Calculating Count for IEnumerable (Non Generic)

Can anyone help me with a Count extension method for IEnumerable (non generic interface).

I know it is not supported in LINQ but how to write it manually?

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;
    }
}

The simplest form would be:

public static int Count(this IEnumerable source)
{
    int c = 0;
    using (var e = source.GetEnumerator())
    {
        while (e.MoveNext())
            c++;
    }
    return c;
}

You can then improve on this by querying for ICollection:

public static int Count(this IEnumerable source)
{
    var col = source as ICollection;
    if (col != null)
        return col.Count;

    int c = 0;
    using (var e = source.GetEnumerator())
    {
        while (e.MoveNext())
            c++;
    }
    return c;
}

Update

As Gerard points out in the comments, non-generic IEnumerable does not inherit IDisposable so the normal using statement won't work. It is probably still important to attempt to dispose of such enumerators if possible - an iterator method implements IEnumerable and so may be passed indirectly to this Count method. Internally, that iterator method will be depending on a call to Dispose to trigger its own try/finally and using statements.

To make this easy in other circumstances too, you can make your own version of the using statement that is less fussy at compile time:

public static void DynamicUsing(object resource, Action action)
{
    try
    {
        action();
    }
    finally
    {
        IDisposable d = resource as IDisposable;
        if (d != null)
            d.Dispose();
    }
}

And the updated Count method would then be:

public static int Count(this IEnumerable source) 
{
    var col = source as ICollection; 
    if (col != null)
        return col.Count; 

    int c = 0;
    var e = source.GetEnumerator();
    DynamicUsing(e, () =>
    {
        while (e.MoveNext())
            c++;
    });

    return c;
}