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    <p>Compile the following code.</p> <pre><code>public static int Main(string[] args) { switch (args[0]) { case "x": return 1; case "y": return 2; case "z": return 3; } return 0; } </code></pre> <p>Now use <a href="http://www.red-gate.com/products/reflector/" rel="noreferrer">Reflector</a> or <a href="http://msdn.microsoft.com/en-us/library/f7dy01k1(VS.80).aspx" rel="noreferrer">ILDASM</a> to examine the IL the C# compiler generates. Keep adding case statements and decompiling and observe the result.</p> <ul> <li>If the number of case statements is small then the compiler emits a sequential equality comparison.</li> <li>If the number of case statements is large then the compiler emits a <code>Dictionary</code> lookup.</li> </ul> <p>I was using the C# 3.0 compiler and I observed that the strategy changes at 7 case statements. I suspect you will see something similiar with C# 4.0 and others.</p> <p><strong>Update:</strong></p> <p>I should point that you will see calls to <code>Dictionary.Add</code> in the IL output where it is building up the dictionary for later use. Do not be fooled into thinking this happens everytime. The compiler is actually generating a separate static class and doing an inline static initialization of it. Pay particular attention to the instruction at L_0026. If the class is already initialized then the branch will skip over the <code>Add</code> calls.</p> <pre><code>L_0021: ldsfld class [mscorlib]System.Collections.Generic.Dictionary`2&lt;string, int32&gt; &lt;PrivateImplementationDetails&gt;{816396DD-F271-4C12-83D0-CC9C9CD67AD6}::$$method0x6000001-1 L_0026: brtrue.s L_0089 L_0028: ldc.i4.7 L_0029: newobj instance void [mscorlib]System.Collections.Generic.Dictionary`2&lt;string, int32&gt;::.ctor(int32) L_002e: dup L_002f: ldstr "x" L_0034: ldc.i4.0 L_0035: call instance void [mscorlib]System.Collections.Generic.Dictionary`2&lt;string, int32&gt;::Add(!0, !1) L_003a: dup L_003b: ldstr "y" L_0040: ldc.i4.1 L_0041: call instance void [mscorlib]System.Collections.Generic.Dictionary`2&lt;string, int32&gt;::Add(!0, !1) L_0046: dup L_0047: ldstr "z" L_004c: ldc.i4.2 L_004d: call instance void [mscorlib]System.Collections.Generic.Dictionary`2&lt;string, int32&gt;::Add(!0, !1) </code></pre> <p>Also, notice that the dictionary actually contains a map from the original string to an integer. This integer is used to formulate a separate switch in IL.</p> <pre><code>L_0089: volatile. L_008b: ldsfld class [mscorlib]System.Collections.Generic.Dictionary`2&lt;string, int32&gt; &lt;PrivateImplementationDetails&gt;{816396DD-F271-4C12-83D0-CC9C9CD67AD6}::$$method0x6000001-1 L_0090: ldloc.2 L_0091: ldloca.s CS$0$0002 L_0093: call instance bool [mscorlib]System.Collections.Generic.Dictionary`2&lt;string, int32&gt;::TryGetValue(!0, !1&amp;) L_0098: brfalse.s L_00da L_009a: ldloc.3 L_009b: switch (L_00be, L_00c2, L_00c6, L_00ca, L_00ce, L_00d2, L_00d6) L_00bc: br.s L_00da L_00be: ldc.i4.1 L_00bf: stloc.1 L_00c0: br.s L_00de L_00c2: ldc.i4.2 L_00c3: stloc.1 L_00c4: br.s L_00de L_00c6: ldc.i4.3 </code></pre> <p><strong>Update 2:</strong></p> <p>For what it is worth VB.NET does not seem to have this same optimization for its <code>Select</code> construct.</p>
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    1. USBrian Gideon
    1. USBrian Gideon
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    1. POAre .Net switch statements hashed or indexed?
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    1. COI remember reading an old article (.Net 1) about this optimization. It also has something to do using the IsInterned statement, and using reference comparison. But I seem unable to find the article.
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    2. COFYI CSC 4.0 seems to switch over at 7 cases too (though one should obviously never rely on this!)
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      1. USRuben Bartelink
    3. CO+1. About update 2: C# `case` statements must be constant expressions, so the C# compiler can use this cached Dictionary approach . VB.Net `Case` statements can be expressions, so the VB compiler *can't* use that strategy in the general case. (@Brian a duplicate of my comment on your other answer on this topic!)
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