Every object you ever created or used in .NET has GetHashCode method along with Equals, GetType, and ToString methods.  This method is an instance method of an Object class from which any other class derives.

GetHashCode() method returns an integer that identifies an object instance.  I also can repeat MSDN documentation but you can read it on your own.

You would want to override this method along with Equals() method in your class if the object of your class is going to be used as a key in Hashtable.

The best way to explain it is to show an example. In my previous post I was using class Point as an example.  I’m going to continue with this class.  So let assume that we have class like this:

   1: public class Point

   2: {

   3:   private readonly int _x;

   4:   private readonly int _y;

   5:  

   6:   public Point(int x, int y)

   7:   {

   8:     _x = x;

   9:     _y = y;

  10:   }

  11: }

Next we want to use Point as a key in a Hashtable.  Remember that the key of the Hasthable must be unique.  In the code bellow we have two identical keys (line 2 & 4). We should expect ArgumentException.

   1: Hashtable hashtable = new Hashtable();

   2: hashtable.Add(new Point(2, 3), "Point 1");

   3: hashtable.Add(new Point(5, 3), "Point 2");

   4: hashtable.Add(new Point(2, 3), "Point 3");

However, no exception was thrown.  There are two reasons why exception is not thrown:

1. Equals() method that is also an instance method of Object class will always indicate that Point in line 2 and line 4 are different. This expected because two objects have different references.
2. GetHashTable() method returns different number for these objects.

Try to execute code bellow:

   1: static void Main(string[] args)

   2:  {

   3:    var point1 = new Point(2, 3);

   4:    var point2 = new Point(2, 3);

   5:    Console.WriteLine("point1 Hash: {0}"

   6:         , point1.GetHashCode());

   7:    Console.WriteLine("point2 Hash: {0}"

   8:         , point2.GetHashCode());

   9:    Console.WriteLine("point1 equal to point2: {0}"

  10:         , point1.Equals(point2));

  11:  }

You will get output similar to this one:

point1 Hash: 58225482
point2 Hash: 54267293
point1 equal to point2: False

You can see that we got different hash codes.

To solve this problem we override Equal() and GetHashTable() in our Point class.

First we override Equals() method:

   1: public override bool Equals(object obj)

   2: {

   3:   if (ReferenceEquals(null, obj)) return false;

   4:   if (ReferenceEquals(this, obj)) return true;

   5:   return ((Point)obj)._x == _x && ((Point)obj)._y == _y;

   6: }

You can see that I use ReferenceEquals() method that is static method of Object class.  It checks if the specified objects are the same.

Next we override GetHashTable() method:

   1: public override int GetHashCode()

   2: {

   3:   unchecked

   4:   {

   5:     return (_x * 397) ^ _y;

   6:   }

   7: }

The whole Point would look like this:

   1: public class Point

   2: {

   3:   private readonly int _x;

   4:   private readonly int _y;

   5:  

   6:   public Point(int x, int y)

   7:   {

   8:     _x = x;

   9:     _y = y;

  10:   }

  11:  

  12:   public override bool Equals(object obj)

  13:   {

  14:     if (ReferenceEquals(null, obj)) return false;

  15:     if (ReferenceEquals(this, obj)) return true;

  16:     return ((Point)obj)._x == _x && ((Point)obj)._y == _y;

  17:   }

  18:  

  19:   public override int GetHashCode()

  20:   {

  21:     unchecked

  22:     {

  23:       return (_x * 397) ^ _y;

  24:     }

  25:   }

  26: }

Now if you try to execute the application the output should look exactly like this:

point1 Hash: 793
point2 Hash: 793
point1 equal to point2: True

Also if we try to add new Point(2, 3) twice, we get an ArgumentException as expected.