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copied!<p>EDIT: Eric Lippert's answer describes (<strong>much</strong> better) the issue - please see his answer for the 'real deal'</p> <p>FINAL EDIT: As unflattering as it is for one to leave a public demonstration of their own ignorance in the wild, there's no gain in veiling ignorance behind a push of the delete button. Hopefully someone else can benefit from my quixotic answer :) </p> <p>Thanks Eric Lippert and svick for being patient and kindly correcting my flawed understanding! </p> <hr> <p>The reason that you are getting the 'wrong' error message here is because of variance and compiler-inference of types combined with how the compiler handles type resolution of named parameters</p> <p>The type of the prime example <code>() => Console.LineWrite( "42" )</code> </p> <p>Through the magic of type inference and covariance, this has the same end result as </p> <p><code>Foo( bar: delegate { Console.LineWrite( "42" ); } );</code></p> <p>The first block could be either of type <code>LambdaExpression</code> or <code>delegate</code>; which it is depends on usage and inference.</p> <p>Given that, is it no wonder that the compiler gets confused when you pass it a parameter that's supposed to be an <code>Action</code> but which could be a covariant object of a different type? The error message is the main key that points toward type resolution being the issue.</p> <p>Let's look at the IL for further clues: All of the examples given compile to this in LINQPad:</p> <pre><code>IL_0000: ldsfld UserQuery.CS$<>9__CachedAnonymousMethodDelegate1 IL_0005: brtrue.s IL_0018 IL_0007: ldnull IL_0008: ldftn UserQuery.<Main>b__0 IL_000E: newobj System.Action..ctor IL_0013: stsfld UserQuery.CS$<>9__CachedAnonymousMethodDelegate1 IL_0018: ldsfld UserQuery.CS$<>9__CachedAnonymousMethodDelegate1 IL_001D: call UserQuery.Foo Foo: IL_0000: ldarg.0 **IL_0001: callvirt System.Action.Invoke** IL_0006: ret <Main>b__0: IL_0000: ldstr "42" IL_0005: call System.Console.WriteLine IL_000A: ret </code></pre> <p>Note the ** around the call to <code>System.Action.Invoke</code>: <code>callvirt</code> is exactly what it seems like: a virtual method call. </p> <p>When you call <code>Foo</code> with a named argument, you're telling the compiler that you're passing an <code>Action</code>, when what you're <strong>really</strong> passing is a <code>LambdaExpression</code>. Normally, this is compiled (note the <code>CachedAnonymousMethodDelegate1</code> in the IL called after the ctor for <code>Action</code>) to an <code>Action</code>, but since you explicitly told the compiler you were passing an action, it attempts to use the <code>LambdaExpression</code> passed in as an <code>Action</code>, instead of treating it as an expression!</p> <p>Short: named parameter resolution fails because of the error in the lambda expression (which is a hard failure in and of itself)</p> <p>Here's the other tell:</p> <pre><code>Action b = () => Console.LineWrite("42"); Foo(bar: b); </code></pre> <p>yields the expected error message.</p> <p>I'm probably not 100% accurate on some of the IL stuff, but I hope I conveyed the general idea</p> <p>EDIT: dlev made a great point in the comments of the OP about the order of overload resolution also playing a part. </p>

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