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Code Inspection: Non-readonly type member referenced in 'GetHashCode()'

MSDN documentation of the GetHashCode() method does not explicitly require that your override of this method returns a value that never changes during the object's lifetime. Specifically, it says:

The GetHashCode method for an object must consistently return the same hash code as long as there is no modification to the object state that determines the return value of the object's Equals method.

On the other hand, it says that the hash code should not change at least when your object is in a collection:

You can override GetHashCode for immutable reference types. In general, for mutable reference types, you should override GetHashCode only if:

  • You can compute the hash code from fields that are not mutable; or
  • You can ensure that the hash code of a mutable object does not change while the object is contained in a collection that relies on its hash code.

But why do you need to override GetHashCode() in the first place? Normally, you will do it if your object is going to be used in a Hashtable, as a key in a dictionary, etc., and it's quite hard to predict when your object will be added to a collection and how long it will be kept there.

With all that said, if you want to be on the safe side make sure that your override of GetHashCode() returns the same value during the object's lifetime. ReSharper will help you here by pointing at each non-readonly field or non-get-only property in your implementation of GetHashCode(). If possible, ReSharper will also suggest quick-fixes to make these members read-only/get-only.

See Also

Last modified: 3 April 2017