Question

I have a list of types (System.Type) which need te be queried on the database.

For each of this types, I need to call the following extensionmethod (which is part of LinqToNhibernate):

Session.Linq<MyType>()

However I do not have MyType, but I want to use a Type instead.

What I have is:

System.Type typeOne;

But I cannot perform the following:

Session.Linq<typeOne>()

How can I use a Type as a Generic parameter?

Answer 1

You can’t, directly. The point of generics is to provide compile-time type safety, where you know the type you’re interested in at compile-time, and can work with instances of that type. In your case, you only know the Type so you can’t get any compile-time checks that any objects you have are instances of that type.

You’ll need to call the method via reflection – something like this:

// Get the generic type definition
MethodInfo method = typeof(Session).GetMethod("Linq", 
                                BindingFlags.Public | BindingFlags.Static);

// Build a method with the specific type argument you're interested in
method = method.MakeGenericMethod(typeOne);
// The "null" is because it's a static method
method.Invoke(null, arguments);

If you need to use this type a lot, you might find it more convenient to write your own generic method which calls whatever other generic methods it needs, and then call your method with reflection.