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POIs there a reason for C#'s reuse of the variable in a foreach?
 

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  1. POIs there a reason for C#'s reuse of the variable in a foreach?
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    <p>When using lambda expressions or anonymous methods in C#, we have to be wary of the <em>access to modified closure</em> pitfall. For example:</p> <pre><code>foreach (var s in strings) { query = query.Where(i =&gt; i.Prop == s); // access to modified closure ... } </code></pre> <p>Due to the modified closure, the above code will cause all of the <code>Where</code> clauses on the query to be based on the final value of <code>s</code>.</p> <p>As explained <a href="http://resnikb.wordpress.com/2009/06/17/c-lambda-and-foreach-variable/" rel="noreferrer">here</a>, this happens because the <code>s</code> variable declared in <code>foreach</code> loop above is translated like this in the compiler:</p> <pre><code>string s; while (enumerator.MoveNext()) { s = enumerator.Current; ... } </code></pre> <p>instead of like this:</p> <pre><code>while (enumerator.MoveNext()) { string s; s = enumerator.Current; ... } </code></pre> <p>As pointed out <a href="https://stackoverflow.com/a/2388644/120955">here</a>, there are no performance advantages to declaring a variable outside the loop, and under normal circumstances the only reason I can think of for doing this is if you plan to use the variable outside the scope of the loop:</p> <pre><code>string s; while (enumerator.MoveNext()) { s = enumerator.Current; ... } var finalString = s; </code></pre> <p>However variables defined in a <code>foreach</code> loop cannot be used outside the loop:</p> <pre><code>foreach(string s in strings) { } var finalString = s; // won't work: you're outside the scope. </code></pre> <p>So the compiler declares the variable in a way that makes it highly prone to an error that is often difficult to find and debug, while producing no perceivable benefits.</p> <p>Is there something you can do with <code>foreach</code> loops this way that you couldn't if they were compiled with an inner-scoped variable, or is this just an arbitrary choice that was made before anonymous methods and lambda expressions were available or common, and which hasn't been revised since then?</p>
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    1. COWhat's wrong with `String s; foreach (s in strings) { ... }`?
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    2. CO@BradChristie the OP is not really talking about `foreach`but about lamda expressions resulting in similar code as shown by the OP...
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    3. CO@BradChristie: Does that compile? (_Error: Type and identifier are both required in a foreach statement_ for me)
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      1. USAustin Salonen
 

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