Calculating Count for IEnumerable (Non Generic)

yourEnumerable.Cast<object>().Count()

To the comment about performance:

I think this is a good example of premature optimization but here you go:

static class EnumerableExtensions
{
    public static int Count(this IEnumerable source)
    {
        int res = 0;

        foreach (var item in source)
            res++;

        return res;
    }
}