Where does the project reference gone?

I have a project (A) that holds reference to other 开发者_开发问答project (B). When I rebuild project A, I get the error:

Error 58 The type or namespace name 'Services' does not exist in the namespace 'MyProj' (are you missing an assembly reference?)

When I add (again) reference to project B, the red lines disappear and everything is fine. When I rebuild again (in order to execute the application) I get again that Error 58.

Where does the project reference gone??

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Seems project's .NET versions conflict. Something very similiar happened to me in those cases.

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Is project B getting a compile time error? This would be your problem. Make sure B compiles or you can't reference it.

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Can you please include the output window content after rebuild? Sometimes the output file cannot be copied and this leads to similar issues

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I think, there is a conflict between your project, i recommend you to change your project B assembly name and then delete its exist assembly in project A and add it again.