConstraints == GenericParameterAttributes.None)
{
    Console.WriteLine("         No special constraints.");
}
else
{
    if (GenericParameterAttributes.None != (sConstraints &
        GenericParameterAttributes.DefaultConstructorConstraint))
    {
        Console.WriteLine("         Must have a parameterless constructor.");
    }
    if (GenericParameterAttributes.None != (sConstraints &
        GenericParameterAttributes.ReferenceTypeConstraint))
    {
        Console.WriteLine("         Must be a reference type.");
    }
    if (GenericParameterAttributes.None != (sConstraints &
        GenericParameterAttributes.NotNullableValueTypeConstraint))
    {
        Console.WriteLine("         Must be a non-nullable value type.");
    }
}

If sConstraints = GenericParameterAttributes.None Then
    Console.WriteLine("         No special constraints.")
Else
    If GenericParameterAttributes.None <> (sConstraints And _
        GenericParameterAttributes.DefaultConstructorConstraint) Then
        Console.WriteLine("         Must have a parameterless constructor.")
    End If
    If GenericParameterAttributes.None <> (sConstraints And _
        GenericParameterAttributes.ReferenceTypeConstraint) Then
        Console.WriteLine("         Must be a reference type.")
    End If
    If GenericParameterAttributes.None <> (sConstraints And _
        GenericParameterAttributes.NotNullableValueTypeConstraint) Then
        Console.WriteLine("         Must be a non-nullable value type.")
    End If
End If

构造泛型类型的实例

泛型类型类似于模板。除非为其泛型类型参数指定实际类型，否则无法创建它的实例。若要在运行时使用反射来做到这一点，需要MakeGenericType方法。

获取表示 Type 泛型类型的对象。以下代码通过两种不同的方式获取泛型类型Dictionary<TKey,TValue>：一种方式是使用带有描述类型的字符串的Type.GetType(String)方法重载，另一种方式是通过调用泛型类型GetGenericTypeDefinition（在 Visual Basic 中为Dictionary\<String, Example>）上的Dictionary(Of String, Example)方法。该方法 MakeGenericType 需要泛型类型定义。

// Use the typeof operator to create the generic type
// definition directly. To specify the generic type definition,
// omit the type arguments but retain the comma that separates
// them.
Type d1 = typeof(Dictionary<,>);

// You can also obtain the generic type definition from a
// constructed class. In this case, the constructed class
// is a dictionary of Example objects, with String keys.
Dictionary<string, Example> d2 = [];
// Get a Type object that represents the constructed type,
// and from that get the generic type definition. The
// variables d1 and d4 contain the same type.
Type d3 = d2.GetType();
Type d4 = d3.GetGenericTypeDefinition();

' Use the GetType operator to create the generic type 
' definition directly. To specify the generic type definition,
' omit the type arguments but retain the comma that separates
' them.
Dim d1 As Type = GetType(Dictionary(Of ,))

' You can also obtain the generic type definition from a
' constructed class. In this case, the constructed class
' is a dictionary of Example objects, with String keys.
Dim d2 As New Dictionary(Of String, Example)
' Get a Type object that represents the constructed type,
' and from that get the generic type definition. The 
' variables d1 and d4 contain the same type.
Dim d3 As Type = d2.GetType()
Dim d4 As Type = d3.GetGenericTypeDefinition()

构造一个类型参数数组，以替换类型参数。数组必须包含正确数量的 Type 对象，其顺序与类型参数列表中显示的顺序相同。在这种情况下，键（第一个类型参数）的类型 String，字典中的值是名为 Example类的实例。
```
Type[] typeArgs = [typeof(string), typeof(Example)];
```
```
Dim typeArgs() As Type = _
    {GetType(String), GetType(Example)}
```

MakeGenericType调用方法将类型参数绑定到类型参数并构造类型。

Type constructed = d1.MakeGenericType(typeArgs);

Dim constructed As Type = _
    d1.MakeGenericType(typeArgs)

使用CreateInstance(Type)方法重载来创建构造类型的对象。以下代码将类的 Example 两个实例存储在生成的 Dictionary<String, Example> 对象中。
```
_ = Activator.CreateInstance(constructed);
```
```
Dim o As Object = Activator.CreateInstance(constructed)
```

示例：

下面的代码示例定义了一个 DisplayGenericType 方法来检查代码中使用的泛型类型定义和构造类型并显示其信息。该 DisplayGenericType 方法演示了如何使用 IsGenericType、IsGenericParameter 属性、GenericParameterPosition 和 GetGenericArguments 方法。

该示例还定义了一个 DisplayGenericParameter 方法来检查泛型类型参数并显示其约束。

该代码示例定义一组测试类型，包括说明类型参数约束的泛型类型，并演示如何显示有关这些类型的信息。

该示例通过创建类型参数数组并调用Dictionary<TKey,TValue>方法，从MakeGenericType类构造类型。该程序将使用Type构造的MakeGenericType对象与使用Type（在 Visual Basic 中为typeof）获得的GetType对象进行比较，以演示它们相同。同样，程序使用 GetGenericTypeDefinition 该方法获取构造类型的泛型类型定义，并将其与 Type 表示 Dictionary<TKey,TValue> 类的对象进行比较。

using System.Reflection;

// Define an example interface.
public interface ITestArgument { }

// Define an example base class.
public class TestBase { }

// Define a generic class with one parameter. The parameter
// has three constraints: It must inherit TestBase, it must
// implement ITestArgument, and it must have a parameterless
// constructor.
public class Test<T> where T : TestBase, ITestArgument, new() { }

// Define a class that meets the constraints on the type
// parameter of class Test.
public class TestArgument : TestBase, ITestArgument
{
    public TestArgument() { }
}

public class Example
{
    // The following method displays information about a generic
    // type.
    private static void DisplayGenericType(Type t)
    {
        Console.WriteLine($"\r\n {t}");
        Console.WriteLine($"   Is this a generic type? {t.IsGenericType}");
        Console.WriteLine($"   Is this a generic type definition? {t.IsGenericTypeDefinition}");

        // Get the generic type parameters or type arguments.
        Type[] typeParameters = t.GetGenericArguments();

        Console.WriteLine($"   List {typeParameters.Length} type arguments:");
        foreach (Type tParam in typeParameters)
        {
            if (tParam.IsGenericParameter)
            {
                DisplayGenericParameter(tParam);
            }
            else
            {
                Console.WriteLine($"      Type argument: {tParam}");
            }
        }
    }

    // Displays information about a generic type parameter.
    private static void DisplayGenericParameter(Type tp)
    {
        Console.WriteLine($"      Type parameter: {tp.Name} position {tp.GenericParameterPosition}");

        foreach (Type iConstraint in tp.GetGenericParameterConstraints())
        {
            if (iConstraint.IsInterface)
            {
                Console.WriteLine($"         Interface constraint: {iConstraint}");
            }
        }

        Console.WriteLine($"         Base type constraint: {tp.BaseType ?? tp.BaseType: None}");

        GenericParameterAttributes sConstraints =
            tp.GenericParameterAttributes &
            GenericParameterAttributes.SpecialConstraintMask;

        if (sConstraints == GenericParameterAttributes.None)
        {
            Console.WriteLine("         No special constraints.");
        }
        else
        {
            if (GenericParameterAttributes.None != (sConstraints &
                GenericParameterAttributes.DefaultConstructorConstraint))
            {
                Console.WriteLine("         Must have a parameterless constructor.");
            }
            if (GenericParameterAttributes.None != (sConstraints &
                GenericParameterAttributes.ReferenceTypeConstraint))
            {
                Console.WriteLine("         Must be a reference type.");
            }
            if (GenericParameterAttributes.None != (sConstraints &
                GenericParameterAttributes.NotNullableValueTypeConstraint))
            {
                Console.WriteLine("         Must be a non-nullable value type.");
            }
        }
    }

    public static void Main()
    {
        // Two ways to get a Type object that represents the generic
        // type definition of the Dictionary class.

        // Use the typeof operator to create the generic type
        // definition directly. To specify the generic type definition,
        // omit the type arguments but retain the comma that separates
        // them.
        Type d1 = typeof(Dictionary<,>);

        // You can also obtain the generic type definition from a
        // constructed class. In this case, the constructed class
        // is a dictionary of Example objects, with String keys.
        Dictionary<string, Example> d2 = [];
        // Get a Type object that represents the constructed type,
        // and from that get the generic type definition. The
        // variables d1 and d4 contain the same type.
        Type d3 = d2.GetType();
        Type d4 = d3.GetGenericTypeDefinition();

        // Display information for the generic type definition, and
        // for the constructed type Dictionary<String, Example>.
        DisplayGenericType(d1);
        DisplayGenericType(d2.GetType());

        // Construct an array of type arguments to substitute for
        // the type parameters of the generic Dictionary class.
        // The array must contain the correct number of types, in
        // the same order that they appear in the type parameter
        // list of Dictionary. The key (first type parameter)
        // is of type string, and the type to be contained in the
        // dictionary is Example.
        Type[] typeArgs = [typeof(string), typeof(Example)];

        // Construct the type Dictionary<String, Example>.
        Type constructed = d1.MakeGenericType(typeArgs);

        DisplayGenericType(constructed);
        _ = Activator.CreateInstance(constructed);

        Console.WriteLine("\r\nCompare types obtained by different methods:");
        Console.WriteLine($"   Are the constructed types equal? {d2.GetType() == constructed}");
        Console.WriteLine($"   Are the generic definitions equal? {d1 == constructed.GetGenericTypeDefinition()}");

        // Demonstrate the DisplayGenericType and
        // DisplayGenericParameter methods with the Test class
        // defined above. This shows base, interface, and special
        // constraints.
        DisplayGenericType(typeof(Test<>));
    }
}

Imports System.Reflection
Imports System.Collections.Generic

' Define an example interface.
Public Interface ITestArgument
End Interface

' Define an example base class.
Public Class TestBase
End Class

' Define a generic class with one parameter. The parameter
' has three constraints: It must inherit TestBase, it must
' implement ITestArgument, and it must have a parameterless
' constructor.
Public Class Test(Of T As {TestBase, ITestArgument, New})
End Class

' Define a class that meets the constraints on the type
' parameter of class Test.
Public Class TestArgument
    Inherits TestBase
    Implements ITestArgument
    Public Sub New()
    End Sub
End Class

Public Class Example
    ' The following method displays information about a generic
    ' type.
    Private Shared Sub DisplayGenericType(ByVal t As Type)
        Console.WriteLine(vbCrLf & t.ToString())
        Console.WriteLine("   Is this a generic type? " _
            & t.IsGenericType)
        Console.WriteLine("   Is this a generic type definition? " _
            & t.IsGenericTypeDefinition)

        ' Get the generic type parameters or type arguments.
        Dim typeParameters() As Type = t.GetGenericArguments()

        Console.WriteLine("   List {0} type arguments:", _
            typeParameters.Length)
        For Each tParam As Type In typeParameters
            If tParam.IsGenericParameter Then
                DisplayGenericParameter(tParam)
            Else
                Console.WriteLine("      Type argument: {0}", _
                    tParam)
            End If
        Next
    End Sub

    ' The following method displays information about a generic
    ' type parameter. Generic type parameters are represented by
    ' instances of System.Type, just like ordinary types.
    Private Shared Sub DisplayGenericParameter(ByVal tp As Type)
        Console.WriteLine("      Type parameter: {0} position {1}", _
            tp.Name, tp.GenericParameterPosition)

        Dim classConstraint As Type = Nothing

        For Each iConstraint As Type In tp.GetGenericParameterConstraints()
            If iConstraint.IsInterface Then
                Console.WriteLine("         Interface constraint: {0}", _
                    iConstraint)
            End If
        Next

        If classConstraint IsNot Nothing Then
            Console.WriteLine("         Base type constraint: {0}", _
                tp.BaseType)
        Else
            Console.WriteLine("         Base type constraint: None")
        End If

        Dim sConstraints As GenericParameterAttributes = _
            tp.GenericParameterAttributes And _
            GenericParameterAttributes.SpecialConstraintMask
        If sConstraints = GenericParameterAttributes.None Then
            Console.WriteLine("         No special constraints.")
        Else
            If GenericParameterAttributes.None <> (sConstraints And _
                GenericParameterAttributes.DefaultConstructorConstraint) Then
                Console.WriteLine("         Must have a parameterless constructor.")
            End If
            If GenericParameterAttributes.None <> (sConstraints And _
                GenericParameterAttributes.ReferenceTypeConstraint) Then
                Console.WriteLine("         Must be a reference type.")
            End If
            If GenericParameterAttributes.None <> (sConstraints And _
                GenericParameterAttributes.NotNullableValueTypeConstraint) Then
                Console.WriteLine("         Must be a non-nullable value type.")
            End If
        End If
    End Sub

    Public Shared Sub Main()
        ' Two ways to get a Type object that represents the generic
        ' type definition of the Dictionary class. 
        '
        ' Use the GetType operator to create the generic type 
        ' definition directly. To specify the generic type definition,
        ' omit the type arguments but retain the comma that separates
        ' them.
        Dim d1 As Type = GetType(Dictionary(Of ,))

        ' You can also obtain the generic type definition from a
        ' constructed class. In this case, the constructed class
        ' is a dictionary of Example objects, with String keys.
        Dim d2 As New Dictionary(Of String, Example)
        ' Get a Type object that represents the constructed type,
        ' and from that get the generic type definition. The 
        ' variables d1 and d4 contain the same type.
        Dim d3 As Type = d2.GetType()
        Dim d4 As Type = d3.GetGenericTypeDefinition()

        ' Display information for the generic type definition, and
        ' for the constructed type Dictionary(Of String, Example).
        DisplayGenericType(d1)
        DisplayGenericType(d2.GetType())

        ' Construct an array of type arguments to substitute for 
        ' the type parameters of the generic Dictionary class.
        ' The array must contain the correct number of types, in 
        ' the same order that they appear in the type parameter 
        ' list of Dictionary. The key (first type parameter)
        ' is of type string, and the type to be contained in the
        ' dictionary is Example.
        Dim typeArgs() As Type = _
            {GetType(String), GetType(Example)}

        ' Construct the type Dictionary(Of String, Example).
        Dim constructed As Type = _
            d1.MakeGenericType(typeArgs)

        DisplayGenericType(constructed)

        Dim o As Object = Activator.CreateInstance(constructed)

        Console.WriteLine(vbCrLf & _
            "Compare types obtained by different methods:")
        Console.WriteLine("   Are the constructed types equal? " _
            & (d2.GetType() Is constructed))
        Console.WriteLine("   Are the generic definitions equal? " _
            & (d1 Is constructed.GetGenericTypeDefinition()))

        ' Demonstrate the DisplayGenericType and 
        ' DisplayGenericParameter methods with the Test class 
        ' defined above. This shows base, interface, and special
        ' constraints.
        DisplayGenericType(GetType(Test(Of )))
    End Sub
End Class

通过

操作说明：使用反射检查和实例化泛型类型

检查泛型类型及其类型参数

构造泛型类型的实例

示例：

另请参阅

其他资源