Type.GetGenericParameterConstraints Method

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Returns an array of Type objects that represent the constraints on the current generic type parameter.

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

Syntax

'Declaration
Public Overridable Function GetGenericParameterConstraints As Type()

public virtual Type[] GetGenericParameterConstraints()

Return Value

Type: array<System.Type[]
An array of Type objects that represent the constraints on the current generic type parameter.

Exceptions

Exception	Condition
InvalidOperationException	The current Type object is not a generic type parameter. That is, the IsGenericParameter property returns false.

Remarks

Each constraint on a generic type parameter is expressed as a Type object. Use the IsClass property to determine whether a constraint is the base class constraint; if the property returns false, the constraint is an interface constraint. If a type parameter has no class constraint and no interface constraints, an empty array is returned.

For a list of the invariant conditions for terms used in generic reflection, see the IsGenericType property remarks.

Examples

The following code example defines a generic type Test with two type parameters that have different constraints. When the program executes, the constraints are examined using the GenericParameterAttributes property and the GetGenericParameterConstraints method.

Note:
To run this example, see Building Examples That Use a Demo Method and a TextBlock Control.

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 Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      ' 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 ,))
      outputBlock.Text += String.Format(vbCrLf & "Examining generic type {0}", def) & vbCrLf

      ' 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

         outputBlock.Text += String.Format(vbCrLf & "Type parameter: {0}", tp.Name) & vbCrLf
         outputBlock.Text &= vbTab & ListGenericParameterAttributes(tp) & vbCrLf

         ' 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
            outputBlock.Text &= vbTab & tpc.ToString() & vbCrLf
         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
' 

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 Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      // 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<,>);
      outputBlock.Text += String.Format("\r\nExamining generic type {0}", def) + "\n";

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

         // 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)
         {
            outputBlock.Text += String.Format("\t{0}", tpc) + "\n";
         }
      }
   }

   // 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
 */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

See Also

Reference

Type Class

System Namespace

GenericParameterAttributes

IsGenericParameter

Other Resources

How to: Examine and Instantiate Generic Types with Reflection