string str3="";
            string str1="aaa:dddd,cccccc";
            string[] myArray=str1.Split(Convert.ToChar(","));
            
            for(int i=0;i<=myArray.Length-1;i++)
            {
                if(myArray[i].IndexOf(":")!=-1)
                {
                    string[] mySub=myArray[i].Split(Convert.ToChar(":"));
                    str3+=mySub[0]+":";
                    
                }
                else
                {
                    str3+=myArray[i];
                }
            }
            MessageBox.Show(str3);

感觉这里的else 用的是多余的·
不过楼主的问题确实得到解决！不错，学习了！

string s1 = "aaa:dddd,cccccc";
            string s2 = "dddd,";

            int startIndex = s1.IndexOf("dddd,");
            int count = s2.Length;

            s1 = s1.Remove(startIndex, count);
            return s1;