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POChanged behavior of string.Empty (or System.String::Empty) in .NET 4.5
 

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    <p><strong>Short version:</strong></p> <p>The C# code</p> <pre><code>typeof(string).GetField("Empty").SetValue(null, "Hello world!"); Console.WriteLine(string.Empty); </code></pre> <p>when compiled and run, gives output <code>"Hello world!"</code> under .NET version 4.0 and earlier, but gives <code>""</code> under .NET 4.5 and .NET 4.5.1.</p> <p>How can a write to a field be ignored like that, or, who resets this field?</p> <p><strong>Longer version:</strong></p> <p>I have never really understood why the <code>string.Empty</code> field (also known as <code>[mscorlib]System.String::Empty</code>) is not <code>const</code> (aka. <code>literal</code>), see "<a href="https://stackoverflow.com/questions/507923/">Why isn&#39;t String.Empty a constant?</a>". This means that, for example, in C# we can't use <code>string.Empty</code> in the following situations:</p> <ul> <li>In a <code>switch</code> statement in the form <code>case string.Empty:</code></li> <li>As the default value of an optional parameter, like <code>void M(string x = string.Empty) { }</code></li> <li>When applying an attribute, like <code>[SomeAttribute(string.Empty)]</code></li> <li>Other situations where a compile-time constant is required</li> </ul> <p>which has implications to the well-known "religious war" over whether to use <code>string.Empty</code> or <code>""</code>, see "<a href="https://stackoverflow.com/questions/263191/">In C#, should I use string.Empty or String.Empty or &quot;&quot; to intitialize a string?</a>".</p> <p>A couple of years ago I amused myself by setting <code>Empty</code> to some other string instance through reflection, and see how many parts of the BCL started behaving strangely because of it. It was quite many. And the change of the <code>Empty</code> reference seemed to persist for the complete life of the application. Now, the other day I tried to repeat that little stunt, but then using a .NET 4.5 machine, and I couldn't do it anymore.</p> <p>(NB! If you have .NET 4.5 on your machine, probably your <code>PowerShell</code> still uses an older version of .NET, so try copy-pasting <code>[String].GetField("Empty").SetValue($null, "Hello world!")</code> into PowerShell to see some effects of changing this reference.)</p> <p>When I tried to search for a reason for this, I stumbled upon the interesting thread "<a href="https://stackoverflow.com/questions/11873798/">What&#39;s the cause of this FatalExecutionEngineError in .NET 4.5 beta?</a>". In the accepted answer to that question, is it noted that through version 4.0, <code>System.String</code> had a static constructor <code>.cctor</code> in which the field <code>Empty</code> was set (in the C# source, that would probably just be a field initializer, of course), while in 4.5 no static constructor exists. In both versions, the field itself looks the same:</p> <pre><code>.field public static initonly string Empty </code></pre> <p>(as seen with IL DASM).</p> <p>No other fields than <code>String::Empty</code> seems to be affected. As an example, I experimented with <code>System.Diagnostics.Debugger::DefaultCategory</code>. This case seems analogous: A sealed class containing a <code>static readonly</code> (<code>static initonly</code>) field of type <code>string</code>. But in this case it works fine to change the value (reference) through reflection.</p> <p><strong>Back to the question:</strong></p> <p>How is it possible, technically, that <code>Empty</code> doesn't seem to change (in 4.5) when I set the field? I have verified that the C# compiler does not "cheat" with the read, it outputs IL like:</p> <pre><code>ldsfld string [mscorlib]System.String::Empty </code></pre> <p>so the actual field ought to be read.</p> <hr> <p><strong>Edit after bounty was put on my question:</strong> Note that the write operation (which needs reflection for sure, since the field is <code>readonly</code> (a.k.a. <code>initonly</code> in the IL)) actually works as expected. It is the <strong><em>read</em></strong> operation which is anomalous. If you read with reflection, as in <code>typeof(string).GetField("Empty").GetValue(null)</code>, everything is normal (i.e. the change of value is seen). See comments below.</p> <p>So the better question is: Why does this new version of the framework cheat when it reads this particular field?</p>
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    1. USJeppe Stig Nielsen
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    1. COIt could also be interesting to hear if this is the same with Mono.
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      1. USJeppe Stig Nielsen
    2. COI think this is more suited to [Programmers](http://programmers.stackexchange.com).
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    3. COWhile the compiler isn't cheating, there's a decent chance the CLR *is*.
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