Very often during development you need somekind of coma or pipe separated string. Since .NET 1.0, framework had string.Join method. So, you can do something like this:
string[] a = new string[] { "Viktar", "Vasya", "Ivan" };
Response.Write(string.Join(",", a));
Howerver, this not really useful, since you usually doesn't have string array. Usually you have list or array of some kind of objects. Let me show you how you can do it .NET 3.5 using LINQ:
List<Person> persons = new List<Person>();
persons.Add(new Person { FirstName = "Viktar", LastName = "Karpach" });
persons.Add(new Person { FirstName = "Vasya", LastName = "Pupkin" });
persons.Add(new Person { FirstName = "Ivan", LastName = "Ivanov" });
Response.Write(string.Join(",", (from p in persons select p.FirstName).ToArray()));