using System;
using System.Collections.Generic;

public class Test
{
    public static void Main()
    {
        var x = new Int16_2D { a = 1, b = 2 }; 
        var set = new HashSet<Int16_2D> { x }; 

        var y = new Int16_2D { a = 1, b = 2 }; 
        Console.WriteLine(set.Contains(y));   // True 

        x.a = 3; 
        Console.WriteLine(set.Contains(y));   // False 
        Console.WriteLine(set.Contains(x));   // False! 

    }
}

public class Int16_2D   
{   
    public Int16 a, b;   
   
    public override bool Equals(Object other)   
    {   
        return other is Int16_2D &&   
        a == ((Int16_2D)other).a &&   
        b == ((Int16_2D)other).b;   
    }

    public override int GetHashCode()
    {
        return a | (int)b * 65536;
    }   
}