Combination of Select((T) => U) & Where((T) => bool) for better ergonomics

SelectWhere.cs

using System.Collections.Generic;

public static class SelectWhereExtension {
  delegate bool SelectWherePredicate<T, U>(T input, out U output);
  
  public static IEnumerable<U> SelectWhere<T, U>(this IEnumerable<T> iter, SelectWherePredicate<T, U> predicate) {
    var enumerator = iter.GetEnumerator();
    while (enumerator.MoveNext()) {
      if (predicate(enumerator.Current, out U product)) {
        yield return product;
      }
    }
  }
}