Setting generic type at runtime

Question

I have a class

public class A<T>
{
   public static string B(T obj)
   {
       return TransformThisObjectToAString(obj);
   }
}

The use of string above is purely exemplary. I can call the static function like this just fine on a known/specified type:

string s= A<KnownType>.B(objectOfKnownType);

How do I make this call, if I don't know T beforehand, rather I have a variable of type Type that holds the type. If I do this:

Type t= typeof(string);
string s= A<t>.B(someStringObject);

I get this compiler error:

Cannot implicitly convert type 't' to 'object'

Answer 1

You can't do this directly, but you can use reflection to provide a type parameter of a class at run-time. I haven't tested this, but something like this should work:

// We want to do something like this: //    object o = "Hello" //    Type t = o.GetType();  // // This is pseudo-code only: //    string s = A<t>.B(o);   string InvokeA(object o) {   // Specify the type parameter of the A<> type   Type genericType = typeof(A<>).MakeGenericType(new Type[] { o.GetType() });   // Get the 'B' method and invoke it:   object res = genericType.GetMethod("B").Invoke(new object[] { o });   // Convert the result to string & return it   return (string)res; } 

Of course, the question is if this is really what you need - If you don't know anything about the object given as an argument, you could as well write the whole code just using object. However, I can imagine some scenarios where this would be useful, so I guess you can try using this.

Answer 2

There's absolutely support for this in the framework and the CLR - just not gracefully in C#. You can accomplish what I think you want, though, with the help of a helper method:

public class A<T>
{
    public static string B(T obj)
    {
        return obj.ToString();
    }
}

public class MyClass
{
    public static void DoExample()
    {
        Console.WriteLine(ExecuteB("Hi"));
        Console.WriteLine(ExecuteB(DateTime.Now));
    }

    public static object ExecuteB(object arg)
    {
        Type arg_type = arg.GetType();
        Type class_type = typeof(MyClass);
        MethodInfo mi = class_type.GetMethod("ExecuteBGeneric", BindingFlags.Static | BindingFlags.Public);
        MethodInfo mi2 = mi.MakeGenericMethod(new Type[] { arg_type });
        return mi2.Invoke(null, new object[] { arg });
    }

    public static object ExecuteBGeneric<T>(T arg)
    {
        return A<T>.B(arg);
    }

Answer 3

You can't. Generic type identifiers have to be known at compile time.

edit

as of other posts, it appears to be possible by dynamicly generating the method and invoking it - which has dangers of course. See Thomas' and Dathan's posts for more inforation.