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POGenerics, overload resolution and delegates (sorry, can't find a better title)
 

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    <blockquote> <p><strong>Possible Duplicate:</strong><br> <a href="https://stackoverflow.com/questions/4573011/why-is-funct-ambiguous-with-funcienumerablet">Why is Func&lt;T&gt; ambiguous with Func&lt;IEnumerable&lt;T&gt;&gt;?</a> </p> </blockquote> <p>I noticed a very weird overload resolution issue with generics...</p> <p>Consider the following methods:</p> <pre><code>static void Foo&lt;TSource&gt;(TSource element, Func&lt;TSource, int&gt; selector) { "int".Dump(); } static void Foo&lt;TSource&gt;(TSource element, Func&lt;TSource, double&gt; selector) { "double".Dump(); } static T Identity&lt;T&gt;(T value) { return value; } </code></pre> <p>(C# 4, tested in LINQPad)</p> <p>If I try to call <code>Foo</code> with a lambda expression as the selector, everything works fine:</p> <pre><code>Foo(42, x =&gt; x); // prints "int" </code></pre> <p>But if I replace <code>x =&gt; x</code> with <code>Identity</code>, the compiler can't decide between the 2 <code>Foo</code> overloads:</p> <pre><code>Foo(42, Identity); // The call is ambiguous between the following methods or properties: // 'UserQuery.Foo&lt;int&gt;(int, System.Func&lt;int,int&gt;)' and // 'UserQuery.Foo&lt;int&gt;(int, System.Func&lt;int,double&gt;)' </code></pre> <p>How can the second overload be a valid candidate ? Type inference correctly determines that <code>TSource</code> is <code>int</code>, so the <code>T</code> parameter for the <code>Identity</code> method has to be <code>int</code> as well, so the return type has to be <code>int</code> too... <code>Identity</code> could be a <code>Func&lt;int,int&gt;</code> or a <code>Func&lt;double,double&gt;</code>, but not a <code>Func&lt;int,double&gt;</code>!</p> <p>And it gets worse! Even if I specify all type parameters explicitly, I still get the same error:</p> <pre><code>Foo&lt;int&gt;(42, Identity&lt;int&gt;); // The call is ambiguous... </code></pre> <p>How can there be any ambiguity here? As far as I can tell, there is no way the overload that takes a <code>Func&lt;int,double&gt;</code> can be a candidate. I guess the explanation must be somewhere in the specifications, but I can't find the relevant bit... or it might be a bug in the compiler, but I guess it's unlikely.</p> <p>Note that it does work if I explicitly create the delegate:</p> <pre><code>Foo(42, new Func&lt;int, int&gt;(Identity)); // prints "int" </code></pre> <p>So, could someone explain what's going on here? Also, why does it work with a lambda but not with a method group?</p>
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    1. COPatiently waiting for Eric Lippert to post *the* answer.
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    2. COWhat happens under C# 3? I suspect this may have something to do with type variance in generics.
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    3. CO@Anon, I didn't try with C#3, but I don't think it has anything to do with variance, because variance doesn't apply to value types
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