Question

I have the following function to get all the child controls of type.

    private IEnumerable<Control> GetControlsOfType(Control control, Type type)
    {
        var controls = control.Controls.Cast<Control>();

        return controls.SelectMany(ctrl => GetControlsOfType(ctrl, type))
                                  .Concat(controls)
                                  .Where(c => c.GetType() == type);
    }

I am trying to convert it using generic type parameter.

    private IEnumerable<T> GetControlsOfType<T>(Control control) where T: Control
    {
        var controls = control.Controls.Cast<Control>();

        return controls.SelectMany(ctrl => GetControlsOfType<T>(ctrl))
                                  .Concat(controls)
                                  .Where(c => c.GetType() == T);
    }

The code has the error on .Concat.

Error 6 ‘System.Collections.Generic.IEnumerable<T>’ does not contain a
definition for ‘Concat’ and the best extension method overload
‘System.Linq.Queryable.Concat<TSource>(System.Linq.IQueryable<TSource>,
System.Collections.Generic.IEnumerable<TSource>)’ has some invalid
arguments

How to fix the issue?

Answer 1

Try adding .OfType<Control>() after GetControlsOfType<T>(ctrl) and instead of your Where

So your code reads:

return controls.SelectMany(ctrl => GetControlsOfType<T>(ctrl).OfType<Control>())
                                  .Concat(controls)
                                  .OfType<T>();