When using the LINQ to SQL designer surface in Visual Studio, each time you add an object (table, procedure, etc), you will end up adding connection string information to your config file.  This can be confusing as the names used for the connection string may be different then what you want to use and you may end up with multiple connection strings, especially if you have multiple developers working on the same project.

To address this issue, you can:

1. Create a partial class for your data context and create a parameter-less constructor as follows
   
   public partial class LinqAppDataContext
       {
           public LinqAppDataContext()
               : base(ConfigurationManager.ConnectionStrings["AppConnectionString"].ToString())
           {
   
           }
       }
   
   In this example, I already have a LinqAppDataContext class, and I am creating a partial class that calls the constructor on the base class that takes a connection string.  I then pull this connection string from the config file.
   
   
2. Step 1 is a one time setup.  The remaining steps need to be followed each time you add a new object to the designer surface
   
   
3. Add a new object to the designer surface (for example, drag a table from the Server Explorer)
   
   
4. Click on an empty section of the designer surface (dbml file)
   
   
5. In the properties window, go to Connection, then clear the information from the Connection String property and then set the Application Settings property to false.  By doing this, the parameter-less constructor from step 1 above will kick in and pull the connection string from your config file.
   
   
6. Verify that no extra connection strings have been added to your projects settings or config files