I am getting this Exception When I am trying to access database from C#.
My Connection String is
Data Source=(localdb)\v11.0;integrated security=true;User Instance = true;AttachDbFileName=C:\Users\UserName\Desktop\DB\TestDB3.mdf
Exception is
The user instance login flag is not allowed when connecting to a user instance of SQL Server. The connection will be closed.
LocalDB instances are all "User Instances", and there's no need to specify User Instance=true in your connection string, in fact it is not even supported (as you can see). Just remove this part and it's going to work fine.
Just to add some more information. If some how this doesn't work for you and it still complains that its not able to locate server. You will be required to set following two attributes on your application pool identity in applicationHost file
<add name="DefaultAppPool">
<processModel identityType="NetworkService" loadUserProfile="true" setProfileEnvironment="true" />
</add>
where these two attributes are required to make sure your account can start LocalDB server.
来源：https://stackoverflow.com/questions/14054254/user-instance-of-sqllocaldb