Adding project reference in visual studio 2010

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Visual-Studio

Hi All,

If i browse the dll and add reference to my project the build is successful.
But if i add the dll through project reference like Reference -> Add Reference -> Projects; i am getting build error, "Namespace not found for some classes".

Also if i add project reference and shift(copy paste) my entire solution to some other path in the hard disk then also build is successful. Only in that particular path with adding project reference is not building.

What is the reason for this? Dll is not in GAC
How do i make solution build successful by adding project reference instead of browsing and adding dll.

Thanks,