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    <p>There's no extra IL code for the <code>var</code> keyword: the resulting IL should be identical for non-anonymous types. If the compiler can't create that IL because it can't figure out what type you intended to use, you'll get a compiler error.</p> <p>The only trick is that <code>var</code> will infer an exact type where you may have chosen an Interface or parent type if you were to set the type manually.</p> <hr> <h3>Update 8 Years Later</h3> <p>I need to update this as my understanding has changed. I now believe it may be possible for <code>var</code> to affect performance in the situation where a method returns an interface, but you would have used an exact type. For example, if you have this method:</p> <pre><code>IList&lt;int&gt; Foo() { return Enumerable.Range(0,10).ToList(); } </code></pre> <p>Consider these three lines of code to call the method: </p> <pre><code>List&lt;int&gt; bar1 = Foo(); IList&lt;int&gt; bar = Foo(); var bar3 = Foo(); </code></pre> <p>All three compile and execute as expected. However, the first two lines are <em>not</em> exactly the same, and the third line will match the second, rather than the first. Because the signature of <code>Foo()</code> is to return an <code>IList&lt;int&gt;</code>, that is how the compiler will build the <code>bar3</code> variable.</p> <p>From a performance standpoint, mostly you won't notice. However, there are situations where <a href="https://stackoverflow.com/questions/7225205/performance-of-direct-virtual-call-vs-interface-call-in-c-sharp">the performance of the third line may not be quite as fast as the performance of the first</a>. As you continue to use the <code>bar3</code> variable, the compiler may not be able to dispatch method calls the same way. </p> <p>Note that it's possible (likely even) the jitter will be able to erase this difference, but it's not guaranteed. <strong>Generally, you should still consider <code>var</code> to be a non-factor in terms of performance.</strong> It's certainly not at all like using a <code>dynamic</code> variable. But to say it never makes a difference at all may be overstating it.</p>
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    1. POWill using 'var' affect performance?
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    1. USJoel Coehoorn
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    1. CONot only should the IL be identical - it *is* identical. var i = 42; compiles to exactly the same code as int i = 42;
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      1. USBrian Rasmussen
    2. CONote that I used var in the IDE after compile and use reflector to extract the code. We get this. string fileName = Console.ReadLine(); string input = Console.ReadLine();
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      1. USAthiwat Chunlakhan
    3. CO@BrianRasmussen: I know your post is old is old, but I assume `var i = 42;` (infers type is int) is NOT identical to `long i = 42;`. So in some cases you may be making incorrect assumptions about the type inference. This could cause elusive/edge case runtime errors if the value doesn't fit. For that reason, it may still be a good idea to be explicit when the value doesn't have an explicit type. So for example, `var x = new List<List<Dictionary<int, string>()>()>()` would be acceptable, but `var x = 42` is somewhat ambiguous and should be written as `int x = 42`. But to each their own...
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      1. USNelson Rothermel
 

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