Dictionary Hashtable IEqualityComparer override virtual

`GetHashCode`

This method implements hashing in .NET. We can override the GetHashCode method that is present on object for custom hashing.

Usually it is best to just leave the GetHashCode implementations alone. Sometimes using a unique string value within a class may be a good choice for GetHashCode.

An example

We can call GetHashCode on a string, but this usually doesn't do much good. This program shows how when we append a char to a string, the hash code keeps changing.

So The hash computation appears to be effective—no collisions are seen when we just append a character.

using System;

class Program
{
    static void Main()
    {
        string value = "";
        for (int i = 0; i < 10; i++)
        {
            value += "x";
            // This calls the optimized hash code implementation.
            Console.WriteLine("GETHASHCODE: " + value.GetHashCode());
        }
    }
}GETHASHCODE: -842352680
GETHASHCODE: -838682664
GETHASHCODE: -126710822
GETHASHCODE: 1988791258
GETHASHCODE: 847333320
GETHASHCODE: 844711880
GETHASHCODE: 1653319342
GETHASHCODE: -1698453842
GETHASHCODE: -795746332
GETHASHCODE: -803610652

Override

Suppose we have a class that contains a unique string value or some other unique value. We could override GetHashCode, and return a hash code based on that value.

Here Just to demonstrate the syntax, we override GetHashCode on a class, and always return 100.

Warning This implementation would make hashing slow because there would be many conflicting hash codes.

using System;
using System.Collections.Generic;

class BlogEntry
{
    public override int GetHashCode()
    {
        // Override the GetHashCode method that is defined on object,
        // which is the base class for all classes.
        return 100;
    }
}

class Program
{
    static void Main()
    {
        var blog = new BlogEntry();
        Console.WriteLine(blog.GetHashCode());

        // The dictionary uses GetHashCode for lookup and addition.
        var dict = new Dictionary<BlogEntry, bool>();
        dict[blog] = true;
    }
}
100

`IEqualityComparer`

This class can specify how a class should compare items and compute identity numbers for hash tables. We can customize the hash code based on specific data.

Summary

GetHashCode is called when we use a Hashtable or Dictionary. All objects implement GetHashCode, and this method can be overridden to change the hash code.