Today while doing some work in Visual Studio 2015, I noticed some odd behavior involving multiline lambdas passed to functions. My code includes invocations of Task.Factory.StartNew with a moderately complex parameterless, void-returning delegate - an Action - passed to it like so:
Task.Factory.StartNew(Sub()
' Do some stuff
' And some things
End Sub)
I was doing some refactoring that involved removing a variable from one function since the capability that required it was being moved somewhere else. After I deleted the declaration line, I noticed that Visual Studio put a red squiggly underline on StartNew, indicating a compilation error. Mousing over the underlined text informed me that the type of the parameter could not be inferred.
That was very strange, since the type of a Sub() lambda is completely unambiguous - it's always just Action with no type parameters. I set it aside for a moment and finished my refactoring of the lambda. Once all references to the nonexistent variable were removed, the error went away.
Assigning the large uncompilable lambda to a variable and then passing the variable to the StartNew function worked as expected; uses of the variable inside the lambda received squiggles. It appears that the Visual Studio 2015 compiler incorrectly lays the blame for all errors inside a lambda-as-an-argument on the calling function as type confusion.
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