System.Type.MakeGenericType method

This article provides supplementary remarks to the reference documentation for this API.

The MakeGenericType method allows you to write code that assigns specific types to the type parameters of a generic type definition, thus creating a Type object that represents a particular constructed type. You can use this Type object to create run-time instances of the constructed type.

Types constructed with MakeGenericType can be open, that is, some of their type arguments can be type parameters of enclosing generic methods or types. You might use such open constructed types when you emit dynamic assemblies. For example, consider the classes Base and Derived in the following code.

public class Base<T, U> { }
public class Derived<V> : Base<int, V> { }

type Base<'T, 'U>() = class end
type Derived<'V>() = inherit Base<int, 'V>()

Public Class Base(Of T, U)
End Class
Public Class Derived(Of V)
    Inherits Base(Of Integer, V)
End Class

To generate Derived in a dynamic assembly, it is necessary to construct its base type. To do this, call the MakeGenericType method on a Type object representing the class Base, using the generic type arguments Int32 and the type parameter V from Derived. Because types and generic type parameters are both represented by Type objects, an array containing both can be passed to the MakeGenericType method.

Note

A constructed type such as Base<int, V> is useful when emitting code, but you cannot call the MakeGenericType method on this type because it is not a generic type definition. To create a closed constructed type that can be instantiated, first call the GetGenericTypeDefinition method to get a Type object representing the generic type definition and then call MakeGenericType with the desired type arguments.

The Type object returned by MakeGenericType is the same as the Type obtained by calling the GetType method of the resulting constructed type, or the GetType method of any constructed type that was created from the same generic type definition using the same type arguments.

Note

An array of generic types is not itself a generic type. You cannot call MakeGenericType on an array type such as C<T>[] (Dim ac() As C(Of T) in Visual Basic). To construct a closed generic type from C<T>[], call GetElementType to obtain the generic type definition C<T>; call MakeGenericType on the generic type definition to create the constructed type; and finally call the MakeArrayType method on the constructed type to create the array type. The same is true of pointer types and ref types (ByRef in Visual Basic).

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

Nested types

If a generic type is defined using C#, C++, or Visual Basic, then its nested types are all generic. This is true even if the nested types have no type parameters of their own, because all three languages include the type parameters of enclosing types in the type parameter lists of nested types. Consider the following classes:

public class Outermost<T>
{
    public class Inner<U>
    {
        public class Innermost1<V> {}
        public class Innermost2 {}
    }
}

Public Class Outermost(Of T)
    Public Class Inner(Of U)
        Public Class Innermost1(Of V)
        End Class
        Public Class Innermost2
        End Class
    End Class
End Class

The type parameter list of the nested class Inner has two type parameters, T and U, the first of which is the type parameter of its enclosing class. Similarly, the type parameter list of the nested class Innermost1 has three type parameters, T, U, and V, with T and U coming from its enclosing classes. The nested class Innermost2 has two type parameters, T and U, which come from its enclosing classes.

If the parameter list of the enclosing type has more than one type parameter, all the type parameters in order are included in the type parameter list of the nested type.

To construct a generic type from the generic type definition for a nested type, call the MakeGenericType method with the array formed by concatenating the type argument arrays of all the enclosing types, beginning with the outermost generic type, and ending with the type argument array of the nested type itself, if it has type parameters of its own. To create an instance of Innermost1, call the MakeGenericType method with an array containing three types, to be assigned to T, U, and V. To create an instance of Innermost2, call the MakeGenericType method with an array containing two types, to be assigned to T and U.

The languages propagate the type parameters of enclosing types in this fashion so you can use the type parameters of an enclosing type to define fields of nested types. Otherwise, the type parameters would not be in scope within the bodies of the nested types. It is possible to define nested types without propagating the type parameters of enclosing types, by emitting code in dynamic assemblies or by using the Ilasm.exe (IL Assembler). Consider the following code for the CIL assembler:

.class public Outer<T> {
    .class nested public Inner<U> {
        .class nested public Innermost {
        }
    }
}

In this example, it is not possible to define a field of type T or U in class Innermost, because those type parameters are not in scope. The following assembler code defines nested classes that behave the way they would if defined in C++, Visual Basic, and C#:

.class public Outer<T> {
    .class nested public Inner<T, U> {
        .class nested public Innermost<T, U, V> {
        }
    }
}

You can use the Ildasm.exe (IL Disassembler) to examine nested classes defined in the high-level languages and observe this naming scheme.