Serialize a HashSet<String> with LinQ

I'd like to take a HashSet<String> and elegantly convert it to a string. I can iterate like so:

HashSet<String> words = new HashSet<string>() { "alpha", "beta", "delta" };

string joined = "";
foreach (var w in words) 
  joined += w + ",";

if(joined.Length > 0)
  joined = joined.SubString(0,joined.Length-1); // remove final comma

Is there a LinQ way to do this elegantly and efficiently?

The only way I can think of doing this is by converting it to an array first:

HashSet<String> words = new HashSet<string>() { "alpha", "beta", "delta" };
string joined = String.Join(",",words开发者_开发问答.ToArray());

But, then I'm doing a double conversion. Would there be some handy-dandy LinQ expression that's efficient and clear?

ANSWER 1 (from marr's idea)

public static string JoinItems(this IEnumerable<string> items, string joiner) {
    StringBuilder sb = new StringBuilder("");

    foreach (var i in items) 
        sb.AppendFormat("{0}{1}",i,joiner);

    if(sb.Length>0) 
        return sb.Remove(sb.Length - joiner.Length, joiner.Length).ToString();
    else
       return sb.ToString();
}

ANSWER 2 using an Enumerator (from Martin's solution)

public static string JoinItems<T>(this IEnumerable<T> items, string delim) {
    var sb = new StringBuilder();
    var i = items.GetEnumerator();
    if (i.MoveNext()) {
        sb.Append(i.Current);
        while (i.MoveNext()) {
            sb.Append(delim);
            sb.Append(i.Current);
        }
    }
    return sb.ToString();
}

I took your method and modified it to not need to remove the last comma. I also changed the AppendFormat to just Append because it avoids all the work of parsing the format each time.

public static string JoinItems(this IEnumerable<string> items, string joiner)
{
    StringBuilder sb = new StringBuilder(); 
    string delim = "";

    foreach (var i in items)
    {
        sb.Append(delim);
        sb.Append(i);
        delim = joiner;
    }

    return sb.ToString(); 
} 

This will do the trick without extra copies or checks on each iteration:

String JoinItems<T>(IEnumerable<T> items) {
  var stringBuilder = new StringBuilder();
  var i = items.GetEnumerator();
  if (i.MoveNext()) {
    stringBuilder.Append(i.Current);
    while (i.MoveNext()) {
      stringBuilder.Append(", ");
      stringBuilder.Append(i.Current);
    }
  }
  return stringBuilder.ToString();
}

I don't see the double conversion in your String.Join() line. I see one conversion ToArray(), which is not terrible, and then it executes String.Join(), which performs well.

There is a String.Join() in .Net 4 that takes an IEnumerable that will work without the conversion. If you're using an older framework version, you can write your own extension method for string that takes a separator as the "this" parameter then joins an IEnumerable. Be sure and use stringbuilder for performance.