Hi Matt,

If you could post your usage of this method, I may be able to explain it in some context.

return sequence.Select( //Loop through each item in sequence
                x =>
                {
                    if (x is TActual) // If the current item TActual is derived from type TBase
                    {
                        var casted = x as TActual; // cast TBase to TActual
                        projection.Invoke(casted); // Invoke the action which you passed, passing our casted TActual as a parameter
                    }
                    return x; // Return the initial TBase
                });

Basically what this method allows us to do is loop through a list of objects and invoke an Action for each item which is of a certain base type without amending the initial list.

The method is extremely generic which may have caused some confusion, but the above is a basic explanation of how it could be used.