What does the C# compiler mean when it prints "an explicit conversion exists"?

I am unable to reproduce the reported behaviour. If it does in fact reproduce, that's a bug. There is no explicit conversion from string to any user-defined interface.

Please update the question with the version number of the compiler you're using and a small program that reproduces the problem, and I'll get a bug entered into the bug database.

Thanks!

UPDATE: Apparently it does not reproduce on the command line, but is alleged to reproduce in VS2008.

I am unable to reproduce it in the RC build of VS2010, so if this was in fact a bug in VS2008, it's probably been fixed. I don't have an installation of VS2008 handy right now to test unfortunately.

Regardless, if you're seeing that diagnostic then odds are very good it is simply a bug in the error reporting heuristics in the semantic analyzer. Clearly there is no explicit conversion from string to IFoo.

There is an explicit conversion from any unsealed type to any interface type because there could be a derived type which implements the interface. But string is sealed, so the error should simply be "no conversion".

I have reproduced this behaviour.

Microsoft Visual Studio 2008

Version 9.0.30729.1 SP

Microsoft .NET Framework

Version 3.5 SP1

Installed Edition: Professional