Archive for the '.net' Category

Microsoft recommends if you overload Equals method you should also overload GetHashCode. Now, how to properly implement GetHashCode? There are many resources on the web that describe it. A good starting point might be this article on Stack Overflow.

Following MSDN guidlines GetHashCode must fulfill these requirements:

• If two objects of the same type represent the same value, the hash function must return the same constant value for either object.
• For the best performance, a hash function must generate a random distribution for all input.
• The hash function must return exactly the same value regardless of any changes that are made to the object.

Sticking to first bullet, you (probably?) should consider the same fields in Equals and GetHashCode methods. Let’s have a look at the example in which I did so:

public class Contact
{
    public int ID { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public override bool Equals(object obj)
    {
        // If parameter is null return false.
        if (obj == null)
        {
            return false;
        }

        // If parameter cannot be cast to Contact return false.
        Contact c = obj as Contact;
        if c == null)
        {
            return false;
        }

        // Return true if the fields match:
        return ID == c.ID
            && FirstName == c.FirstName
            && LastName == c.LastName;;
    }

    public override int GetHashCode()
    {
        return ID.GetHashCode()
            + FirstName.GetHashCode()
            + LastName.GetHashCode();
    }
}

Now, what is wrong with this example of GetHashCode? There’s one drawback here. The hash is calculated as a sum of three integer values, which might give a value that is greater than int.MaxValue and that will result in OverflowException.

Continue reading ‘C#: GetHashCode() might cause OverflowException’