Rule Definition
Direct access to the database from the User Interface does not respect the multi-layer architecture principles making the application more difficult to change. Furthermore, accessing database elements directly from the User Interface prevents access control at the database level . E.g.: use of non-optimized query against the database and can be the source of performance issue.

Remediation
Use business layer and data layer instead

Violation Code Sample

public partial class Form1 : Form // Form is in one of the namespaces for GUI
    {

        private void Form1_Load(object sender, EventArgs e)
        {
            SqlConnection myConnection = new SqlConnection(myConnectionString);
            SqlCommand myCommand = new SqlCommand(mySelectQuery, myConnection); // access to the DB => VIOLATION
            myConnection.Open();
            SqlDataReader myReader = myCommand.ExecuteReader(CommandBehavior.CloseConnection);
            while (myReader.Read())
            {
                Console.WriteLine(myReader.GetString(0));
            }
            myReader.Close();

        }