You can handle the CurrentCellValidaung event. There use the grid.CurrentCell.ControlText to get teh new value, and set e.Cancel if it is not a proper value. You can also set grid.CurrentCell.ErrorMessage if you want to see a message displayed. Here is a snippet that assume column 5 is an double, and requires any new entry be larger than teh old entry.
private void gridControl1_CurrentCellValidating(object sender, CancelEventArgs e)
{
GridCurrentCell cc = this.gridControl1.CurrentCell;
object oldValue = this.gridControl1[cc.RowIndex, cc.ColIndex].CellValue;
string newValue = cc.Renderer.ControlText;
//some check on column 5 - assumes column 5 is of CellType == typeof(double)
if(cc.ColIndex == 5 && oldValue != null && oldValue.ToString().Length > 0)
{
double d;
if(double.TryParse(newValue, System.Globalization.NumberStyles.Any, null, out d))
{
if(d < (double) oldValue)
{
e.Cancel = true;
cc.ErrorMessage = string.Format("value must be larger than {0:F3}", oldValue);
}
}
}
}