how to fix 'T' is a 'type parameter' but is used like a 'variable' compile error

Question

I need to check if a generic type parameter T is MyEntity or a subclass of it.

Code below causes this compiler error:

'T' is a 'type parameter' but is used like a 'variable'

how to fix?

public class MyEntity { }

static void Test<T>()
{
    // Error    34  'T' is a 'type parameter' but is used like a 'variable'
    if (T is MyEntity)
    {
    }
}

Answer 1

You can use IsAssignableFrom method on Type to check whether one Type can be assigned to another.

if (typeof(MyEntity).IsAssignableFrom(typeof(T)))

Note: if you want that T can only be MyEntity or it's subclasses, you can define generic constraint, like this:

static void Test<T>() where T : MyEntity
{

}

And the code like Test<object>() won't even compile

---

You can check IsAssignableFrom with this code:

public static void F<T>()
{
    var isAssignable = typeof(IEnumerable).IsAssignableFrom(typeof(T));

    Console.WriteLine ("{0} is {1} IEnumerable", typeof(T).Name, isAssignable ? "" : "not");
}

Examples:

F<IList>();       //prints IList is IEnumerable
F<IEnumerable>(); //prints IEnumerable is IEnumerable
F<object>();      //prints object is not IEnumerable