Remove value from string in C# windows application

string your_string = "123456789123456789000001,RSSI=-31.2";
string[] parts= your_string.Split(',');
parts[0]= //this will be your required string that is "123456789123456789000001"

If you have charArray convert it to string then split it using char ','.
You will get 2 string values.
Convert the first string to charArray and that your required output.

string arrayString = Convert.ToString(charArray);
                string[] splitString = arrayString.Split(new char[] { ',' }, StringSplitOptions.None);
                char[] output = splitString[0].ToCharArray();