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如何：使用反射检查和实例化泛型类型

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更新：2007 年 11 月

与其他类型的信息一样，泛型类型的信息的获取方式为：检查表示泛型类型的 Type 对象。主要的差异在于，泛型类型具有一组表示其泛型类型参数的 Type 对象。本部分的第一个步骤是检查泛型类型。

通过将类型变量绑定到泛型类型定义的类型参数，可以创建表示构造类型的 Type 对象。第二个步骤演示此过程。

检查泛型类型及其类型参数

  1. 获取表示泛型类型的 Type 实例。在下面的代码中，使用 C# 的 typeof 运算符（在 Visual Basic 中为 GetType，在 Visual C++ 中为 typeid）获取类型。有关获取 Type 对象的其他方法，请参见 Type 类主题。注意，余下的步骤中，类型包含在名为 t 的方法参数中。

    Dim d1 As Type = GetType(Dictionary(Of ,))

    Type d1 = typeof(Dictionary<,>);

  2. 使用 IsGenericType 属性确定类型是否为泛型，然后使用 IsGenericTypeDefinition 属性确定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? {0}",
    t.IsGenericType);
Console.WriteLine("   Is this a generic type definition? {0}",
    t.IsGenericTypeDefinition);

  3. 使用 GetGenericArguments 方法获取包含泛型类型参数的数组。

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

    Type[] typeParameters = t.GetGenericArguments();

  4. 对每个类型变量，使用 IsGenericParameter 属性确定其是不是类型参数（例如，在泛型类型定义中），是不是已为类型参数指定的类型（例如，在构造类型中）。

    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

    Console.WriteLine("   List {0} type arguments:", 
    typeParameters.Length);
foreach( Type tParam in typeParameters )
{
    if (tParam.IsGenericParameter)
    {
        DisplayGenericParameter(tParam);
    }
    else
    {
        Console.WriteLine("      Type argument: {0}",
            tParam);
    }
}

  5. 在类型系统中，和普通类型一样，泛型类型参数是由 Type 的实例表示的。下面的代码演示表示泛型类型参数的 Type 对象的名称和参数位置。此处，参数位置无足轻重；它在检查类型参数（用作其他泛型类型的类型变量）时更有价值。

    Private Shared Sub DisplayGenericParameter(ByVal tp As Type)
    Console.WriteLine("      Type parameter: {0} position {1}", _
        tp.Name, tp.GenericParameterPosition)

    private static void DisplayGenericParameter(Type tp)
{
    Console.WriteLine("      Type parameter: {0} position {1}", 
        tp.Name, tp.GenericParameterPosition);

  6. 通过使用 GetGenericParameterConstraints 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。不保证约束处于任何特定顺序。

    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

    Type classConstraint = null;

foreach(Type iConstraint in tp.GetGenericParameterConstraints())
{
    if (iConstraint.IsInterface)
    {
        Console.WriteLine("         Interface constraint: {0}",
            iConstraint);
    }
}

if (classConstraint != null)
{
    Console.WriteLine("         Base type constraint: {0}", 
        tp.BaseType);
}
else
    Console.WriteLine("         Base type constraint: None");

  7. 使用 GenericParameterAttributes 属性获取类型参数的特殊约束，如要求其为引用类型。该属性还包含表示方差的值，该值可屏蔽，如下面的代码所示。

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

    GenericParameterAttributes sConstraints = 
    tp.GenericParameterAttributes & 
    GenericParameterAttributes.SpecialConstraintMask;

  8. 特殊约束的属性为标志，表示没有任何特殊约束的标志 (GenericParameterAttributes.None) 还表示没有协变或逆变。因此，若要测试其中一个条件，必须使用适当的屏蔽。在此情况下，请使用 GenericParameterAttributes.SpecialConstraintMask 隔离特殊约束标志。

    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

    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.");
    }
}

构造泛型类型的实例

泛型类型和模板类似。除非指定其泛型类型参数的实际类型，否则不能创建泛型类型的实例。若要在运行时使用反射创建实例，需要使用 MakeGenericType 方法。

构造泛型类型的实例

  1. 获取表示泛型类型的 Type 对象。下面的代码以两种不同方式获取泛型类型 Dictionary<TKey, TValue>：一种方法使用 Type.GetType(String) 方法重载和描述类型的字符串，另一种方法调用构造类型 Dictionary<String, Example>（在 Visual Basic 中为 Dictionary(Of String, Example)）的 GetGenericTypeDefinition 方法。MakeGenericType 方法需要泛型类型定义。

    ' 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()

    // 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 = new Dictionary<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.
Type d3 = d2.GetType();
Type d4 = d3.GetGenericTypeDefinition();

  2. 构造一组用于替换类型参数的类型变量。数组必须包含正确数目的 Type 对象，并且顺序和对象在类型参数列表中的顺序相同。在这种情况下，键（第一个类型参数）的类型为 String，字典中的值是名为 Example 的类的实例。

    Dim typeArgs() As Type = _
    { GetType(String), GetType(Example) }

    Type[] typeArgs = {typeof(string), typeof(Example)};

  3. 调用 MakeGenericType 方法将类型变量绑定到类型参数，然后构造类型。

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

    Type constructed = d1.MakeGenericType(typeArgs);

  4. 使用 CreateInstance(Type) 方法重载来创建构造类型的对象。下面的代码在生成的 Dictionary<String, Example> 对象中存储 Example 类的两个实例。

    Dim o As Object = Activator.CreateInstance(constructed)

    object o = Activator.CreateInstance(constructed);

示例

下面的代码示例定义 DisplayGenericType 方法来检查泛型类型定义和代码中使用的构造类型，并显示它们的信息。DisplayGenericType 方法演示如何使用 IsGenericType、IsGenericParameter 和 GenericParameterPosition 属性以及 GetGenericArguments 方法。

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

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，并演示如何显示这些类型的信息。

示例通过创建一组类型参数并调用 MakeGenericType 方法，从 Dictionary<TKey, TValue> 类构造类型。程序对使用 MakeGenericType 构造的 Type 对象和使用 typeof（Visual Basic 中为 GetType）获取的 Type 对象进行比较，演示这两个对象是相同的。类似地，程序使用 GetGenericTypeDefinition 方法获取构造类型的泛型类型定义，并将其与表示 Dictionary<TKey, TValue> 类的 Type 对象进行比较。

Imports System
Imports System.Reflection
Imports System.Collections.Generic
Imports System.Security.Permissions

' 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

    <PermissionSetAttribute(SecurityAction.Demand, Name:="FullTrust")> _
    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

using System;
using System.Reflection;
using System.Collections.Generic;
using System.Security.Permissions;

// 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 {0}", t);
        Console.WriteLine("   Is this a generic type? {0}",
            t.IsGenericType);
        Console.WriteLine("   Is this a generic type definition? {0}",
            t.IsGenericTypeDefinition);

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

        Console.WriteLine("   List {0} type arguments:", 
            typeParameters.Length);
        foreach( Type tParam in typeParameters )
        {
            if (tParam.IsGenericParameter)
            {
                DisplayGenericParameter(tParam);
            }
            else
            {
                Console.WriteLine("      Type argument: {0}",
                    tParam);
            }
        }
    }

    // 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 static void DisplayGenericParameter(Type tp)
    {
        Console.WriteLine("      Type parameter: {0} position {1}", 
            tp.Name, tp.GenericParameterPosition);

        Type classConstraint = null;

        foreach(Type iConstraint in tp.GetGenericParameterConstraints())
        {
            if (iConstraint.IsInterface)
            {
                Console.WriteLine("         Interface constraint: {0}",
                    iConstraint);
            }
        }

        if (classConstraint != null)
        {
            Console.WriteLine("         Base type constraint: {0}", 
                tp.BaseType);
        }
        else
            Console.WriteLine("         Base type constraint: 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.");
            }
        }
    }

    [PermissionSetAttribute(SecurityAction.Demand, Name="FullTrust")]
    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 = new Dictionary<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.
        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);

        object o = Activator.CreateInstance(constructed);

        Console.WriteLine("\r\nCompare types obtained by different methods:");
        Console.WriteLine("   Are the constructed types equal? {0}",
            (d2.GetType()==constructed));
        Console.WriteLine("   Are the generic definitions equal? {0}",
            (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<>));
    }
}

编译代码

  • 代码包含编译所需的 C# using 语句（在 Visual Basic 中为 Imports）。

  • 不需要其他程序集引用。

  • 使用 csc.exe、vbc.exe 或 cl.exe 在命令行编译代码。若要在 Visual Studio 中编译代码，请将其置于控制台应用程序项目模板中。

请参见

概念

查看类型信息

.NET Framework 中的泛型概述

参考

Type

MethodInfo

其他资源

反射类型和泛型类型