Cannot Open .mdf Database in Windows Application

A .mdf connection is almost always local. No remote access needed.

To answer your question, if you want to enable remote access for SQL Server, run the SQL Server Configuration Manager and configure & enable the desired protocol under SQL Server Network Configuration. The TCP/IP default port is 1433. Also check that the desired protocol is enabled under SQL Native Client [version #] Configuration.   The firewall on both ends of a connection will need to allow the specified port.

You might try building your application and running the .exe without debugging. It might offer a clue as to the issue.

Good luck!

attached error message?

Error messages is now attached.
EE-SQL.png

Error 26 is the common "can't find server" error.

You can look at this link to begin troubleshooting the issue.

You can also post your connection string (don't compromise security) so we can look for some obvious error. Also, if you show us how you are using the connection, it might help.

-> funwithdotnet:
Your remark 'A .mdf connection is almost always local. No remote access needed.' is according to my experience with the same application on my previous pc where I ran Visual Studio 2010 (and SQL Server 2008 Management Studio) - here there was no problems to connect.

I used two connectionstrings (I have renamed database and folders):

connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=E:\2. Kims data - applikationsudvikling\WinApp\foldername1\foldername\db.mdf;Integrated Security=True;User Instance=True" providerName="System.Data.SqlClient"

And this if I would connect to the .mdf in the same folder a the .exe:

connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=E:\2. Kims data - applikationsudvikling\WinApp\foldername1\foldername2\db.mdf;Integrated Security=True;User Instance=True" providerName="System.Data.SqlClient"

It solved the problem and was suggested as solution on a Danish IT forum.