مشکل حل شد و کد درست و بدون اشتباه این است
SqlConnection conn = new SqlConnection("Data Source=(local);Initial Catalog=fdata;Integrated security=True");
SqlCommand comm = new SqlCommand();
{
comm.CommandText = "select CourseName,NameOfMaster,TypeClassroom from mahdodiyat where Time='A'";
dgvA.AutoSizeColumnsMode = DataGridViewAutoSizeColumnsMode.DisplayedCells;
conn.Open();
comm.Parameters.AddWithValue("@time", "A");
comm.Connection = conn;
DataTable at = new DataTable();
at.Load(comm.ExecuteReader());
dgvA.DataSource = at;
conn.Close();
}