.NET: Combine array items into a string (deliminated with comma)

So, here is the quick way to achieve it:

ArrayList arr = new ArrayList();
arr.Add("test1");
arr.Add("test2");
arr.Add("test3");

//be sure all elements are string
string str = String.Join(",", (string[]) arr.ToArray(Type.GetType("System.String")));

MessageBox.Show(str); // test1,test2,test3