Question

If I want to use objects as the keys for a Dictionary, what methods will I need to override to make them compare in a specific way?

Say I have a a class which has properties:

class Foo {     public string Name { get; set; }     public int FooID { get; set; }      // elided }  

And I want to create a:

Dictionary<Foo, List<Stuff>> 

I want Foo objects with the same FooID to be considered the same group. Which methods will I need to override in the Foo class?

To summarize: I want to categorize Stuff objects into lists, grouped by Foo objects. Stuff objects will have a FooID to link them to their category.

Answer 1

By default, the two important methods are GetHashCode() and Equals(). It is important that if two things are equal (Equals() returns true), that they have the same hash-code. For example, you might ‘return FooID;’ as the GetHashCode() if you want that as the match. You can also implement IEquatable<Foo>, but that is optional:

class Foo : IEquatable<Foo> {     public string Name { get; set;}     public int FooID {get; set;}      public override int GetHashCode() {         return FooID;     }     public override bool Equals(object obj) {         return Equals(obj as Foo);     }     public bool Equals(Foo obj) {         return obj != null && obj.FooID == this.FooID;     } } 

Finally, another alternative is to provide an IEqualityComparer<T> to do the same.