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POWhy isn't inlining a method equivalent to declaring it explicitly?
 

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    <h2>Do you have a blind spot in programming?</h2> <p>I mean is there a common technique or language feature that you can't really get used to. Well, I have one (or probably more than one) and mine is usage of <code>delegate</code>. Hands up! Who else doesn't feel comfortable with delegates? Be honest!</p> <h2>So what's a delegate?</h2> <p>Since my courses at university introduced me to C, I know about function pointers. Function pointers are handy if you want to pass methods as arguments. So in my mind a delegate is something like a function pointer. Eureka! I got it. I have not!</p> <h2>A concrete scenario?</h2> <p>I would like to remove any line from a text file that matches a <a href="http://en.wikipedia.org/wiki/Regular_expression" rel="nofollow noreferrer">regular expression</a>. Assuming I have a collection of lines, <code>List&lt;T&gt;</code> has method <code>RemoveAll</code> which seems to be perfectly suitable for that purpose. <code>RemoveAll</code> expects an evaluation method as argument for deciding on whether to remove or leave a list element. And there it is: The function pointer!</p> <h2>Any code here?</h2> <pre><code>public static int RemoveLinesFromFile(string path, string pattern) { List&lt;string&gt; lines = new List&lt;string&gt;(File.ReadAllLines(path)); int result = lines.RemoveAll(DoesLineMatch); File.WriteAllLines(path, lines.ToArray()); return result; } </code></pre> <p>So I'm looking for a function <code>DoesLineMatch</code> which evaluates if a line matches a pattern.</p> <h2>Do you see the problem?</h2> <p><code>RemoveAll</code> expects a delegate <code>Predicate&lt;string&gt; match</code> as argument. I would have coded it like this:</p> <pre><code>private static bool DoesLineMatch(string line, string pattern) { return Regex.IsMatch(line, pattern); } </code></pre> <p>But then I'm getting an error "Expected a method with 'bool DoesLineMatch(string)' signature". What am I missing here?</p> <h2>Does it work at all?</h2> <p>This is how I finally got it working:</p> <pre><code>public static int RemoveLinesFromFile(string path, string pattern) { List&lt;string&gt; lines = new List&lt;string&gt;(File.ReadAllLines(path)); int result = lines.RemoveAll(delegate(string line) { return Regex.IsMatch(line, pattern); }); File.WriteAllLines(path, lines.ToArray()); return result; } </code></pre> <p>I'm happy that it works but I don't understand it.</p> <h2>And what is the question?</h2> <p>What I did to get it working is simply inlining the method. As far as I understand inlining, it is just some kind of use-once-and-destroy-code. If you use a variable or method only once you may inline it, but inlining is always equivalent to declaring it explicitly.</p> <p><strong>Is there a way to declare the method explicitly? How would I do it?</strong></p> <p>PS.: Pardon me that my question is somewhat lengthy.</p> <p>PPS.: As soon as I get this delegate thing I will make the leap from 2.0 to 3.0 and learn lambdas.</p> <p>PPPS.: Following <a href="https://stackoverflow.com/questions/1508774/why-isnt-inlining-a-method-equivalent-to-declaring-it-explicitly/1508802#1508802">Jon's hint</a> on efficiency of <code>Regex.IsMatch(string, string)</code> I modified my code:</p> <pre><code> int result = lines.RemoveAll(delegate(string line) { Regex regex = new Regex(pattern); return regex.IsMatch(line); }); </code></pre> <p>That isn't of much help regarding efficiency matters. So I followed <a href="http://en.wikipedia.org/wiki/ReSharper" rel="nofollow noreferrer">ReSharper</a>'s proposal and moved the Regex instantiation to the outer scope:</p> <pre><code> Regex regex = new Regex(pattern); int result = lines.RemoveAll(delegate(string line) { return regex.IsMatch(line); }); </code></pre> <p>Now ReSharper urged me to replace this with a method group:</p> <pre><code> Regex regex = new Regex(pattern); int result = lines.RemoveAll(regex.IsMatch); </code></pre> <p>And that is quite similar to the answers proposed here. Not what I asked for, but again I'm amazed how ReSharper (and Stack Overflow of course) helps learning.</p>
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