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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)

public enum GenericParameterAttributes
/** @attribute FlagsAttribute() */ 
public enum GenericParameterAttributes
public enum GenericParameterAttributes

 Member nameDescription
ContravariantThe generic type parameter is contravariant. A contravariant type parameter can appear as a parameter type in method signatures.  
CovariantThe 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. 
DefaultConstructorConstraintA type can be substituted for the generic type parameter only if it has a parameterless constructor. 
NoneThere are no special flags. 
NotNullableValueTypeConstraintA type can be substituted for the generic type parameter only if it is a value type and is not nullable. 
ReferenceTypeConstraintA type can be substituted for the generic type parameter only if it is a reference type. 
SpecialConstraintMaskSelects 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
VarianceMaskSelects 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.

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 Main()
        // 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<,>);
        Console.WriteLine("\r\nExamining generic type {0}", def);

        // Get the type parameters of the generic type definition,
        // and display them.
        Type[] defparams = def.GetGenericArguments();
        foreach (Type tp in defparams)
            Console.WriteLine("\r\nType parameter: {0}", tp.Name);

            // 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)
                Console.WriteLine("\t{0}", tpc);

    // List the variance and special constraint flags. 
    private static string ListGenericParameterAttributes(Type t)
        string retval;
        GenericParameterAttributes gpa = t.GenericParameterAttributes;
        GenericParameterAttributes variance = gpa & 

        // Select the variance flags.
        if (variance == GenericParameterAttributes.None)
            retval = "No variance flag;";
            if ((variance & GenericParameterAttributes.Covariant) != 0)
                retval = "Covariant;";
                retval = "Contravariant;";

        // Select 
        GenericParameterAttributes constraints = gpa & 

        if (constraints == GenericParameterAttributes.None)
            retval += " No special constraints";
            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.

Type parameter: U
        No variance flag; ReferenceTypeConstraint DefaultConstructorConstraint

Windows 98, Windows Server 2000 SP4, Windows Millennium Edition, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0

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