Invoke generic method with the generic type from a System.Type

Question

below is a code example and the question, please note that I can NOT use C# 4.0 and the dynamic keyword.

static class TestClass
{
    static void Main(string[] args)
    {
        Object o = "Previous value";
        Test(ref o);
        Trace.WriteLine(o);
    }

    static public void Test<T>(ref T obj)
    {
        //  The goal is to somehow invoke Test2 with the real type of obj, i.e the type in obj.GetType() 

        //  1st try:
        Test2(ref obj); // This doesn't work because the type in Test2 will be the same as T here.

        //  2nd try:
        MethodInfo mi = typeof(TestClass).GetMethod("Test2");
        mi = mi.MakeGenericMethod(new Type[] { obj.GetType() });
        mi.Invoke(null, new Object[] { obj }); // obj is no longer by reference so we need to store the object array and copy back the result after the call

        //  3rd try, successful implementation by the smartest mind of stack overflow :)
    }

    static public void Test2<T>(ref T s)
    {
        if (typeof(T) == typeof(String))
        {
            s = (T)(Object)"Hello world!";
        }
    }
}

I also tried some more methods using Delegate.CreateDelegate but without any luck. Is this at all possible?

Edit: I'm not afraid of using a dynamic method (and MSIL assembler) but my knowledge in this area is very limited.

Edit2: Here's an example that is closer to what I really trying to do:

public static class TypeHandler<T>
{
    public delegate void ProcessDelegate(ref T value);

    public static readonly ProcessDelegate Process = Init();

    private static ProcessDelegate Init()
    {
        //  Do lot's of magic stuff and returns a suitable delegate depending on the type
        return null;
    }
}


static class TestClass
{
    static public void Main(string[] args)
    {
        Object o = "Previous value";
        Test(ref o);
        Trace.WriteLine(o);
    }

    static public void Test<T>(ref T obj)
    {
        if (obj is T)
        {
        //  Optimized, common case
            TypeHandler<T>.Process(ref obj);
            return;
        }
        Type t = obj.GetType();
        //  How to call the delegate found in TypeHandler<t>.Process ? (I can get delegate but I can't call it).


    }

}

Answer 1

Your comment looks like you already understand how to do it:

MethodInfo mi = typeof(TestClass).GetMethod("Test2");
mi = mi.MakeGenericMethod(new Type[] { obj.GetType() });
object[] args = new object[] { obj };
mi.Invoke(null, args);
obj = (T) args[0];

That's really just turning your comment into code. Does that somehow not do what you want?

Answer 2

The main questions, in my opinion is, what do you want to do?

If you just want to assign a string to a reference object you could try this:

Generics can be defined during runtime, but it's not verry comfortable and has to be done by reflection ... in my oppinion it is a "no go", but just an oppinion

Try using the object.GetType() to get the current Type of the Object.

static class TestClass {
    static void Main(string[] args) {
        Object o = "Previous value";
        Test(ref o);
        Console.WriteLine(o);
        Console.ReadLine();
    }

    static public void Test<T>(ref T obj) {
        Object o = (Object)obj;
        Test2(ref o);
        obj = (T)o;
    }

    static public void Test2(ref object s) {
        if (s.GetType().Equals(typeof(String))) {
            s = "Hello world!";
        }
    }
}

Answer 3

Update 3: OK, since you're fine with an ugly solution, you may want to check out the undocumented __refvalue and __makeref keywords.

---

It seems your issue is that you want to be able to specify the type for a ref object parameter to be converted or changed to.

The problem with that is that you can't just arbitrarily assign a variable of any type T to a string, for example. So you'd need to pass in a ref object or ref string for that to work at all.

It looks to me like oberfreak nailed down what you were trying to achieve (I originally failed to notice you had initialized o as a string and thus clearly wanted its actual type to influence the behavior of the Test2 function). His answer has the right approach for you.

Update: you mention in a comment that what you're trying to do is have dynamic behavior which could be achieved using a dictionary. I'm guessing that looks something like this?

Update 2: updated this example based on your updated example.

public static class TypeHandler // note: get rid of generic T parameter
{
    delegate void ProcessDelegate(ref object obj); // again, not generic

    static Dictionary<Type, ProcessDelegate> processors = new Dictionary<Type, ProcessDelegate>()
    {
        { typeof(string), (ref object obj) => { obj = "Hello, world!"; } }
        // etc.
    };

    public static void Process(ref object obj)
    {
        processors[obj.GetType()].Invoke(ref obj);
    }
}

That should work. But you can't really achieve the same thing with generics because there's no way to do this (as you know):

//          not allowed
//               |
//          -----------
//         |           |
TypeHandler<o.GetType()>.Process(ref o);

If there were, then you'd be all set. But the only possible way you can do that is by using reflection, which is ugly and expensive for something like this and would clearly defeat your intention of keeping this simple and ensuring good performance.

Answer 4

The right way to implement method Test2 would be

static public void Test2<T>(ref T s)
    {
        if (s is string)
        {
            s = (T)(Object)"Hello world!";
        }
}

Or am I missing something here?

Answer 5

If you can change the ref to a regular return, you can cheat massively in 4.0 via dynamic:

dynamic foo = obj;
Test(foo);

that is now:

Test<TheActualTypeOfObj>(obj);

Full example:

static void Main(string[] args)
{
    object o = "Previous value";
    o = Test2((dynamic)o);
    Trace.WriteLine(o);
}

static public T Test2<T>(T s)
{
    if (typeof(T) == typeof(string))
    {
        s = (T)(object)"Hello world!";
    }
    return s;
}

which writes "Hello world!"