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The Black Box of .NET

Thursday, May 17, 2012

Don't implement GetHashCode() on mutable fields/properties

CodeProject You shouldn't ever implement GetHashCode on mutable properties (properties that could be changed by someone) - i.e. non-private setters.   I've seen this done in several places and it results in very difficult to find bugs.

Here's why - imagine this scenario:
  1. You put an instance of your object in a collection which uses GetHashCode() "under the covers" or directly (Hashtable).
  2. Then someone changes the value of the field/property that you've used in your GetHashCode() implementation.
Guess what...your object is permanently lost in the collection since the collection uses GetHashCode() to find it! You've effectively changed the hashcode value from what was originally placed in the collection. Probably not what you wanted.

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