Question

When i debug my application, it gives an error that SelectCommand.Connection Property is not been initialized. i dont know what am i doing here wrong :s. I actually want to add a filter over my search on the textchanged event of a textbox.

public class ConnectionClass
{
    static SqlConnection cn;

    public static SqlConnection Connection()
    {
        string myConnection = ConfigurationManager.ConnectionStrings["_uniManagementConnectionString1"].ConnectionString;
        if (cn != null)
        {
            cn = new SqlConnection(myConnection);
            cn.Open();
        }
        return cn;
    }

}    

public class ClassDataManagement 
{ 
    SqlConnection cn = ConnectionClass.Connection();

    public DataTable GetData(string SQL)
    {
         SqlCommand cmd = new SqlCommand(SQL, cn);
         SqlDataAdapter da = new SqlDataAdapter(cmd);
         DataTable dt = new DataTable();
         da.Fill(dt);
         return dt;
    } 
}

protected void TextBoxFilterText_TextChanged(object sender, EventArgs e)
{
   ClassDataManagement dm = new ClassDataManagement();
   string query = "Select CourseCode from _Courses where coursecode like'%" + TextBoxFilterText.Text.TrimEnd() + "%'";
   dm.GetData(query);
   GridViewCourses.DataBind();
}

Answer 1

That’s because your cn variable is null, and not getting initialized. Yet another example why it’s a bad idea to initialize and open database connections in a static method.

Try this:

public class ClassDataManagement 
{ 
   public DataTable GetData(string SQL)
    {
          string YourConnectionString = ConfigurationManager.ConnectionStrings["_uniManagementConnectionString1"].ConnectionString;     
          DataTable dt = new DataTable();             

          using (SqlConnection cn = new SqlConnection(YourConnectionString))
          using (SqlCommand cmd = new SqlCommand(SQL, cn))
          using (SqlDataAdapter da = new SqlDataAdapter(cmd))
          {
              da.Fill(dt);
          }
          return dt;
    } 
}

With the SqlDataAdapter class, you don’t need to explicitly call SqlConnection.Open(). The SqlDataAdapter.Fill() method handles all of the connection opening (and closing).

MSDN Reference on SqlDataAdapter.Fill()

As per the above reference, quoted:

The connection object associated with the SELECT statement must be valid, but it does not need to be open. If the connection is closed before Fill is called, it is opened to retrieve data, then closed. If the connection is open before Fill is called, it remains open.