DRAG DROP
There is MS Visual Studio 2010 and MS .NET Framework 4 application PassGuideApp.
PassGuideApp connects to a MS SQL Server Express 2008 database PassGuideDB.
The primary database file of PassGuideDB is PassGuideDB.mdf.
PassGuideDB.mdf is stored in the App_data folder.
You want to set up the connection between PassGuideApp and PassGuideDB.
What do you need to do?
Answer: 
Explanation:
Here’s is the first result that comes up when you google ‘mdf connection string’:
http://forums.asp.net/t/1246174.aspx?ConnectionString+to+access+mdf+file+in+App_Data+folder
And the connection string specified in that link is similar to the one specified by the answer here, so I’ll just go ahead and assume the answer is correct.
I did a bit of extra research in the following link as well:
http://msdn.microsoft.com/en-us/library/jj653752%28v=vs.110%29.aspx
And didn’t find any issues with the answer, so it looks good.
Also good to know, that the |DataDirectory| is only for Web application to select the App_Data directory of the web application. Thas why web.config is picked in the answer.
However, you can prepare you desktop application to be able to consume a connection sting with a |DataDirectory| part in it:
http://stackoverflow.com/questions/3500829/sql-express-connection-string-relative-to-application-location