SQL Commands are not recognizing mdf Database in Visual Studio Project

I recently added an Existing mdf database file to my C# Windows Application project. A Connection was created succesfully and I can view the database tables in visual studio.

However, when I try to execute SQL commands in my code it seems its not recognizing the database. For example:

BUFETEConnectionString =" Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\BUFETE.mdf;Integrated Security=True;User Instance=True"

That is the connection string Visual Studio created when I added the existing mdf. Now when I try this:

connectionStr = Properties.Settings.Default.BUFETEConnectionString;
connection = new SqlConnection();
connection.ConnectionString = connectionStr;

connection.Open();

System.Data.SqlClient.SqlCommand Command;
Command = new System.Data.SqlClient.SqlCommand("USE BUFETE");
Command.Connection = connection;

DataReader = Command.ExecuteReader();

I get a System.Data.SqlClient.SqlException, which message reads:

Database 'BUFETE' does not exist. Make sure that the name is entered correctly.

Why is this? I don't understand. I've been battling with this for hours and I just can't think of anything.

I tried executing the List Tables Store procedure and it is indeed not showing the database BUFETE.

What am I missing...? Please help.