I recently helped migrate a ton of code from Visual Studio 2005 to 2008, and .NET 2.0 to 3.5. Most of it went very smoothly; it touches every .sln, .csproj, and .Designer.cs file, and puts a bunch of junk in Web.Configs, but rarely encountered errors. One thing I didn't expect was that even for a project running in VS 2008 but targeting .NET Framework 2.0, it will still use the v3.5 C# compiler. As such, it does behave a bit differently than the 2.0 compiler, even when targeting the 2.0 Framework.
One piece of code used an internal custom EventArgs class, that was consumed via a public delegate. This code compiled fine using the 2.0 C# compiler, but the 3.5 compiler threw this error:
error CS0059: Inconsistent accessibility: parameter type 'MyApp.Namespace.MyEventArgs' is less accessible than delegate 'MyApp.Namespace.MyEventHandler'
It's a goofy situation, the error makes perfect sense, and it was easy to correct (I made both internal), but I expected VS 2008 would use the compiler to match whatever the target .NET Framework version was. I wouldn't have expected any compilation errors it didn't have before conversion, not until I changed the targeted Framework version.
Another funny error happened around code analysis. Code analysis ran fine in VS 2005, but in VS 2008, it threw this error (compilation error, not a code analysis warning):
Running Code Analysis...
C:\Program Files\Microsoft Visual Studio 9.0\Team Tools\Static Analysis Tools\FxCop\FxCopCmd.exe /outputCulture:1033 /out:"bin\Debug\MyApp.Namespace.MyProject.dll.CodeAnalysisLog.xml" /file:"bin\Debug\MyApp.Namespace.MyProject.dll" /directory:"C:\MyStuff\MyApp.Namespace.MyProject\bin\Debug" /directory:"c:\WINDOWS\Microsoft.NET\Framework\v2.0.50727" /directory:"..\..\..\Lib" /rule:"C:\Program Files\Microsoft Visual Studio 9.0\Team Tools\Static Analysis Tools\FxCop\Rules" /ruleid:-Microsoft.Design#CA1012 /ruleid:-Microsoft.Design#CA2210 ... /searchgac /ignoreinvalidtargets /forceoutput /successfile /ignoregeneratedcode /saveMessagesToReport:Active /targetframeworkversion:v2.0 /timeout:120 MSBUILD : error : Invalid settings passed to CodeAnalysis task. See output window for details.
Code Analysis Complete -- 1 error(s), 0 warning(s)
Done building project "MyApp.Namespace.MyProject.csproj" -- FAILED.
I especially like the "See output window for details," which 1. screams of a Visual Studio hack as it is, and 2. doesn't actually give me any more details in this particular case, though Google tells me that other people do get more information in the output window.
I noticed Debug and Release modes both had code analysis enabled (I think switching Framework versions swapped them on me and I accidentally enabled it in Release mode), and Release mode wasn't erroring out but Debug was. I looked at the difference in the csproj file, and in the FxCopCmd.exe calls, and the key seemed to be the /ruleid parameters (bolded), of which there were a ton in Debug but not Release. Presumably this is because I disabled some of the rules in the project properties, so I tried enabling them all. The number of /ruleid params went down, but it still gave the same error. The Code Analysis tab in project properties looked the same between Debug and Release.
Finally I unloaded the project, edited the csproj file (I'm glad I found out how to do this within VS, instead of exiting VS and editing it in Notepad), and removed this line, which was present in the Debug PropertyGroup element but not the Release one:
Code analysis then ran successfully. I imagine this solution isn't ideal for everyone, if you want to enable/disable particular rules, and it's not ideal for us long-term, but it did allow us to keep code analysis enabled without the build failing.