CODEWITHSUNDEEP
c#dictionaryequals
Masoud
Masoud

Reputation: 1405

Dictionary class in C# - Equality of two object

I have a class named Class1 I override its Equals function Now I have an instance of Dictionary And I added an instance of Class1 named OBJ1 to it. I have another instance of Class1 named OBJ2. the code returns true for OBJ1.Equals(OBJ2). But I can't find OBJ2 in dictionary.

Here is pseudo code

Class1 OBJ1 = new Class1(x, y, z);
Class1 OBJ2 = new Class1(a, b, c);
Dictionary<Class1, int> dic1 = new Dictionary<Class1, int>();
dic1.Add(OBJ1, 3);
OBJ1.Equals(OBJ2) -------------> return true
Dictionary.ContainsKey(OBJ2) --------------> return false

why is this happening? any help would be highly welcomed

Upvotes: 4

Views: 5510

Answers (7)

Ani
Ani

Reputation: 113472

2 possibilities:

  1. GetHashCode has not been overridden correctly. You might want to take a look at Why is it important to override GetHashCode when Equals method is overriden in C#?
  2. OBJ1 has been mutated after it has been added to the dictionary in a way that impacts its hashcode. In this case, the bucket it is placed in will no longer be correct - ContainsKey will end up hunting for it in a different bucket.

From Dictionary<TKey, TValue>:

As long as an object is used as a key in the Dictionary, it must not change in any way that affects its hash value.

Upvotes: 13

ThatBlairGuy
ThatBlairGuy

Reputation: 2462

Make certain Class1 overrides GetHashCode(). The return from that method is the first thing checked when comparing equality. The default implementation is unique for each object.

Upvotes: 3

Daniel DiPaolo
Daniel DiPaolo

Reputation: 56428

You need to override GetHashCode as well, but also don't forget that you may need to pass in a custom Comparer to the Dictionary constructor as well as pointed out in this SO question

Upvotes: 3

Jahan Zinedine
Jahan Zinedine

Reputation: 14874

Did you override the GetHashCode either?

Upvotes: 3

BrokenGlass
BrokenGlass

Reputation: 161012

You probably did not override GetHashcode in your class. When you override Equals you must override GetHashcode as well, else Dictionary won't work for you.

Upvotes: 4

Frederik Gheysels
Frederik Gheysels

Reputation: 56984

Did you override GetHashCode as well ? Can you display the implementation of the Equals method ?

Upvotes: 3

Jon Skeet
Jon Skeet

Reputation: 1503954

Chances are you haven't overridden GetHashCode in a manner consistent with Equals.

The contract of GetHashCode requires that if OBJ1.Equals(OBJ2) returns true, then OBJ1.GetHashCode() must return the same value as OBJ2.GetHashCode().

IIRC, you'll get a compiler error (or at least a warning) if you override Equals without overriding GetHashCode().

Another possibility is that you haven't actually overridden Equals, but overloaded it by adding a new signature, e.g.

public bool Equals(Class1 other)

In general, to provide a "natural" value equality comparison you should:

  • Override Equals(object)
  • Override GetHashCode
  • Strongly consider implementing IEquatable<T>
  • Consider overloading == and !=

Upvotes: 12

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