C# How to Initialize Generic class with object of type "Type"

Question

I recently had this problem.

doSomething(typeof(int));
doSomething(typeof(MyClassA));
doSomething(typeof(MyClassB));

public void doSomething(Type _type)
{
    var myGenObj = new MyGenericClass<_type>();  // Error.  Really I'd want MyGenericClass<int>, MyGenericClass<MyClassA>, etc depending on what's passed in.
    myGenObj.doSomeGenStuff();
    // more stuff...

}

I think that this can be done with reflection somehow.. Possibly there's an easier way. I've been somewhat confused on how Type works vs Classes under the covers. Anyways thanks for any help.

Thanks.

Answer 1

You want Type.MakeGenericType and then Activator.CreateInstance... but then calling a method on the newly-created object will be tricky. Ideally you could have a non-generic base class or interface containing those members:

public interface IFoo
{
    void CallSomeMethod();
}

public class MyGenericClass<T> : IFoo
{
    ...
}

// Names changed to be more conventional
public void DoSomething(Type type)
{
    var genericType = typeof(MyGenericClass<>).MakeGenericType(type);
    var instance = (IFoo) Activator.CreateInstance(genericType);
    instance.CallSomeMethod();
}

If you do need to call a method which depends on the type parameter, you'll need to do that with reflection, or with dynamic which can streamline reflection-based code.

EDIT: As cdhowie says, if you always actually do know the type at compile-time, you can use a generic method which would make things much simpler. You'd then call the method like this:

DoSomething<int>();
DoSomething<MyClassA>();
DoSomething<MyClassB>();