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    <h2>It's more complicated than that though!</h2> <p>I was having related problem (writing front-end .exe that will call the back-end with all parameters passed + some extra ones) and so i looked how people do that, ran into your question. Initially all seemed good doing it as you suggest <code>arg.Replace (@"\", @"\\").Replace(quote, @"\"+quote)</code>.</p> <p>However when i call with arguments <code>c:\temp a\\b</code>, this gets passed as <code>c:\temp</code> and <code>a\\b</code>, which leads to the back-end being called with <code>"c:\\temp" "a\\\\b"</code> - which is incorrect, because there that will be two arguments <code>c:\\temp</code> and <code>a\\\\b</code> - not what we wanted! We have been overzealous in escapes (windows is not unix!).</p> <p>And so i read in detail <a href="http://msdn.microsoft.com/en-us/library/system.environment.getcommandlineargs.aspx" rel="noreferrer">http://msdn.microsoft.com/en-us/library/system.environment.getcommandlineargs.aspx</a> and it actually describes there how those cases are handled: backslashes are treated as escape <strong>only</strong> in front of double quote. </p> <p>There is a twist to it in how multiple <code>\</code> are handled there, the explanation can leave one dizzy for a while. I'll try to re-phrase said unescape rule here: say we have a substring of <strong><em>N</em></strong> <code>\</code>, followed by <code>"</code>. When unescaping, we replace that substring with <strong><em>int(N/2)</em></strong> <code>\</code> and iff <strong><em>N</em></strong> was odd, we add <code>"</code> at the end.</p> <p>The encoding for such decoding would go like that: for an argument, find each substring of 0-or-more <code>\</code> followed by <code>"</code> and replace it by twice-as-many <code>\</code>, followed by <code>\"</code>. Which we can do like so:</p> <pre><code>s = Regex.Replace(arg, @"(\\*)" + "\"", @"$1$1\" + "\""); </code></pre> <p>That's all...</p> <p>PS. ... <strong>not</strong>. Wait, wait - there is more! :)</p> <p>We did the encoding correctly but there is a twist because you are enclosing all parameters in double-quotes (in case there are spaces in some of them). There is a boundary issue - in case a parameter ends on <code>\</code>, adding <code>"</code> after it will break the meaning of closing quote. Example <code>c:\one\ two</code> parsed to <code>c:\one\</code> and <code>two</code> then will be re-assembled to <code>"c:\one\" "two"</code> that will me (mis)understood as one argument <code>c:\one" two</code> (I tried that, i am not making it up). So what we need in addition is to check if argument ends on <code>\</code> and if so, <em>double</em> the number of backslashes at the end, like so:</p> <pre><code>s = "\"" + Regex.Replace(s, @"(\\+)$", @"$1$1") + "\""; </code></pre>
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    1. CO+1 for explaining this insanity. However shouldn't the `*` and the `+` be *inside* the grouping parentheses in the above match expressions? Otherwise the `$1` replacement will only ever be a single backslash.
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    2. COActually I *think* the two replacements can be combined into: `"\""+Regex.Replace(s, "(\\\\*)(\\\\$|\")", "$1$1\\$2")+"\""`. However my brain is beginning to sink now so appreciated if you could check correctness :-)
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    3. COFYI: http://blogs.msdn.com/b/twistylittlepassagesallalike/archive/2011/04/23/everyone-quotes-arguments-the-wrong-way.aspx
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