@Bogdanp maybe this is another way to express it:

public static class EnumerableExtensions {

public static IEnumerable<TR> SelectNotNull<T, TR>( this IEnumerable<T> source, Func<T, TR?> fn) where TR : class { return source.Select(fn) .Where(it => it != null) .OfType<T>(); }

public static IEnumerable<TR> SelectNotNull<T, TR>( this IEnumerable<T> source, Func<T, TR?> fn) where TR : struct { return source.Select(fn) .Where(it => it != null) .Select(item => item.Value); } }