using System;

​

void SwapStrings( string s1, string s2) {

    var tmp = s1;

    s1 = s2;

    s2 = tmp;

}

​

var s1 = "A";

var s2 = "B";

SwapStrings(s1, s2);

//s1 is "B", s2 is "A"

​

Console.WriteLine($"s1: {s1}, s2: {s2}");