[Update: BizTalk 2010 support for .Net 4.5 is added in CU7. If you encounter the below error, install the cumulative update.]
Recently, I did Windows updates on a BizTalk 2010 server and let it install the .Net 4.5 Framework. A short time later, I was moving some existing code over and noticed a compiler warning when I ran a build on a project.
'System.Runtime.CompilerServices.ExtensionAttribute' in assembly System.Core is already defined; the first definition is in assembly mscorlib
The workarounds suggested on the connect.microsoft site don’t seem to work with BizTalk projects and the BizTalk forums have nothing worthwhile to add, so the fix seems to be uninstalling .Net 4.5 and then reinstalling .Net 4.