C # The referenced Excel file cannot find the type or namespace name "Excel" (is the using instruction or assembly reference missing ?)

The Excel reference problem occurs in the project. One project has referenced Excel Object 11.0, which works well.
However, when you want to reference excel in another project, you cannot use the Excel namespace and the prompt "cannot find the type or namespace name" Excel "(whether the using command or ProgramSet Reference ?) ".
Of course, some colleagues have said this method by downloading a patch on the following websites and installing it, but even though I have installed it several times, it cannot solve my problem. Finally, I went to the Microsoft. office space and found an Excel file under. InterOP. So I added using Excel = Microsoft. Office. InterOP. Excel to the class. The problem was solved now!
Appendix: Microsoft patches.
Http://www.microsoft.com/downloads/thankyou.aspx? Familyid = 3c9a983a-ac14-4125-8ba0-d36d67e0f4ad & displaylang = en