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.
Assembly: mscorlib (in mscorlib.dll)
| Member name | Description | |
|---|---|---|
| Contravariant | The generic type parameter is contravariant. A contravariant type parameter can appear as a parameter type in method signatures. | |
| Covariant | The 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. | |
| DefaultConstructorConstraint | A type can be substituted for the generic type parameter only if it has a parameterless constructor. | |
| None | There are no special flags. | |
| NotNullableValueTypeConstraint | A type can be substituted for the generic type parameter only if it is a value type and is not nullable. | |
| ReferenceTypeConstraint | A type can be substituted for the generic type parameter only if it is a reference type. | |
| SpecialConstraintMask | Selects 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. | |
| VarianceMask | Selects 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() = _ 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 '
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1