C# - Убрать слова менее пяти буков

using System;
using System.Text;
using System.Collections.Generic;
class Practic {
  public static string var15(string str) {
    string[] words = str.Split(' ');
    for(int i = 0; i<words.Length; ++i){
        if(words[i].Length < 5) words[i] = "";
    }
    str = String.Join(" ", words);
    return str;
  }
  public static void Main() {
    List < string > str = new List < string > ();
    Console.WriteLine("Введите строки");
    string input = "";
    while ((input = Console.ReadLine()) != "")
      str.Add(input);
    foreach(var s in str) {
      Console.WriteLine(var15(s));

    }
  }
}