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GenericParameterAttributes Enumeration

Describes the constraints on a generic type parameter of a generic type or method.

This enumeration has a FlagsAttribute attribute that allows a bitwise combination of its member values.

Namespace:  System.Reflection
Assembly:  mscorlib (in mscorlib.dll)

[FlagsAttribute]
public enum class GenericParameterAttributes

Member nameDescription
Supported by Portable Class LibraryNoneThere are no special flags.
Supported by Portable Class LibraryVarianceMaskSelects the combination of all variance flags. This value is the result of using logical OR to combine the following flags: Contravariant and Covariant.
Supported by Portable Class LibraryCovariantThe generic type parameter is covariant. A covariant type parameter can appear as the result type of a method, the type of a read-only field, a declared base type, or an implemented interface.
Supported by Portable Class LibraryContravariantThe generic type parameter is contravariant. A contravariant type parameter can appear as a parameter type in method signatures.
Supported by Portable Class LibrarySpecialConstraintMaskSelects the combination of all special constraint flags. This value is the result of using logical OR to combine the following flags: DefaultConstructorConstraint, ReferenceTypeConstraint, and NotNullableValueTypeConstraint.
Supported by Portable Class LibraryReferenceTypeConstraintA type can be substituted for the generic type parameter only if it is a reference type.
Supported by Portable Class LibraryNotNullableValueTypeConstraintA type can be substituted for the generic type parameter only if it is a value type and is not nullable.
Supported by Portable Class LibraryDefaultConstructorConstraintA type can be substituted for the generic type parameter only if it has a parameterless constructor.

The members of the GenericParameterAttributes enumeration are divided into two groups, the variance group and the special constraints group. To test a GenericParameterAttributes value for variance flags, first perform a bitwise AND operation with VarianceMask. If the result is None, there are no variance flags. Similarly, use SpecialConstraintMask to test for constraint flags.

The following code example defines a generic type Test with two type parameters. The second type parameter has a base class constraint and a reference type constraint. When the program executes, the constraints are examined using the Type::GenericParameterAttributes property and the Type::GetGenericParameterConstraints method.


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
 */


.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
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