GenericParameterAttributes 枚举

定义

命名空间:

System.Reflection

程序集:

System.Runtime.dll

描述泛型类型或方法的泛型类型参数的约束。

此枚举支持其成员值的按位组合。

public enum class GenericParameterAttributes

[System.Flags]
public enum GenericParameterAttributes

[<System.Flags>]
type GenericParameterAttributes = 

Public Enum GenericParameterAttributes

继承

Object

ValueType

Enum

GenericParameterAttributes

属性

FlagsAttribute

字段

Contravariant	2	泛型类型参数是逆变的。 逆变类型参数可以在方法签名中显示为参数类型。
Covariant	1	泛型类型参数是协变的。 协变类型参数可以显示为方法的结果类型、只读字段的类型、声明的基类型或实现的接口。
DefaultConstructorConstraint	16	仅当类型具有无参数构造函数时，才能将类型替换为泛型类型参数。
None	0	没有特殊标志。
NotNullableValueTypeConstraint	8	仅当类型为值类型且不可为 null 时，才能将类型替换为泛型类型参数。
ReferenceTypeConstraint	4	仅当类型为引用类型时，才能将类型替换为泛型类型参数。
SpecialConstraintMask	28	选择所有特殊约束标志的组合。 此值是使用逻辑 OR 合并以下标志的结果：DefaultConstructorConstraint、ReferenceTypeConstraint和 NotNullableValueTypeConstraint。
VarianceMask	3	选择所有方差标志的组合。 此值是使用逻辑 OR 合并以下标志的结果：Contravariant 和 Covariant。

示例

下面的代码示例使用两个类型参数定义泛型类型 Test。 第二个类型参数具有基类约束和引用类型约束。 执行程序时，将使用 Type.GenericParameterAttributes 属性和 Type.GetGenericParameterConstraints 方法检查约束。

using namespace System;
using namespace System::Collections;
using namespace System::Reflection;

// Define a sample interface to use as an interface constraint.
interface class ITest{};

// Define a base type to use as a class constraint.
public ref class Base{};

// Define the generic type to examine. The first generic type parameter,
// T, derives from the class Base and implements ITest. This demonstrates
// a base class constraint and an interface constraint. In the .NET
// Framework version 2.0, C++ has no way of expressing special constraints.
// See the C# example code.
//
generic <typename T, typename U>
   where T :  Base, ITest
ref class Test {};

// Define a type that derives from Base and implements interface
// ITest. This type satisfies the constraint on T in class Test.
public ref class Derived: public Base, public ITest {};

public ref class Example
{
public:
   static void Main()
   {
      // Create a constructed type from Test<T,U>, and from it
      // get the generic type definition.
      //
      Type^ def = Test::typeid;
      Console::WriteLine( L"\r\nExamining generic type {0}", def );
      
      // Get the type parameters of the generic type definition,
      // and display them.
      //
      for each (Type^ tp in def->GetGenericArguments())
      {
         Console::WriteLine( L"\r\nType parameter: {0}", tp);
         Console::WriteLine( L"\t{0}", 
            ListGenericParameterAttributes( tp ) );
         
         // List the base class and interface constraints. The
         // constraints do not appear in any particular order. If
         // there are no class or interface constraints, an empty
         // array is returned.
         //
         for each (Type^ constraint in tp->GetGenericParameterConstraints())
         {
            Console::WriteLine( L"\t{0}", constraint );
         }
      }
   }

private:

   // List the variance and special constraint flags. 
   //
   static String^ ListGenericParameterAttributes( Type^ t )
   {
      String^ retval;
      GenericParameterAttributes gpa = t->GenericParameterAttributes;

      // Select the variance flag.
      GenericParameterAttributes variance =
         static_cast<GenericParameterAttributes>(
            gpa & GenericParameterAttributes::VarianceMask );

      if ( variance == GenericParameterAttributes::None )
            retval = L"No variance flag;";
      else
      {
         if ( (variance & GenericParameterAttributes::Covariant) !=
               GenericParameterAttributes::None )
            retval = L"Covariant;";
         else
            retval = L"Contravariant;";
      }

      // Select the special constraint flags.
      GenericParameterAttributes constraints =
         static_cast<GenericParameterAttributes>(
            gpa & GenericParameterAttributes::SpecialConstraintMask);

      if ( constraints == GenericParameterAttributes::None )
            retval = String::Concat( retval, L" No special constraints" );
      else
      {
         if ( (constraints & GenericParameterAttributes::ReferenceTypeConstraint) !=
               GenericParameterAttributes::None )
            retval = String::Concat( retval, L" ReferenceTypeConstraint" );

         if ( (constraints & GenericParameterAttributes::NotNullableValueTypeConstraint) !=
               GenericParameterAttributes::None )
            retval = String::Concat( retval, L" NotNullableValueTypeConstraint" );

         if ( (constraints & GenericParameterAttributes::DefaultConstructorConstraint) !=
               GenericParameterAttributes::None )
            retval = String::Concat( retval, L" DefaultConstructorConstraint" );
      }

      return retval;
   }
};

int main()
{
   Example::Main();
}

/* This example produces the following output:

Examining generic type Test`2[T,U]

Type parameter: T
        No variance flag; No special constraints
        Base
        ITest

Type parameter: U
        No variance flag; No special constraints
 */

using System;
using System.Reflection;

// Define a sample interface to use as an interface constraint.
public interface ITest {}

// Define a base type to use as a base class constraint.
public class Base {}

// Define the generic type to examine. The first generic type parameter,
// T, derives from the class Base and implements ITest. This demonstrates
// a base class constraint and an interface constraint. The second generic 
// type parameter, U, must be a reference type (class) and must have a 
// default constructor (new()). This demonstrates special constraints.
//
public class Test<T,U> 
    where T : Base, ITest 
    where U : class, new() {}

// Define a type that derives from Base and implements ITest. This type
// satisfies the constraints on T in class Test.
public class Derived : Base, ITest {}

public class Example
{
    public static void Main()
    {
        // To get the generic type definition, omit the type
        // arguments but retain the comma to indicate the number
        // of type arguments. 
        //
        Type def = typeof(Test<,>);
        Console.WriteLine("\r\nExamining generic type {0}", def);

        // Get the type parameters of the generic type definition,
        // and display them.
        //
        Type[] defparams = def.GetGenericArguments();
        foreach (Type tp in defparams)
        {
            Console.WriteLine("\r\nType parameter: {0}", tp.Name);
            Console.WriteLine("\t{0}", 
                ListGenericParameterAttributes(tp));

            // List the base class and interface constraints. The
            // constraints are returned in no particular order. If 
            // there are no class or interface constraints, an empty
            // array is returned.
            //
            Type[] tpConstraints = tp.GetGenericParameterConstraints();
            foreach (Type tpc in tpConstraints)
            {
                Console.WriteLine("\t{0}", tpc);
            }
        }
    }

    // List the variance and special constraint flags. 
    //
    private static string ListGenericParameterAttributes(Type t)
    {
        string retval;
        GenericParameterAttributes gpa = t.GenericParameterAttributes;
        GenericParameterAttributes variance = gpa & 
            GenericParameterAttributes.VarianceMask;

        // Select the variance flags.
        if (variance == GenericParameterAttributes.None)
        {
            retval = "No variance flag;";
        }
        else
        {
            if ((variance & GenericParameterAttributes.Covariant) != 0)
                retval = "Covariant;";
            else
                retval = "Contravariant;";
        }

        // Select 
        GenericParameterAttributes constraints = gpa & 
            GenericParameterAttributes.SpecialConstraintMask;

        if (constraints == GenericParameterAttributes.None)
        {
            retval += " No special constraints";
        }
        else
        {
            if ((constraints & GenericParameterAttributes.ReferenceTypeConstraint) != 0)
                retval += " ReferenceTypeConstraint";
            if ((constraints & GenericParameterAttributes.NotNullableValueTypeConstraint) != 0)
                retval += " NotNullableValueTypeConstraint";
            if ((constraints & GenericParameterAttributes.DefaultConstructorConstraint) != 0)
                retval += " DefaultConstructorConstraint";
        }

        return retval;
    }
}
/* This example produces the following output:

Examining generic type Test`2[T,U]

Type parameter: T
        No variance flag; no special constraints.
        Base
        ITest

Type parameter: U
        No variance flag; ReferenceTypeConstraint DefaultConstructorConstraint
 */

Imports System.Reflection

' Define a sample interface to use as an interface constraint.
Public Interface ITest
End Interface 

' Define a base type to use as a base class constraint.
Public Class Base
End Class 

' Define the generic type to examine. The first generic type parameter,
' T, derives from the class Base and implements ITest. This demonstrates
' a base class constraint and an interface constraint. The second generic 
' type parameter, U, must be a reference type (Class) and must have a 
' default constructor (New). This demonstrates special constraints.
'
Public Class Test(Of T As {Base, ITest}, U As {New, Class}) 
End Class

' Define a type that derives from Base and implements ITtest. This type
' satisfies the constraints on T in class Test.
Public Class Derived
    Inherits Base
    Implements ITest
End Class 

Public Class Example
    
    Public Shared Sub Main() 
        ' To get the generic type definition, omit the type
        ' arguments but retain the comma to indicate the number
        ' of type arguments. 
        '
        Dim def As Type = GetType(Test(Of ,))
        Console.WriteLine(vbCrLf & "Examining generic type {0}", def)
        
        ' Get the type parameters of the generic type definition,
        ' and display them.
        '
        Dim defparams() As Type = def.GetGenericArguments()
        For Each tp As Type In defparams

            Console.WriteLine(vbCrLf & "Type parameter: {0}", tp.Name)
            Console.WriteLine(vbTab & ListGenericParameterAttributes(tp))
            
            ' List the base class and interface constraints. The
            ' constraints do not appear in any particular order. An
            ' empty array is returned if there are no constraints.
            '
            Dim tpConstraints As Type() = _
                tp.GetGenericParameterConstraints()
            For Each tpc As Type In  tpConstraints
                Console.WriteLine(vbTab & tpc.ToString())
            Next tpc
        Next tp
    
    End Sub 
    
    ' List the variance and special constraint flags.
    '
    Private Shared Function ListGenericParameterAttributes(ByVal t As Type) As String 
        Dim retval As String
        Dim gpa As GenericParameterAttributes = t.GenericParameterAttributes

        ' Select the variance flags.
        Dim variance As GenericParameterAttributes = _
            gpa And GenericParameterAttributes.VarianceMask
        
        If variance = GenericParameterAttributes.None Then
            retval = "No variance flag;"
        Else
            If (variance And GenericParameterAttributes.Covariant) <> 0 Then
                retval = "Covariant;"
            Else
                retval = "Contravariant;"
            End If
        End If 

        ' Select the constraint flags.
        Dim constraints As GenericParameterAttributes = _
            gpa And GenericParameterAttributes.SpecialConstraintMask
        
        If constraints = GenericParameterAttributes.None Then
            retval &= " no special constraints."
        Else
            If (constraints And GenericParameterAttributes.ReferenceTypeConstraint) <> 0 Then
                retval &= " ReferenceTypeConstraint"
            End If
            If (constraints And GenericParameterAttributes.NotNullableValueTypeConstraint) <> 0 Then
                retval &= " NotNullableValueTypeConstraint"
            End If
            If (constraints And GenericParameterAttributes.DefaultConstructorConstraint) <> 0 Then
                retval &= " DefaultConstructorConstraint"
            End If
        End If 
        Return retval
    
    End Function 
End Class 
' This example produces the following output:
'
'Examining generic type Test`2[T,U]
'
'Type parameter: T
'        No variance flag; no special constraints.
'        Base
'        ITest
'
'Type parameter: U
'        No variance flag; ReferenceTypeConstraint DefaultConstructorConstraint
'

注解

GenericParameterAttributes 枚举的成员分为两个组：方差组和特殊约束组。 若要测试方差标志的 GenericParameterAttributes 值，请先使用 VarianceMask 执行按位 AND 操作。 如果结果为 None，则没有方差标志。 同样，使用 SpecialConstraintMask 测试约束标志。