How To Validate The Text In Edit Mode Of Gridtextcolumn In Datagrid(Sfdatagrid)?

public class GridCellTextBoxRendererExt : GridCellTextBoxRenderer
{
       string oldvalue = null;
       string newvalue = null;

       public override void OnInitializeEditElement(DataColumnBase dataColumn, TextBox uiElement, object dataContext)
       {
           base.OnInitializeEditElement(dataColumn, uiElement, dataContext);
           uiElement.KeyDown += UiElement_KeyDown;
           uiElement.KeyUp += UiElement_KeyUp;
       }

       private void UiElement_KeyUp(object sender, System.Windows.Input.KeyEventArgs e)
       {
           TextBox textBox = (e.OriginalSource as TextBox);
           newvalue = textBox.Text;
           Console.WriteLine("NewValue: " + newvalue + "\n\noldvalue: " + oldvalue);
           if (newvalue == "Sample")
           {
               textBox.Text = oldvalue;
               textBox.SelectionStart = textBox.Text.Length;
           }
       }

       private void UiElement_KeyDown(object sender, System.Windows.Input.KeyEventArgs e)
       {
           oldvalue = (e.OriginalSource as TextBox).Text;
       }
}