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copied!<p>Yes, it is important if your item will be used as a key in a dictionary, or <code>HashSet<T></code>, etc - since this is used (in the absence of a custom <code>IEqualityComparer<T></code>) to group items into buckets. If the hash-code for two items does not match, they may <em>never</em> be considered equal (<code>Equals</code> will simply never be called).</p> <p>The <code>GetHashCode()</code> method should reflect the <code>Equals</code> logic; the rules are:</p> <ul> <li>if two things are equal (<code>Equals(...) == true</code>) then they <em>must</em> return the same value for <code>GetHashCode()</code></li> <li>if the <code>GetHashCode()</code> is equal, it is <em>not</em> necessary for them to be the same; this is a collision, and <code>Equals</code> will be called to see if it is a real equality or not.</li> </ul> <p>In this case, it looks like "<code>return FooId;</code>" is a suitable <code>GetHashCode()</code> implementation. If you are testing multiple properties, it is common to combine them using code like below, to reduce diagonal collisions (i.e. so that <code>new Foo(3,5)</code> has a different hash-code to <code>new Foo(5,3)</code>):</p> <pre><code>int hash = 13; hash = (hash * 7) + field1.GetHashCode(); hash = (hash * 7) + field2.GetHashCode(); ... return hash; </code></pre> <p>Oh - for convenience, you might also consider providing <code>==</code> and <code>!=</code> operators when overriding <code>Equals</code> and <code>GetHashCode</code>.</p> <hr> <p>A demonstration of what happens when you get this wrong is <a href="https://stackoverflow.com/questions/638761/c-gethashcode-override-of-object-containing-generic-array/639098#639098">here</a>.</p>

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