Can Generic Classes Be Instantiated with Runtime-Determined Type Parameters?

Runtime Instantiation of Generic Classes

This article explores the challenge of instantiating a generic class with a type parameter determined at runtime. Directly using a runtime-determined Type variable as a generic type parameter is impossible due to compile-time constraints. The compiler needs the concrete type at compile time.

Attempting this directly, as shown below, results in a compiler error:

string typeName = "<read type name somewhere>"; // Runtime type name
Type myType = Type.GetType(typeName);

MyGenericClass<myType> myGenericClass = new MyGenericClass<myType>(); // Compiler error

The solution involves leveraging reflection. The following example demonstrates this:

using System;
using System.Reflection;

public class Generic<T>
{
    public Generic()
    {
        Console.WriteLine($"T={typeof(T)}");
    }
}

class Test
{
    static void Main()
    {
        string typeName = "System.String";
        Type typeArgument = Type.GetType(typeName);

        Type genericClass = typeof(Generic<>); // Note the <> here
        Type constructedClass = genericClass.MakeGenericType(typeArgument);

        object created = Activator.CreateInstance(constructedClass);
    }
}

The crucial step is using Type.MakeGenericType(). This method dynamically creates a new type representing a generic instance of Generic<T>, substituting T with the runtime typeArgument. Activator.CreateInstance() then instantiates this newly constructed type. Note the use of Generic<> to specify the open generic type.

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