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POCurious null-coalescing operator custom implicit conversion behaviour
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copied!<p><strong>Note: this appears to have been fixed in <a href="https://github.com/dotnet/roslyn" rel="noreferrer">Roslyn</a></strong></p> <p>This question arose when writing my answer to <a href="https://stackoverflow.com/questions/6238074">this one</a>, which talks about the associativity of the <a href="http://msdn.microsoft.com/en-us/library/ms173224.aspx" rel="noreferrer">null-coalescing operator</a>.</p> <p>Just as a reminder, the idea of the null-coalescing operator is that an expression of the form</p> <pre><code>x ?? y </code></pre> <p>first evaluates <code>x</code>, then:</p> <ul> <li>If the value of <code>x</code> is null, <code>y</code> is evaluated and that is the end result of the expression</li> <li>If the value of <code>x</code> is non-null, <code>y</code> is <em>not</em> evaluated, and the value of <code>x</code> is the end result of the expression, after a conversion to the compile-time type of <code>y</code> if necessary</li> </ul> <p>Now <em>usually</em> there's no need for a conversion, or it's just from a nullable type to a non-nullable one - usually the types are the same, or just from (say) <code>int?</code> to <code>int</code>. However, you <em>can</em> create your own implicit conversion operators, and those are used where necessary.</p> <p>For the simple case of <code>x ?? y</code>, I haven't seen any odd behaviour. However, with <code>(x ?? y) ?? z</code> I see some confusing behaviour.</p> <p>Here's a short but complete test program - the results are in the comments:</p> <pre><code>using System; public struct A { public static implicit operator B(A input) { Console.WriteLine("A to B"); return new B(); } public static implicit operator C(A input) { Console.WriteLine("A to C"); return new C(); } } public struct B { public static implicit operator C(B input) { Console.WriteLine("B to C"); return new C(); } } public struct C {} class Test { static void Main() { A? x = new A(); B? y = new B(); C? z = new C(); C zNotNull = new C(); Console.WriteLine("First case"); // This prints // A to B // A to B // B to C C? first = (x ?? y) ?? z; Console.WriteLine("Second case"); // This prints // A to B // B to C var tmp = x ?? y; C? second = tmp ?? z; Console.WriteLine("Third case"); // This prints // A to B // B to C C? third = (x ?? y) ?? zNotNull; } } </code></pre> <p>So we have three custom value types, <code>A</code>, <code>B</code> and <code>C</code>, with conversions from A to B, A to C, and B to C.</p> <p>I can understand both the second case and the third case... but <em>why</em> is there an extra A to B conversion in the first case? In particular, I'd <em>really</em> have expected the first case and second case to be the same thing - it's just extracting an expression into a local variable, after all.</p> <p>Any takers on what's going on? I'm extremely hesistant to cry "bug" when it comes to the C# compiler, but I'm stumped as to what's going on...</p> <p>EDIT: Okay, here's a nastier example of what's going on, thanks to configurator's answer, which gives me further reason to think it's a bug. EDIT: The sample doesn't even need two null-coalescing operators now...</p> <pre><code>using System; public struct A { public static implicit operator int(A input) { Console.WriteLine("A to int"); return 10; } } class Test { static A? Foo() { Console.WriteLine("Foo() called"); return new A(); } static void Main() { int? y = 10; int? result = Foo() ?? y; } } </code></pre> <p>The output of this is:</p> <pre><code>Foo() called Foo() called A to int </code></pre> <p>The fact that <code>Foo()</code> gets called twice here is hugely surprising to me - I can't see any reason for the expression to be <em>evaluated</em> twice.</p>

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