CODEWITHSUNDEEP
c#genericsdictionarygethashcode
Ed Guiness
Ed Guiness

Reputation: 35267

What must be done to use the value of a reference type as a dictionary key?

Suppose I have a class T that I want to use as a key in a Dictionary<T,U> collection.

What must I implement in T so that these keys are based on values of T rather than T references?

I'm hoping it's just GetHashCode().

Upvotes: 3

Views: 163

Answers (4)

driis
driis

Reputation: 164291

You must implement GetHashCode() and Equals().

Dictionary is a Hashtable below the covers, so you might want to read this: Pitfalls Of Equals/GetHashCode – How Does A Hash Table Work?

Upvotes: 5

Jon Skeet
Jon Skeet

Reputation: 1500505

Either implement Equals and GetHashCode or create an appropriate IEqualityComparer<T> which has the right form of equality matching for your map.

I rather like the IEqualityComparer<T> route: in many cases there isn't one obviously-right form of equality - you want to treat objects as equal in different ways depending on the situation. In that case, a custom equality comparer is just what you need. Of course, if there is a natural equality operation, it makes sense to implement IEquatable<T> in the type itself... if you can. (Another benefit of IEqualityComparer<T> is that you can implement it for types you have no control over.)

Upvotes: 3

Julien Lebosquain
Julien Lebosquain

Reputation: 41233

If you don't pass any IEqualityComparer<T> in the dictionary constructor, it will use EqualityComparer<T>.Default which is defined by MSDN as :

The Default property checks whether type T implements the System.IEquatable(Of T) interface and, if so, returns an EqualityComparer(Of T) that uses that implementation. Otherwise, it returns an EqualityComparer(Of T) that uses the overrides of Object.Equals and Object.GetHashCode provided by T.

So implementing IEquatable<T> would be my choice (if you implement it it also makes sense to override Equals and GetHashCode anyway).

Upvotes: 3

C. Ross
C. Ross

Reputation: 31848

You need to override Equals(object obj). It is always expected that you implement GetHashCode when you modify Equals. Read this article at MSDN.

Upvotes: 2

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