Polymorphism runtime object type check

Question

Consider the following classes:

class A
{
    public virtual string Name { get { return "A"; } }
}

class B : A
{
    public override string Name { get { return "B"; } }
}

class C : A
{
    public override string Name { get { return "C"; } }
}

And a list containing two objects of type B and one of type C:

List<A> l = new List<A>();
l.Add(new B());
l.Add(new C());
l.Add(new B());

Is there some way to force a check of the type at runtime and iterate only over the objects of type B in this list (short of writing a custom implementation of a list)? Something like:

foreach (B obj in l)               // runtime error
    Console.WriteLine(obj.Name);

// desired output:
//   B
//   B

I haven't run into this problem in a project, but the thought just occurred to me and I'm curious if this can be done. I'm aware the need for this feature could indicate a design flaw.

Check out [`Enumerable.OfType`](https://msdn.microsoft.com/en-us/library/vstudio/bb360913(v=vs.100).aspx) — juharr, Aug 03 '15 at 16:56
Possible duplicate of [Type Checking: typeof, GetType, or is?](http://stackoverflow.com/questions/983030/type-checking-typeof-gettype-or-is) — Liam, Nov 06 '15 at 08:40

score 7 · Accepted Answer · answered Aug 03 '15 at 16:56

7

You can use .OfType<T>():

foreach (B obj in l.OfType<B>())
    Console.WriteLine(obj.Name);

If you want only Bs, and not any subclasses of B to be listed, you can use the following:

foreach (B obj in l.Where(o => o.GetType() == typeof(B)))
    Console.WriteLine(obj.Name);

answered Aug 03 '15 at 16:56

Jashaszun

9,207
3
29
57

1

Awesome, `OfType` was exactly what I was looking for. Thanks for the caveat about the subclasses as well. – drew.cuthbert Aug 03 '15 at 18:44

score 4 · Answer 2 · answered Aug 03 '15 at 16:57

4

foreach (A obj in l) 
{
    if (obj is B)
    {
       Console.WriteLine(obj.Name);
    }

}

answered Aug 03 '15 at 16:57

ro-E

279
1
5
16

This is the simple approach I should have been thinking of myself. Thanks. – drew.cuthbert Aug 03 '15 at 18:43

Polymorphism runtime object type check

2 Answers2