I am hoping someone can help me...I am totally frustrated! I am using Visual C# 2010 Express (which might be the problem right there, but I wouldn't think so). I have added a .dbml (LINQ to SQL) file to my project and I am about to create a new connection. This works fine when I use ASP.NET. When I right-click on "Data Connections" and choose "Add Connection," the dialog box appears. The first field is: "Data Source." Normally, when you click the "Change" button, you have a choice which includes "Microsoft Sql Server Database (Sql Client)." This choice does NOT appear - instead, I am forced to use "Microsoft Sql Server Database File," which makes me choose an .mdf file. But this won't work - when I attempt to test the connection, I get the following error message: "A network-related or instance-specific error occurred while establishing a connection to Sql Server..." It appears as though it's trying to ATTACH the DB file, but, of course, it's already attached! So I can't understand why I can't just select the database, like I can with an ASP.NET project. Why won't the correct choice appear in the dialog? Has anyone else experienced this problem????