C# Generics

Question

I'm trying to create an instance of a generic class, without knowing what type to cast it to it until runtime. I've written the following code

        Type pType = propertyInfo.GetType();
        ObjectComparer<pType> oc = new  ObjectComparer<pType>();

Hopefully that gives you an idea what I'm trying to do, however it wont compile it just says

"the type or namespace pType could not be found".

Is there any easy way of doing this?

Thanks

Gavin

Answer 1

Type type = typeof(ObjectComparer<>).MakeGenericType(pType);
object obj = Activator.CreateInstance(type);

However, unless you cast to a non-generic interface (IComparer, maybe), you lose the ability to use it as a generic type.

---

The most common answer here is a non-generic base class/interface:

interface ISomeInterface {
    object SomeMethod(object value);
}
interface ISomeInterface<T> : ISomeInterface {
    T SomeMethod(T value);
}

You can then invoke via the non-generic ISomeInterface without having to jump through any extra hoops.

Example using MakeGenericMethod below - but a non-generic base interface would more efficient:

class Program {
    static void Main() {
        int i = 123;
        typeof(Program).GetMethod("Foo",
                BindingFlags.Static | BindingFlags.NonPublic)
            .MakeGenericMethod(i.GetType())
            .Invoke(null, new object[] { i });
    }
    static void Foo<T>(T value) {
        ObjectComparer<T> comparer = new ObjectComparer<T>();
        comparer.Bar(value);
    }
}
class ObjectComparer<T> {
    public void Bar(T value) {
        Console.WriteLine(typeof(T).Name + " = " + value);
    }
}

Answer 2

Something like this:

Type someType = typeof(ObjectComparer<>);
Type toConstruct = someType.MakeGenericType (pType);

object o = Activator.CreateInstance (toConstruct);

Seems that i'm to slow.