Type.GetGenericParameterConstraints Method

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

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

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

Syntax

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.

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

Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Platforms

Windows Phone

See Also

Reference

Type Class

System Namespace

GenericParameterAttributes

IsGenericParameter

Other Resources

How to: Examine and Instantiate Generic Types with Reflection