1. Classic Method

 Declare an additional temporary variable, copy the value or reference of one of the two variables into it, and then perform the swap.

var a = 10;
var b = 20;
var temp = a;
a = b;
b = temp;
    

2. Modern Method (C# 7.0 or later)

 Using a tuple, you can achieve the swap in a single line.

(b, a) = (a, b)