Question

What happens if you have a collection of objects that implement various interfaces and you do a foreach on that collection for a specific interface (which only some members of the collection implement)? Is it possible to skip the members that don’t implement that interface?

interface IFoo {}
interface IBar {}

class Foo : IFoo {}
class Baz : IFoo, IBar {}

…

var foos = new List<IFoo> ();

foos.Add(new Foo());
foos.Add(new Baz());

foreach (IBar bar in foos)
{
    // What happens now?
}

Answer 1

foreach (IBar bar in foo)
{
    // What happens now?
}

// What happens now?

Nothing happens now, as you already got an InvalidCastException in the first row…

WHY?

foreach statements are being translated to something like:

foreach (object f in foo)
{
    IBar bar = (IBar) f;

    ...
}

There is an implicit cast in the foreach statement
Which allows you to write stupid things like the following without Compilation time error:

var foo  = new List<string>{ "111", "222","333"};
foreach (IBar bar in foo) // InvalidCastException at runtime.
{
    ...
}

You can use LINQ as suggested by @Erno to get only the objects that implement the IBar interface:

foreach(IBar bar in foo.OfType<IBar>())

Which is like:

foo.Where(f => f is IBar)