Best way to find out if an (upcast) instance doesn't implement a particular interface

Maybe the need to do this is a 'design smell' but thinking about another question, I was wondering what the cleanest way to implement the inverse of this:

foreach(ISomethingable somethingableClass in collectionOfRelatedObjects)
{
  somethingableClass.DoSomething();
}

i.e. How to get/iterate through all the objects that don't implement a particular interface?

Presumably you'd need to start by upcasting to the highest level:

foreach(ParentType parentType in collectionOfRelatedObjects)
{
  // TODO: iterate through everything which *doesn't* implement ISomethingable 
}

Answer by solving the TODO: in the cleanest/simplest and/or most efficient way

From stackoverflow Gordon Hartley

• Something like this?

  foreach (ParentType parentType in collectionOfRelatedObjects) {
      if (!(parentType is ISomethingable)) {
      }
  }
  

  From J D OConal

• Probably best to go all the way and improve the variable names:

  foreach (object obj in collectionOfRelatedObjects)
  {
      if (obj is ISomethingable) continue;
  
      //do something to/with the not-ISomethingable
  }
  

  From Tim Erickson

• J D OConal's is the best way to do this but as a side note, you can use the as keyword to cast an object, and it'll return null if its not of that type.

  So something like:

  foreach (ParentType parentType in collectionOfRelatedObjects) {
      var obj = (parentType as ISomethingable);
      if (obj == null)  {
      }
  }
  

  From sontek

• this should do the trick:

  collectionOfRelatedObjects.Where(o => !(o is ISomethingable))
  

  J D OConal : Nice. I haven't done too much with these new methods.
  From Jay Bazuzi

• With some help from the LINQ extension method OfType<>(), you can write:

  using System.Linq;
  
  ...
  
  foreach(ISomethingable s in collection.OfType<ISomethingable>())
  {
    s.DoSomething();
  }
  

  From Yacoder