删除C#中的额外逗号

string input = "A,B,C,";
string output = input.Replace(",,", ",");

string input = "A,B,C,";
string output = Regex.Replace(input, ",+", ",");

string input = "A,B,C,";
string[] parts = input.Split(',');
string output = string.Join(",", parts.Where(p => !string.IsNullOrEmpty(p)));