Question

I have a generic method which adds all the html controls I have on a page into a generic list using a series of foreach loops which works. Is it possible to convert this into a LINQ expression?

private List<T> GetControls<T>() where T : HtmlControl
         {
            List<T> c = new List<T>();

            foreach (HtmlControl c1 in Controls)
            {
                foreach (HtmlControl c2 in c1.Controls)
                {
                    if (c2.GetType() == typeof(HtmlForm))
                    {
                        foreach (Control c3 in c2.Controls)
                        {
                            if (c3.GetType() == typeof(ContentPlaceHolder))
                            {
                                foreach (HtmlControl c4 in c3.Controls)
                                {
                                    if (c4.GetType() == typeof(T))
                                    {
                                        c.Add((T)c4);
                                    }
                                    if (c4.GetType() == typeof(PlaceHolder))
                                    {
                                        foreach (HtmlControl c5 in c4.Controls)
                                        {
                                            if (c5.GetType() == typeof(T))
                                            {
                                                c.Add((T)c5);
                                            }
                                        }
                                    }
                                }
                            }
                        }
                    }
                }
            }
            return c;
        }

Answer 1

This should do it:

List<T> c = this.Controls.Cast<Control>()
    .SelectMany(c1 => c1.Controls.Cast<Control>())
    .OfType<HtmlForm>()
    .SelectMany(c2 => c2.Controls.Cast<Control>())
    .OfType<ContentPlaceHolder>()
    .SelectMany(c3 => c3.Controls.Cast<Control>())
    .SelectMany(c4 =>
        {
            if (c4 is T)
                return new[] { (T)c4 };
            if (c4 is PlaceHolder)
                return c4.Controls.Cast<Control>().OfType<T>();
            return Enumerable.Empty<T>();
        })
    .ToList();

However note I’ve used is instead of a type comparison here. This is deliberate, because this is what the OfType LINQ method also uses internally.

If you’re sure you want exact types rather than objects that pass an is comparison, you’ll have to implement your own OfType (or just use .Where(x => x.GetType == typeof(whatever)) instead.)

(Also note that I’ve used Control instead of HtmlControl, in case some of your HtmlControls contain regular Controls.)