Error : ColumnCount property cannot be set on a data-bound DataGridView control

hi,

In my DatagridView i am haveing a problem to to load data on button click when i first click the the button values shows ingridview 

but on second buttton click its gives an exception

ColumnCount property cannot be set on a data-bound DataGridView control

here is my code

under Button_click()
-----------------------------------------------------------
try
{
    if (dropClass.Text == "Select" || dropsections.Text == "Select" || dropTerm.Text == "Select")
    {
    }
    else
    {
        SqlConnection con = new SqlConnection();
        con = new SqlConnection(connectionString);
        {
            DataGridView grv = new DataGridView();

            using (SqlCommand cmd = new SqlCommand("SelectFromTbl_StudentRegistration_studentEntry", con))
            {
                cmd.CommandType = CommandType.StoredProcedure;
                using (SqlDataAdapter sda = new SqlDataAdapter(cmd))
                {

                    cmd.Parameters.AddWithValue("@Class", dropClass.Text.ToString());
                    cmd.Parameters.AddWithValue("@Section", dropsections.Text.ToString());

                    using (DataTable dt = new DataTable())
                    {
                        sda.Fill(dt);
                        //Set AutoGenerateColumns False

                        gridviewattendence.AutoGenerateColumns = false;
                        gridviewattendence.ColumnCount = 5;

                        //    Set Columns Count
                        gridviewattendence.Columns[2].HeaderText = "REGISTRATION NO";
                        gridviewattendence.Columns[2].DataPropertyName = "Student_Reg_No";


                        gridviewattendence.Columns[3].HeaderText = "ADM NO";
                        gridviewattendence.Columns[3].DataPropertyName = "ADM_NO";


                        gridviewattendence.Columns[4].HeaderText = "STUDENT NAME";
                        gridviewattendence.Columns[4].DataPropertyName = "Student_Name";

                        gridviewattendence.DataSource = dt;
                        gridviewattendence.Refresh();

                    }
                }
            }
        }
    }
}
catch (Exception ex)
{
}