What is the purpose of the Gethashcode() method?

Question

I’m using Resharper to automate the method override Equals and the operator ==. One of the methods the tool has overwritten is GetHashCode:

public override int GetHashCode()
{
    unchecked
    {
        return (Id.GetHashCode() * 397) ^ (Name != null ? Name.GetHashCode() : 0);
     }
}

What is the function of this method and what is its role in comparing objects in the .NET Framework? What is the function of the reserved word unchecked? Why multiply the value of HashCode of property Id by 397? (The project is configured for the version 4.5.1 of .NET Framework).

Complete code of the class:

public class Class1 : IEquatable<Class1>
{
    public Guid Id { get; set; }
    public string Name { get; set; }

    public bool Equals(Class1 other)
    {
        if (ReferenceEquals(null, other)) return false;
        if (ReferenceEquals(this, other)) return true;
        return Id.Equals(other.Id) && string.Equals(Name, other.Name);
    }

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj)) return false;
        if (ReferenceEquals(this, obj)) return true;
        if (obj.GetType() != this.GetType()) return false;
        return Equals((Class1) obj);
    }

    public override int GetHashCode()
    {
        unchecked
        {
            return (Id.GetHashCode() * 397) ^ (Name != null ? Name.GetHashCode() : 0);
        }
    }

    public static bool operator ==(Class1 left, Class1 right)
    {
        return Equals(left, right);
    }

    public static bool operator !=(Class1 left, Class1 right)
    {
        return !Equals(left, right);
    }
}

Answer 1

Most of the question has already been answered in What is Hashcode and what is its purpose?. It says it’s mainly for use in tables hash where the most prominent type is the Dictionary.

You need a code that avoids too many collisions, so you need to use a number that is easy to generate different codes. A prime number is quite obvious, it cannot be too low, nor too large to have a good distribution.

There are controversies of the ideal number or if only a simple form using multiplication and xor are sufficient. On the other hand if you make a very complicated formula, in addition to being able to slow down you may end up having the opposite effect.

The algorithm must consider all members that form the identity of the object and it is frequent to use the hash code of the members to form the whole object code.

Some people say it was a mistake to require every object to be hashable, and think there should be an interface that allows this where it makes sense.

See about the unchecked.

using System;
using static System.Console;
using System.Collections.Generic;

public class Program {
    public static void Main() {
        var dict = new Dictionary<Class1, int> { { new Class1 { Id = new Guid(), Name = "João" }, 0 }};
        foreach (var item in dict) WriteLine(item.Key.Name);
    }
}

public class Class1 : IEquatable<Class1> {
    public Guid Id { get; set; }
    public string Name { get; set; }

    public bool Equals(Class1 other) {
        if (ReferenceEquals(null, other)) return false;
        if (ReferenceEquals(this, other)) return true;
        return Id.Equals(other.Id) && string.Equals(Name, other.Name);
    }

    public override bool Equals(object obj) {
        if (ReferenceEquals(null, obj)) return false;
        if (ReferenceEquals(this, obj)) return true;
        if (obj.GetType() != this.GetType()) return false;
        return Equals((Class1) obj);
    }

    public override int GetHashCode() {
        unchecked {
            int hash = (int) 2166136261;
            hash = (hash * 16777619) ^ Id.GetHashCode();
            return hash = (hash * 16777619) ^ (Name != null ? Name.GetHashCode() : 0);
        }
    }

    public static bool operator ==(Class1 left, Class1 right) => Equals(left, right);

    public static bool operator !=(Class1 left, Class1 right) => !Equals(left, right);
}

Behold working in the ideone. And in the .NET Fiddle. Also put on the Github for future reference.

Jon Skeet’s response with an example.

Answer 2

The method GetHashCode() is used to compare object values, unlike the method Equals() inherited and unclassified from the Object class that compares entities by reference (compares the memory address of two objects).

The same code of hash can be generated for two different objects, although it rarely occurs. Mr Maniero’s reply explains the use of 397 in general in order to avoid frequent collisions.

The use of keyword unchecked ignores any exception generated by arithmetic overflow (the result of this multiplication could potentially exceed memory limits according to the type resulting from the operation).