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GenericParameterAttributes Enumeration
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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
Assemblies:   mscorlib (in mscorlib.dll)
  System.Reflection.Primitives (in System.Reflection.Primitives.dll)

<FlagsAttribute> _
Public Enumeration GenericParameterAttributes

Member nameDescription
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 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 LibraryDefaultConstructorConstraintA type can be substituted for the generic type parameter only if it has a parameterless constructor.
Supported by Portable Class LibraryNoneThere are no special flags.
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 LibraryReferenceTypeConstraintA type can be substituted for the generic type parameter only if it is a reference type.
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 LibraryVarianceMaskSelects the combination of all variance flags. This value is the result of using logical OR to combine the following flags: Contravariant and Covariant.

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.

Imports System
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() = _
            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;" 
            If (variance And GenericParameterAttributes.Covariant) <> 0 Then
                retval = "Covariant;" 
                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." 
            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 

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library
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