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POCustom Methods in Contract.Ensures
 

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    <p>I'm trying to understand Code Contracts in a bit more detail. I've got the following contrived example, where I'm trying to assert the invariant of a try/get pattern that if it returns <code>true</code> then the <code>out</code> object is non-null, otherwise if it returns <code>false</code>.</p> <pre><code> public static bool TryParseFruit(string maybeFruit, out Fruit fruit) { Contract.Requires(maybeFruit != null); Contract.Ensures((Contract.Result&lt;bool&gt;() &amp;&amp; Contract.ValueAtReturn(out fruit) != null) || (!Contract.Result&lt;bool&gt;() &amp;&amp; Contract.ValueAtReturn(out fruit) == null)); // Elided for brevity if (ICanParseIt()) { fruit = SomeNonNullValue; return true; } else { fruit = null; return false; } } </code></pre> <p>I don't like the duplication inside <code>Contract.Ensures</code> so I wanted to factor out my own method for this.</p> <pre><code>[Pure] public static bool Implies(bool a, bool b) { return (a &amp;&amp; b) || (!a &amp;&amp; !b); } </code></pre> <p>Then I changed my invariant in <code>TryParseFruit</code> to</p> <pre><code>Contract.Ensures(Implies(Contract.Result&lt;bool&gt;(), Contract.ValueAtReturn(out fruit) != null); </code></pre> <p>But this generates warnings that the "ensures is unproven". If I then perform the inline refactoring on my <code>Implies</code> method then everything is OK again.</p> <p>Could someone explain to me why this happens? I'm guessing it's because <code>Contract.ValueAtReturn</code> is used magically in some way that means I can't just pass it's result off to another function and expect it to work.</p> <p>(Update #1)</p> <p>I think that all of the following <code>Contract.Ensures</code> express the same thing (namely that if the function returns <code>true</code> then <code>fruit</code> is non-null, otherwise <code>fruit</code> is <code>null</code>). Note that I am only using one of these at a time :)</p> <pre><code>Contract.Ensures(Implies(Contract.Result&lt;bool&gt;(), Contract.ValueAtReturn(out fruit) != null)); Contract.Ensures(Contract.Result&lt;bool&gt;() == (Contract.ValueAtReturn(out fruit) != null)); Contract.Ensures(Contract.Result&lt;bool&gt;() ^ (Contract.ValueAtReturn(out fruit) == null)); </code></pre> <p>However, none of the above <code>Contract.Ensures</code> lead to a clean compile of the code below. I want the Code.Contracts to express that <code>fruit.Name</code> cannot be a null reference.</p> <pre><code> Fruit fruit; while (!TryGetExample.TryParseFruit(ReadLine(), out fruit)) { Console.WriteLine("Try again..."); } Console.WriteLine(fruit.Name); </code></pre> <p>I am only able to get the completely clean compile with code contracts if I use the long winded way of expressing this detailed above. My question is why!</p>
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