using System;

bool Equals(string left, string right) => left?.Equals(right) ?? (right is null);

Console.WriteLine(Equals("ab", "ab") == true);
Console.WriteLine(Equals("ab", "zx") == false);
Console.WriteLine(Equals(null, null) == true);
Console.WriteLine(Equals("ab", null) == false);
Console.WriteLine(Equals(null, "ab") == false);