Overriding GetHashCode [duplicate]

Question

This question already has answers here:

What is the best algorithm for overriding GetHashCode? (19 answers)

Closed 2 years ago.

As you know, GetHashCode returns a semi-unique value that can be used to identify an object instance in a collection. As a good practice, it is recommended to override this method and implement your own.

My question is - do you override this method when working on custom objects? If so, what algorithm do you use to generate the unique ID?

I was thinking about generating a GUID and then getting integer data from that identificator.

Answer 1

I would normally override hashcode and equality checking methods for data classes (i.e. classes where the value semantics makes sense). Have a look at this question for a common implementation. If you do override hashcode override equals. Using a GUID is a pretty terrible idea because you want two objects which are different instances but have the same value to have the same hashcode and for equals to return true.

Answer 2

you only need to override GetHashCode if you are overriding Equals. The default GetHashCode is implemented by the runtime in a similar way you wanted to do it - every object has a hidden field assigned by the runtime.

How to override GetHashCode

Actually your IDE should do this for you - when you type "override GetHashCode" the IDE should generate this boilerplate code. Visual Studio does not do it but SharpDevelop does.

Answer 3

If you use resharper it can generate the GetHashCode(), Equals and operator method bodies for you.

Access this menu by pressing Alt+Insert.

http://www.jetbrains.com/resharper/webhelp/Code_Generation__Equality_Members.html

Answer 4

When you override GetHashCode() you also need to override Equals(), operator== and operator!= . And be very careful to meet all the requirements for those methods.

The guidelines are here on MSDN. Most important quote:

It is not a good idea to override operator == in non-immutable types.

Answer 5

Generally I use the aggregated GetHashCode from the component properties of the class. E.g.

public class Test
{
  public string Text { get; set; }
  public int Age { get; set; }

  public override GetHashCode()
  {
    int result = 
      string.IsNullOrEmpty(Text) ? 0 : Text.GetHashCode()
      + Age.GetHashCode();

    return result;
  }
}

Answer 6

In my personal usage, I only override when overriding equals method too. Generally, I do this for objects I know that I might run a LINQ to Objects query on, or some other comparison operation.

I usually return, if say a LINQ to SQL entity or DTO object, the primary key value. Whatever you return, if you don't store the value locally, it may produce an unexpected result.

HTH.