String.Join. This method can take a string array and a separator and then return that as a single string

// First Convert the long array to string arrayString[] strArray = Array.ConvertAll<long, string>(lngArray, new Converter<long, string>(delegate(long lNum) {return lNum.ToString();}));


// Now use the String.Join to get a comma seperated list of the array’s items

string strArrayList = String.Join(“,”, strArray );