using System;

var firstNumber = "1";
var numberList = "";
switch (firstNumber) {
    case "1":
        numberList = "1";
        goto case "2";
    case "2":
        numberList += "-2";
        goto case "3";
    case "3":
        numberList += "-3";
        break;
}       
//numbersList = "1-2-3"

Console.WriteLine(numberList);