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POIs this object-lifetime-extending-closure a C# compiler bug?
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  1. POIs this object-lifetime-extending-closure a C# compiler bug?
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    copied!<p>I was answering a <a href="https://stackoverflow.com/questions/8417470/private-field-captured-in-anonymous-delegate">question</a> about the possibility of closures (legitimately) extending object-lifetimes when I ran into some <em>extremely</em> curious code-gen on the part of the C# compiler (4.0 if that matters). </p> <p>The shortest repro I can find is the following:</p> <ol> <li>Create a lambda that captures a local while calling a <em>static</em> method of the containing type.</li> <li>Assign the generated delegate-reference to an <em>instance</em> field of the containing object.</li> </ol> <p>Result: The compiler creates a closure-object that references the object that created the lambda, when it has no reason to - the 'inner' target of the delegate is a <em>static</em> method, and the lambda-creating-object's instance members needn't be (and aren't) touched when the delegate is executed. Effectively, the compiler is acting like the programmer has captured <code>this</code> without reason.</p> <pre><code>class Foo { private Action _field; public void InstanceMethod() { var capturedVariable = Math.Pow(42, 1); _field = () =&gt; StaticMethod(capturedVariable); } private static void StaticMethod(double arg) { } } </code></pre> <p>The generated code from a release build (decompiled to 'simpler' C#) looks like this:</p> <pre><code>public void InstanceMethod() { &lt;&gt;c__DisplayClass1 CS$&lt;&gt;8__locals2 = new &lt;&gt;c__DisplayClass1(); CS$&lt;&gt;8__locals2.&lt;&gt;4__this = this; // What's this doing here? CS$&lt;&gt;8__locals2.capturedVariable = Math.Pow(42.0, 1.0); this._field = new Action(CS$&lt;&gt;8__locals2.&lt;InstanceMethod&gt;b__0); } [CompilerGenerated] private sealed class &lt;&gt;c__DisplayClass1 { // Fields public Foo &lt;&gt;4__this; // Never read, only written to. public double capturedVariable; // Methods public void &lt;InstanceMethod&gt;b__0() { Foo.StaticMethod(this.capturedVariable); } } </code></pre> <p>Observe that <code>&lt;&gt;4__this</code> field of the closure object is populated with an object reference but is never read from (there is no reason).</p> <p>So what's going on here? Does the language-specification allow for it? Is this a compiler bug / oddity or is there a good reason (that I'm clearly missing) for the closure to reference the object? This makes me anxious because this looks like a recipe for closure-happy programmers (like me) to unwittingly introduce strange memory-leaks (imagine if the delegate were used as an event-handler) into programs. </p>
 

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