The Type.MakeGenericType(Type[]) 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 Type.MakeGenericType(Type[]) 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.

code reference: System.Type.MakeGenericType#1

To generate Derived in a dynamic assembly, it is necessary to construct its base type. To do this, call the Type.MakeGenericType(Type[]) method on a Type object representing the class Base, using the generic type arguments int 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 Type.MakeGenericType(Type[]) method.

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

For a list of the invariant conditions for terms used in generic reflection, see the Type.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:

code reference: System.Type.MakeGenericType#2

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 Type.MakeGenericType(Type[]) 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 Type.MakeGenericType(Type[]) method with an array containing three types, to be assigned to T, U, and V. To create an instance of Innermost2, call the Type.MakeGenericType(Type[]) 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 MSIL Assembler (Ilasm.exe). Consider the following code for the MSIL 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 MSIL Disassembler (Ildasm.exe) to examine nested classes defined in the high-level languages and observe this naming scheme.

The following example uses Type.GetType and Type.MakeGenericType to create a constructed type from the generic Dictionary type. The constructed type represents a Dictionary of Test objects with string keys.

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

public class Test
{
	public static void Main()
	{
		Console.WriteLine("\n--- Create a constructed type from the generic  Dictionary type.");

		// Create a type object representing the generic Dictionary 
		// type.       
		Type generic = Type.GetType("System.Collections.Generic.Dictionary");

		DisplayTypeInfo(generic);

		// Create an array of types to substitute for the type
		// parameters of Dictionary. The key is of type string, and
		// the type to be contained in the Dictionary is Test.
		Type[] typeArgs = { typeof(string), typeof(Test) };
		Type constructed = generic.MakeGenericType(typeArgs);

		DisplayTypeInfo(constructed);

		// Compare the type objects obtained above to type objects
		// obtained using typeof() and GetGenericTypeDefinition().
		Console.WriteLine("\n--- Compare types obtained by different methods:");

		Type t = typeof(Dictionary<string, Test>);

		Console.WriteLine("\tAre the constructed types equal? {0}", t == constructed);
		Console.WriteLine("\tAre the generic types equal? {0}", t.GetGenericTypeDefinition() == generic);
	}

	private static void DisplayTypeInfo(Type t)
	{
		Console.WriteLine("\n{0}", t);
		Console.WriteLine("\tIs this a generic type definition? {0}", t.IsGenericTypeDefinition);
		Console.WriteLine("\tDoes it have generic type arguments? {0}", t.HasGenericArguments);

		Type[] typeArguments = t.GetGenericArguments();

		Console.WriteLine("\tList type arguments ({0}):", typeArguments.Length);
		foreach (Type tParam in typeArguments)
		{
			Console.WriteLine("\t\t{0}", tParam);
		}
	}
}

/* This example produces the following output:

--- Create a constructed type from the generic Dictionary type.

System.Collections.Generic.Dictionary[KeyType,ValueType]
        Is this a generic type definition? True
        Does it have generic type arguments? True
        List type arguments (2):
                K
                V

System.Collections.Generic.Dictionary[System.String, Test]
        Is this a generic type definition? False
        Does it have generic type arguments? True
        List type arguments (2):
                System.String
                Test

--- Compare types obtained by different methods:
        Are the constructed types equal? True
        Are the generic types equal? True
 */