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POCode Contracts warn of "ensures unproven" when locks are involved
 

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  1. POCode Contracts warn of "ensures unproven" when locks are involved
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    <p>I'm trying to work out how .NET Code Contracts interact with the <code>lock</code> keyword, using the following example:</p> <pre class="lang-cs prettyprint-override"><code>public class TestClass { private object o1 = new object(); private object o2 = new object(); private void Test() { Contract.Requires(this.o1 != null); Contract.Requires(this.o2 != null); Contract.Ensures(this.o1 != null); lock (this.o2) { this.o1 = new object(); } } } </code></pre> <p>When I run the Code Contract static analysis tool, it prints a a warning: <code>Ensures unproven: this.o1 != null</code></p> <p>If I do any of:</p> <ul> <li>change the <code>o2</code> in the <code>lock</code> to <code>o1</code>,</li> <li>change the <code>o1</code> inside the <code>lock</code> block to <code>o2</code>,</li> <li>adding a second line inside the <code>lock</code> block assigning a <code>new</code> <code>object</code> to <code>o2</code></li> <li>change <code>lock (this.o2)</code> to <code>if (this.o2 != null)</code>,</li> <li>remove the <code>lock</code> statement entirely.</li> </ul> <p>the warning disappears.</p> <p>However, changing the line inside the <code>lock</code> block to <code>var temp = new object();</code> (thus removing all references to <code>o1</code> from the method) still causes the warning:</p> <pre class="lang-cs prettyprint-override"><code>private void Test() { Contract.Requires(this.o1 != null); Contract.Requires(this.o2 != null); Contract.Ensures(this.o1 != null); lock (this.o2) { var temp = new object(); } } </code></pre> <hr> <p>So there are two questions:</p> <ol> <li>Why is this warning occurring?</li> <li>What can be done to prevent it (bearing in mind this is a toy example and there's actually stuff happening inside the <code>lock</code> in the real code)?</li> </ol>
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    1. USPhilip C
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    1. COApparently Code Contracts doesn't consider the instantiation of `o1` that automatically occurs during constructor execution as sufficient evidence ensuring o1!=null. Ensures is a postcondition, and the `lock` implies that the method will be called from multiple threads.
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      1. USRobert Harvey
    2. CO@RobertHarvey But as I said, if you change the `this.o1 = new object()` into `this.o2 = new object()`, it's fine. It can't ensure anything in that case that can't also be ensured in the original case, and it still involves a lock. And the `Requires` should be enough evidence that `o1` isn't null at the start of the method.
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