I was working on a Visual Studio 2010 VB.Net Windows Service application.  I added a reference to an in-house IO library and then used the Imports keyword to import the namespace from the referenced assembly.  IntelliSense recognized the namespace and I had access to the methods in the IO library.

I then tried to compile the application.  It failed.  My Imports statement for the IO library and the methods I used with that library generated error messages.  Along with the errors, I noticed a warning message.  The warning message provided the clue to the solution.  It mentioned that the referenced IO assembly could not be resolved because it had a dependency on System.Data.OracleClient.

Here is the link where I found the solution (thanks Xiaoyu!).  It turns out that when I added the new Windows Service project, the targeted framework was for the .NET Framework 4 Client Profile.  That framework does not include the System.Data.OracleClient.dll. 

The fix: Change the target framework to .NET Framework 4 (Project Properties -> Compile tab -> Advanced Compile Options… -> Target framework drop down).