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    <p>It's not necessarily important; it depends on the size of your collections and your performance requirements and whether your class will be used in a library where you may not know the performance requirements. I frequently know my collection sizes are not very large and my time is more valuable than a few microseconds of performance gained by creating a perfect hash code; so (to get rid of the annoying warning by the compiler) I simply use:</p> <pre><code> public override int GetHashCode() { return base.GetHashCode(); } </code></pre> <p>(Of course I could use a #pragma to turn off the warning as well but I prefer this way.)</p> <p>When you are in the position that you <em>do</em> need the performance than all of the issues mentioned by others here apply, of course. <strong>Most important</strong> - otherwise you will get wrong results when retrieving items from a hash set or dictionary: <strong>the hash code must not vary with the life time of an object</strong> (more accurately, during the time whenever the hash code is needed, such as while being a key in a dictionary): for example, the following is wrong as Value is public and so can be changed externally to the class during the life time of the instance, so you must not use it as the basis for the hash code:</p> <pre><code> class A { public int Value; public override int GetHashCode() { return Value.GetHashCode(); //WRONG! Value is not constant during the instance's life time } } </code></pre> <p>On the other hand, if Value can't be changed it's ok to use:</p> <pre><code> class A { public readonly int Value; public override int GetHashCode() { return Value.GetHashCode(); //OK Value is read-only and can't be changed during the instance's life time } } </code></pre>
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    1. POWhy is it important to override GetHashCode when Equals method is overridden?
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    1. USILoveFortran
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    1. CODownvoted. This is plain wrong. Even Microsoft states in MSDN (http://msdn.microsoft.com/en-us/library/system.object.gethashcode.aspx) that the value of GetHashCode MUST change when the object state changes in a way that may affect the return value of a call to Equals(), and even in its examples it also shows GetHashCode implementations that fully depend on publicly changeable values.
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      1. USSebastian P.R. Gingter
    2. COSebastian, I disagree: If you add an object to a collection that uses hash codes it will be put in a bin dependent on the hash code. If you now change the hash code you will not find the object again in the collection as the wrong bin will be searched. This is, in fact, something that has happened in our code and that's why I found it necessary to point that out.
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      1. USILoveFortran
    3. COSebastian, In addition, I cannot see a statement in the link (http://msdn.microsoft.com/en-us/library/system.object.gethashcode.aspx) that GetHashCode() must change. In contrary - it must NOT change as long as Equals returns the same value for the same argument: "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. " This statement does not imply the opposite, that it must change if the return value for Equals changes.
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