问题
I am using the MagicLibrary for a few components. I knew it was written in Visual Studio 2003 and recompiled it in Visual C# Express 2010. It compiled just fine and I could see the library in the folder.
I then created a new project in Visual C# Express 2010 and added the components MagicalLocalLibrary.dll to the Toolbox. They appeared fine with no errors. I put in a control from the Toolbox to my form, and it seemed fine. I then debugged (to see if any errors would occur) and I saw that the reference MagicalLibrary disappeared and I got the error:
The type or namespace name 'Crownwood' could not be found (are you missing a using directive or an assembly reference?)
The thing is that, the reference shows fine under References in Solution Explorer but I can't import it with using or use it in the code (simply is not listed in InstelliSense either) but if i go into the Object Browser and choose "My Solution" it is not listed. I can click on the Reference in Solution Explorer and just change the Copy Local property to True or False, then it will appear in the Object Browser but whenever I debug my project again it disappear.
Any idea why it does this? Thanks in advance.
回答1:
Check that you are using the .Net 4 framework and not the client profile version
You find that setting when you right-click on your project, choose properties. And under application there is a setting for target framework. If you have chosen the .Net 4 framework Client Profile, then this behavior that you described can happen
回答2:
I ran into a very similar issue once. In my case the problem turned out to be a missing dependency for the "vanishing" namespace's assembly. What finally gave me the information I needed was the suggestion in this StackOverflow answer to raise the MSBuild output verbosity level. After raising the verbosity setting the IDE should give you a more specific reason why the compilation fails.
来源：https://stackoverflow.com/questions/7260592/reference-disappear-upon-debug-build-c-vb-net-vs2010