How to call generic method with a given Type object?

Question

I want to call my generic method with a given type object.

void Foo(Type t)
{
     MyGenericMethod<t>();
}

obviously doesn't work.

How can I make it work?

Answer 1

This approach will not work. The reason why is that Type is an object who's type is determined at runtime. However you are trying to use it to call a generic method. A generic method call's type is established at compile time. Hence a Type object can't ever be used for a type parameter on a generic method.

Answer 2

You need to use reflection, unfortunately (for the reasons Jared mentioned). For example:

MethodInfo method = typeof(Foo).GetMethod("MyGenericMethod");
method = method.MakeGenericMethod(t);
method.Invoke(this, new object[0]);

Obviously you'd want more error checking in reality :)

---

Side note: my local MSDN doesn't specify that the parameter from MakeGenericMethod is a parameter array, so I'd have expected to require:

method = method.MakeGenericMethod(new Type[] { t });

but it seems it is a parameter array in reality, and the online MSDN docs agree. Odd.

Answer 3

Your code sample won't work, because the generic method expects a type identifier, not a an instance of the Type class. You'll have to use reflection to do it:

public class Example {

    public void CallingTest()
    {
        MethodInfo method = typeof (Example).GetMethod("Test");
        MethodInfo genericMethod = method.MakeGenericMethod(typeof (string));
        genericMethod.Invoke(this, null);

    }

    public void Test<T>()
    {
        Console.WriteLine(typeof (T).Name);
    }
}

Do keep in mind that this is very brittle, I'd rather suggest finding another pattern to call your method.

Another hacky solution (maybe someone can make it a bit cleaner) would be to use some expression magic:

public class Example {

    public void CallingTest()
    {
        MethodInfo method = GetMethod<Example>(x => x.Test<object>());
        MethodInfo genericMethod = method.MakeGenericMethod(typeof (string));
        genericMethod.Invoke(this, null);

    }

    public static MethodInfo GetMethod<T>(Expression<Action<T>> expr)
    {
        return ((MethodCallExpression) expr.Body)
            .Method
            .GetGenericMethodDefinition();
    }

    public void Test<T>()
    {
        Console.WriteLine(typeof (T).Name);
    }
}

Note passing the 'object' type identifier as a generic type argument in the lambda. Couldn't figure out so quickly how to get around that. Either way, this is compile-time safe I think. It just feels wrong somehow :/