Convert Array List to Comma Separated String in C#

Guessing from the name of your variable (arrayList), you've got List<string[]> or an equivalent type there.

The issue here is that you're calling ToString() on the array. Try this instead:

for (i = 0; i < xxx; i++)
{
    var array = arrayList[i];
    MailingList = string.Join(", ", array);
    Response.Write(MailingList);
}

EDIT: If arrayList is simply an ArrayList containing strings, you can just do

Response.Write(string.Join(", ", arrayList.OfType<string>()));

Personally I would avoid using nongeneric collections (such as ArrayList) if possible and use strongly-typed collections from System.Collections.Generic such as List<string>. For example, if you have a piece of code that depends on that all contents of the ArrayList are strings, it will suffer catastrophically if you accidentally add an item that's not a string.

EDIT 2: If your ArrayList actually contains System.Web.UI.WebControls.ListItems like you mentioned in your comment: arrayList.AddRange(ListBox.Items);, then you'll need to use this instead:

Response.Write(string.Join(", ", arrayList.OfType<ListItem>()));

The second parameter for String.Join needs to be an IEnumerable. Replace MailingList.ToString() with arrayList and it should work.

Initialization:

string result = string.Empty;

For value types:

if (arrayList != null) {
   foreach(var entry in arrayList){
      result += entry + ',';
   }
}

For reference types:

if (arrayList != null) {
   foreach(var entry in arrayList){
      if(entry != null)
         result += entry + ',';
   }
}

And cleanup:

if(result == string.Empty)
   result = null;
else
   result = result.Substring(0, result.Length - 1);

most of the answers are already there, still posting a complete - working snippet

string[] emailListOne = { "[email protected]", "[email protected]", "[email protected]", "[email protected]" };
string[] emailListTwo = { "[email protected]", "[email protected]", "[email protected]", "[email protected]" };
string[] emailListThree = { "[email protected]", "[email protected]", "[email protected]", "[email protected]" };
string[] emailListFour = { "[email protected]", "[email protected]", "[email protected]", "[email protected]" };

List<string[]> emailArrayList = new List<string[]>();
emailArrayList.Add(emailListOne);
emailArrayList.Add(emailListTwo);
emailArrayList.Add(emailListThree);
emailArrayList.Add(emailListFour);

StringBuilder csvList = new StringBuilder();
int i = 0;
foreach (var list in emailArrayList)
{
   csvList.Append(string.Join(",", list));
   if(i < emailArrayList.Count - 1)
      csvList.Append(",");
   i++;
}
Response.Write(csvList.ToString());