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Type.GenericParameterPosition Property

Gets the position of the type parameter in the type parameter list of the generic type or method that declared the parameter, when the Type object represents a type parameter of a generic type or a generic method.

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

public virtual int GenericParameterPosition { get; }

Property Value

Type: System.Int32
The position of a type parameter in the type parameter list of the generic type or method that defines the parameter. Position numbers begin at 0.

ExceptionCondition
InvalidOperationException

The current type does not represent a type parameter. That is, IsGenericParameter returns false.

The GenericParameterPosition property returns the position of a type parameter in the parameter list of the generic type definition or generic method definition where the type parameter was originally defined. The DeclaringType and DeclaringMethod properties identify the generic type or method definition:

  • If the DeclaringMethod property returns a MethodInfo, that MethodInfo represents a generic method definition, and the current Type object represents a type parameter of that generic method definition.

  • If the DeclaringMethod property returns null, then the DeclaringType property always returns a Type object representing a generic type definition, and the current Type object represents a type parameter of that generic type definition.

To provide the correct context for the value of the GenericParameterPosition property, it is necessary to identify the generic type or method a type parameter belongs to. For example, consider the return value of the generic method GetSomething in the following code:

public class B<T, U> { }
public class A<V>
{
    public B<V, X> GetSomething<X>()
    {
        return new B<V, X>();
    }
}

The type returned by GetSomething depends on the type arguments supplied to class A and to GetSomething itself. You can obtain a MethodInfo for GetSomething, and from that you can obtain the return type. When you examine the type parameters of the return type, GenericParameterPosition returns 0 for both. The position of V is 0 because V is the first type parameter in the type parameter list for class A. The position of X is 0 because X is the first type parameter in the type parameter list for GetSomething.

NoteNote

Calling the GenericParameterPosition property causes an exception if the current Type does not represent a type parameter. When you examine the type arguments of an open constructed type, use the IsGenericParameter property to tell which are type parameters and which are types. The IsGenericParameter property returns true for a type parameter; you can then use the GenericParameterPosition method to obtain its position and use the DeclaringMethod and DeclaringType properties to determine the generic method or type definition that defines it.

The following example defines a generic class with two type parameters and defines a second generic class that derives from the first class. The derived class's base class has two type arguments: the first is Int32, and the second is a type parameter of the derived type. The example displays information about these generic classes, including the positions reported by the GenericParameterPosition property.

using System;
using System.Reflection;
using System.Collections.Generic;

// Define a base class with two type parameters. 
public class Base<T, U> { }

// Define a derived class. The derived class inherits from a constructed 
// class that meets the following criteria: 
//   (1) Its generic type definition is Base<T, U>. 
//   (2) It specifies int for the first type parameter. 
//   (3) For the second type parameter, it uses the same type that is used 
//       for the type parameter of the derived class. 
// Thus, the derived class is a generic type with one type parameter, but 
// its base class is an open constructed type with one type argument and 
// one type parameter. 
public class Derived<V> : Base<int, V> { }

public class Test
{
    public static void Main()
    {
        Console.WriteLine(
            "\r\n--- Display a generic type and the open constructed");
        Console.WriteLine("    type from which it is derived.");

        // Create a Type object representing the generic type definition  
        // for the Derived type, by omitting the type argument. (For 
        // types with multiple type parameters, supply the commas but 
        // omit the type arguments.)  
        //
        Type derivedType = typeof(Derived<>);
        DisplayGenericTypeInfo(derivedType);

        // Display its open constructed base type.
        DisplayGenericTypeInfo(derivedType.BaseType);
    }

    private static void DisplayGenericTypeInfo(Type t)
    {
        Console.WriteLine("\r\n{0}", t);

        Console.WriteLine("\tIs this a generic type definition? {0}", 
            t.IsGenericTypeDefinition);

        Console.WriteLine("\tIs it a generic type? {0}", 
            t.IsGenericType);

        Console.WriteLine("\tDoes it have unassigned generic parameters? {0}", 
            t.ContainsGenericParameters);

        if (t.IsGenericType)
        {
            // If this is a generic type, display the type arguments. 
            //
            Type[] typeArguments = t.GetGenericArguments();

            Console.WriteLine("\tList type arguments ({0}):", 
                typeArguments.Length);

            foreach (Type tParam in typeArguments)
            {
                // IsGenericParameter is true only for generic type 
                // parameters. 
                // 
                if (tParam.IsGenericParameter)
                {
                    Console.WriteLine(
                        "\t\t{0}  (unassigned - parameter position {1})",
                        tParam,
                        tParam.GenericParameterPosition);
                }
                else
                {
                    Console.WriteLine("\t\t{0}", tParam);
                }
            }
        }
    }
}

/* This example produces the following output:

--- Display a generic type and the open constructed
    type from which it is derived.

Derived`1[V]
        Is this a generic type definition? True
        Is it a generic type? True
        Does it have unassigned generic parameters? True
        List type arguments (1):
                V  (unassigned - parameter position 0)

Base`2[System.Int32,V]
        Is this a generic type definition? False
        Is it a generic type? True
        Does it have unassigned generic parameters? True
        List type arguments (2):
                System.Int32
                V  (unassigned - parameter position 0)
 */

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

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

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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