class DataGridViewScrollTextBoxEditingControl : TextBox, IDataGridViewEditingControl
    {
        DataGridView dataGridView;
        private bool valueChanged = false;
        int rowIndex;
//////////
        public bool ReadOnly { get { return base.ReadOnly; } set { base.ReadOnly = value; } }
...
    public class ScrollTextBoxCell : DataGridViewTextBoxCell
    {
        public override void InitializeEditingControl(int rowIndex, object initialFormattedValue, DataGridViewCellStyle dataGridViewCellStyle)
        {
            base.InitializeEditingControl(rowIndex, initialFormattedValue, dataGridViewCellStyle);
            DataGridViewScrollTextBoxEditingControl ctl = DataGridView.EditingControl as DataGridViewScrollTextBoxEditingControl;
            ctl.Text = (string)this.Value;
            ctl.ReadOnly = true;
        }
...