Entity Manager Rejecting Reject Changes

   Hello People

I am facing problem with method RejectChanges

When I cancelled one event but enitity manager still have

that changes and using value to save on next event.

I attaching following code anyone can help me

Thanks in Advance

private void btnRemove_Click(object sender, RoutedEventArgs e)

        {

            if (DataGrid.SelectedRowsCount != 0)

            {

                STUDENT stud= (STUDENT )DataGrid.FocusedDataRow;

                STUDENT_GROUP.STUDENT.Remove(stud);

             }

            else

                MessageBox.Show("Select a Profession to remove.");

        }

private void CancelButton_Click(object sender, RoutedEventArgs e)

        {

            _em.RejectChanges();

            this.DialogResult = false;

        }