Problem with C# Return After Converting From List to Array

I am attempting to solve a problem. There is a bunch of gibberish, but after every vowel is a letter that is used to piece together a legible word. My code always returns either

System.Char[]
System.Char[]
System.Char[]

or

System.Collections.Generic.List`1[System.Char]
System.Collections.Generic.List`1[System.Char]
System.Collections.Generic.List`1[System.Char]

for some random reason. I know it’s a simple problem but I’m having so much trouble with it.

Here is my code currently:

using System;
using System.Linq; 
using System.Collections.Generic;

public class Program
{
  public static string Decypher(string encryptedMessage)
  {
    List<char> letters = new List<char>();
    char[] vowels = {'a', 'e', 'i', 'o', 'u'};
    for (int i = 0; i <= encryptedMessage.Count() - 1; i++)
      {
        if (vowels.Contains(encryptedMessage[i]))
        {
          letters.Add(encryptedMessage[i + 1]);
          i++;
        }
      }

    return letters.ToString();
  }
  
  public static void Main(string[] args)
  {
    string testCase1 = "fksahnlgueyilfhnalfkjnhdssaokjfhndsfiwaourhnfdjgbalfkjshedfnsf";
    string testCase2 = "mkjmnacioudhrieeqwthyiugueresjfgwatfhwghfnhgnffn";
    string testCase3 = "elruoqywicwnjksakvfbsgyohuehnghiefhggadfgsfsfs";

    Console.WriteLine(Decypher(testCase1));
    Console.WriteLine(Decypher(testCase2));
    Console.WriteLine(Decypher(testCase3));
  }
}

>Solution :

Some types have a custom ToString method but most just inherit the default implementation from the Object class, which is to return the name of the type. That’s exactly what you’re seeing from this:

return letters.ToString();

If what you actually want is a string containing the characters from letters then the most obvious option is to use the string constructor that takes a char array as an argument:

return new string(letters.ToArray());