Error encountered while reading module 'Microsoft.Office.Tools.Common.v9.0

I encountered the following error when I created a Unit Test project in Visual Studio 2008 and tried to add a module to the test project.

The following error was encountered while reading module 'Microsoft.Office.Tools.Common.v9.0': Could not resolve type reference: [office]Microsoft.Office.Core.IRibbonExtensibility.

I instantly got the resolution of the problem at http://blogs.msdn.com/b/tolong/archive/2007/12/02/missing-office-microsoft-office-core-reference.aspx when  I googled.

That's easy. Just add COM library, Microsoft Office 12.0 Object Library, to the project.

But I had to struggle to work the unit test project for because I added the COM library to the unit test project instead of my project. I thought the test project needs the library because I didn't have such error when I add/modify my program...

Now, I can use the unit test project in Visual Studio 2008 after add the library my Project rather than the unit test project.