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POEqualityComparer<T>.Default isn't clever enough
 

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  1. POEqualityComparer<T>.Default isn't clever enough
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    <p>I was reading the source code of <code>EqualityComparer&lt;T&gt;.Default</code> and found that it's not so clever. Here is an example:</p> <pre><code>enum MyEnum : int { A, B } EqualityComparer&lt;MyEnum&gt;.Default.Equals(MyEnum.A, MyEnum.B) //is as fast as EqualityComparer&lt;int&gt;.Default.Equals(0, 1) enum AnotherEnum : long { A = 1L, B = 2L } //is 8x slower than EqualityComparer&lt;long&gt;.Default.Equals(1L, 2L) </code></pre> <p>The reason is obvious from the source code of the private method in EqualityComparer.</p> <pre><code>private static EqualityComparer&lt;T&gt; CreateComparer() { //non-important codes are ignored if (c.IsEnum &amp;&amp; (Enum.GetUnderlyingType(c) == typeof(int))) { return (EqualityComparer&lt;T&gt;) RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType) typeof(EnumEqualityComparer&lt;int&gt;), c); } return new ObjectEqualityComparer&lt;T&gt;(); } </code></pre> <p>We can see <code>EqualityComparer&lt;int&gt;.Default</code>,<code>EqualityComparer&lt;MyEnum&gt;.Default</code> and <code>EqualityComparer&lt;long&gt;.Default</code> get a wise comparer whose <code>Equals</code> method looks like:</p> <pre><code>public static bool Equals(int x, int y) { return x == y; //or return x.Equals(y); here //I'm not sure, but neither causes boxing } public static bool Equals(MyEnum x, MyEnum y) { return x == y; //it's impossible to use x.Equals(y) here //because that causes boxing } </code></pre> <p>The above two are <em>clever</em>, but <code>EqualityComparer&lt;AnotherEnum&gt;.Default</code> is unlucky, from the method we can see at last it gets a <code>ObjectEqualityComparer&lt;T&gt;()</code>, whose <code>Equals</code> method probably looks like:</p> <pre><code>public static bool Equals(AnotherEnum x, AnotherEnum y) { return x.Equals(y); //too bad, the Equals method is from System.Object //and it's not override, boxing here! //that's why it's so slow } </code></pre> <p>I think this condition <code>Enum.GetUnderlyingType(c) == typeof(int)</code> is pointless, if the underlying type of an enum is of type int, the method can <em>convert</em> the default comparer of int to this enum. But why can't an enum based on long? It's not so hard i think? Any special reason? Constructing a comparer like <code>x == y</code> isn't so hard for enum, right? Why at last it gives a slow <code>ObjectEqualityComparer&lt;T&gt;</code> for enums(even it works correctly)?</p>
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    1. COas a side note it is possible to write your own which does the right thing. But I haven't seen a way to do it without either code generation or C++/CLI. I couldn't find a way, even with unsafe{} to allow reinterpret casts via void* in c# with generic types.
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    2. COI just published a small library on GitHub that lets you make the default equality comparer as clever as you want: https://github.com/smartcaveman/undefault
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