[RESOLVED] Type equality...?

[NickThissen]

Hey,

I need to check if a certain Type instance:
1. Is equal to a type, and
2. Is equal to another type or derives from that type.

A little hard to explain, so let's bring in an example (actually, the real thing in this case as it's simple enough to understand).


I have one abstract class Shape, and many deriving classes, such as RectangleShape, EllipseShape, TriangleShape, etc.
Now, I need to check two different things:

Suppose I have a Type instance 't'. I need to check if

1. this type is exactly another type (such as RectangleShape). In english: "Is t a RectangleShape, but not some class deriving RectangleShape".

2. this type t is either one of the deriving Shapes (in english: "Does t derive from Shape?")


In code, I have a method that accepts a Type, and I need to check number 2 first (as a check whether the Type is actually a Shape, whatever Shape it is), and number 1 later (to check which kind of shape it actually is).

private int GetNextFreeNumber(Type t)
{
    if (t derives from Shape)
    {
        foreach (Shape s in this)
        {
            if (t is the type of s, and not a type deriving from s, nor any other Shape type)
            {
                //...
            }
        }
    }
}

How do I do this? I know I can use

if (t == typeof(Shape))

and I also know of the 'is' operator, something like

if (someInstance is Shape)

but I can't use the 'is' operator because I haven't got an instance of Shape, I only got a Type (and that Type may be Button or Form or whatever, in which case I don't even need to do the foreach loop).

So basically I know of only one 'type equality' check in C#, but obviously I can't use it for both... Does my method do check number 1 or number 2, and how can I do the other check?

Googling for 'type equality' has turned out pretty useless, all I get is 'reference type equality' showing how to implement an IEquatable interface, and stuff like that.

Thanks!

[jmcilhinney]

E.g.

Type c = typeof (Control);
Type f = typeof (Form);
Type b = typeof (Button);

MessageBox.Show(c == typeof (Control) ? "c is Control" : "c is not Control");

MessageBox.Show(f == typeof (Control) ? "f is Control" : "f is not Control");

MessageBox.Show(b == typeof (Control) ? "b is Control" : "b is not Control");

MessageBox.Show(c.IsSubclassOf(typeof (Control))
                    ? "c is derived from Control"
                    : "c is not derived from Control");

MessageBox.Show(f.IsSubclassOf(typeof (Control))
                    ? "f is derived from Control"
                    : "f is not derived from Control");

MessageBox.Show(b.IsSubclassOf(typeof (Control))
                    ? "b is derived from Control"
                    : "b is not derived from Control");

MessageBox.Show((typeof(Control)).IsAssignableFrom(c)
                    ? "Control is assignable from c"
                    : "Control is not assignable from c");

MessageBox.Show((typeof(Control)).IsAssignableFrom(f)
                    ? "Control is assignable from f"
                    : "Control is not assignable from f");

MessageBox.Show((typeof(Control)).IsAssignableFrom(b)
                    ? "Control is assignable from b"
                    : "Control is not assignable from b");

[NickThissen]

Ah, I didn't know the IsSubclassOf method, that seems to do what I want. I was looking for an operator instead of a method, that explains why I didn't find it

Thanks!

[avrail]

hey,
about the IsSubclassOf, i just wanna share this
http://msdn.microsoft.com/en-us/libr...ubclassof.aspx