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HOW TO：使用反映檢視和執行個體化泛型型別

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更新：2007 年 11 月

取得泛型型別相關資訊的方式，與取得其他型別的資訊一樣：檢查表示泛型型別的 Type 物件。基本的差異在於泛型型別有一個 Type 物件清單，其表示它的泛型型別參數。本章節中的第一個程序會檢查泛型型別。

您可以將型別引數繫結到泛型型別定義的型別參數，藉以建立表示建構的型別之 Type 物件。第二個程序將示範如何進行這個處理。

若要檢查泛型型別和它的型別參數

  1. 取得表示此泛型型別的 Type 執行個體。在下列程式碼中，會使用 C# typeof 運算子 (Visual Basic 中為 GetType，Visual C++ 中為 typeid) 取得此型別。請參閱 Type 類別主題，以了解取得 Type 物件的其他方式。請注意此程序的其餘部分中，此型別會包含在名為 t 的方法參數中。

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

    Type d1 = typeof(Dictionary<,>);

  2. 使用 IsGenericType 屬性來判斷此型別是否為泛型，並使用 IsGenericTypeDefinition 屬性來判斷此型別是否為泛型型別定義。

    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 方法取得包含泛型型別引數的陣列。

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

    Type[] typeParameters = t.GetGenericArguments();

  4. 使用 IsGenericParameter 屬性，針對每一個型別引數判斷它是否為型別參數 (例如，在泛型型別定義中)，或是否為已指定給型別參數的型別 (例如，在建構的型別中)。

    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. 在型別系統中，泛型型別參數是由 Type 的執行個體所表示，就如同一般型別一樣。下列程式碼將顯示表示泛型型別參數的 Type 物件之名稱和參數位置；參數位置在這裡並不是重要的資訊，當您正在檢查當做另一個泛型型別之型別引數使用的型別參數時，它才會比較重要。

    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. 使用 GetGenericParameterConstraints 方法來取得單一陣列中的所有條件約束，以判斷泛型型別參數的基底型別條件約束和介面條件約束。不保證條件約束一定會以任何特定的順序出現。

    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 屬性來探索型別參數上的特殊條件約束，例如，要求它必須是參考型別。這個屬性也包含了表示變異數的值，您可以如下列程式碼中所示加以遮蔽。

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

    GenericParameterAttributes sConstraints = 
    tp.GenericParameterAttributes & 
    GenericParameterAttributes.SpecialConstraintMask;

  8. 特殊條件約束的屬性 (Attribute) 為旗標，而代表沒有特殊條件約束的同一旗標 (GenericParameterAttributes.None) 也用來代表沒有 Covariance 或 Contravariance。因此，若要測試這些條件中的任何一項，您必須使用適當的遮罩。在這種情況下，請使用 GenericParameterAttributes.SpecialConstraintMask 隔離特殊條件約束旗標。

    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.");
    }
}

建構泛型型別的執行個體

泛型型別就像是樣板一樣，除非您為它的泛型型別參數指定真實的型別，否則將無法建立它的執行個體。若要在執行階段使用反映 (Reflection) 進行這項處理，必須使用 MakeGenericType 方法。

若要建構泛型型別的執行個體

  1. 取得表示泛型型別的 Type 物件。下列程式碼會以兩個不同方式取得泛型型別 Dictionary<TKey, TValue>：將 Type.GetType(String) 方法多載與描述此型別的字串一起使用，以及呼叫建構的型別 Dictionary<String, Example> (Visual Basic 中為 Dictionary(Of String, Example)) 上的 GetGenericTypeDefinition 方法。MakeGenericType 方法需要泛型型別定義。

    ' 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. 建構型別引數陣列來替代型別參數，此陣列必須包含正確的 Type 物件數目，且這些物件的出現順序必須與在型別參數清單中的順序相同。在此情況下，索引鍵 (第一個型別參數) 具有型別 String，且字典中的值會是名為 Example 的類別之執行個體。

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

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

  3. 呼叫 MakeGenericType 方法，將型別引數繫結到型別參數，並建構此型別。

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

    Type constructed = d1.MakeGenericType(typeArgs);

  4. 使用 CreateInstance(Type) 方法多載來建立建構的型別之物件。下列程式碼會將 Example 類別的兩個執行個體儲存到產生的 Dictionary<String, Example> 物件中。

    Dim o As Object = Activator.CreateInstance(constructed)

    object o = Activator.CreateInstance(constructed);

範例

下列程式碼範例會定義 DisplayGenericType 方法來檢查用於程式碼中的泛型型別定義和建構的型別，並顯示其資訊。DisplayGenericType 方法會示範如何使用 IsGenericType、IsGenericParameter 和 GenericParameterPosition 屬性及 GetGenericArguments 方法。

此範例也會定義 DisplayGenericParameter 方法來檢查泛型型別參數，並顯示其條件約束。

此程式碼範例會定義一組測試型別 (包括可說明型別參數條件約束的泛型型別)，以及示範如何顯示與這些型別有關的資訊。

此範例會從 Dictionary<TKey, TValue> 類別建構型別，透過的方式是建立型別引數的陣列，以及呼叫 MakeGenericType 方法。程式會將使用 MakeGenericType 建構的 Type 物件與使用 typeof (Visual Basic 中為 GetType) 取得之 Type 物件相比較，以示範兩者是相同的。同樣地，程式會使用 GetGenericTypeDefinition 方法來取得建構之型別的泛型型別定義，並將它與表示 Dictionary<TKey, TValue> 類別的 Type 物件相比較。

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<>));
    }
}

編譯程式碼

  • 此程式碼含有進行編譯所需的 C# using 陳述式 (Visual Basic 中的 Imports)。

  • 不需要其他的組件參考。

  • 使用 csc.exe、vbc.exe 或 cl.exe，在命令列上編譯程式碼；若要在 Visual Studio 中編譯程式碼，請將它放置在主控台應用程式專案範本中。

請參閱

概念

檢視型別資訊

.NET Framework 中的泛型概觀

參考

Type

MethodInfo

其他資源

反映和泛用型別