using DevExpress.XtraGrid.Views.Grid;
using DevExpress.XtraEditors.Controls;

private void gridView1_ValidatingEditor(object sender, BaseContainerValidateEditorEventArgs e) {
    GridView view = sender as GridView;
    if(view.FocusedColumn.FieldName == "Discount") {
        //Get the currently edited value
        double discount = Convert.ToDouble(e.Value);
        //Specify validation criteria
        if(discount < 0) {
            e.Valid = false;
            e.ErrorText = "Enter a positive value";
        }
        if(discount > 0.2) {
            e.Valid = false;
            e.ErrorText = "Reduce the amount (20% is maximum)";
        }
    }
}

private void gridView1_InvalidValueException(object sender, InvalidValueExceptionEventArgs e) {
    //Do not perform any default action
    e.ExceptionMode = DevExpress.XtraEditors.Controls.ExceptionMode.NoAction;
    //Show the message with the error text specified
    MessageBox.Show(e.ErrorText);
}

Imports DevExpress.XtraGrid.Views.Grid
Imports DevExpress.XtraEditors.Controls

Private Sub GridView1_ValidatingEditor(ByVal sender As Object, _
ByVal e As BaseContainerValidateEditorEventArgs) Handles GridView1.ValidatingEditor
    Dim View As GridView = sender
    If View.FocusedColumn.FieldName = "Discount" Then
        'Get the currently edited value
        Dim discount As Double = Convert.ToDouble(e.Value)
        'Specify validation criteria
        If discount < 0 Then
            e.Valid = False
            e.ErrorText = "Enter a positive value"
        End If
        If discount > 0.2 Then
            e.Valid = False
            e.ErrorText = "Reduce the amount (20% is maximum)"
        End If
    End If
End Sub

Private Sub GridView1_InvalidValueException(ByVal sender As Object, _
ByVal e As InvalidValueExceptionEventArgs) _
Handles GridView1.InvalidValueException
    'Do not perform any default action
    e.ExceptionMode = DevExpress.XtraEditors.Controls.ExceptionMode.NoAction
    'Show the message with the error text specified
    MessageBox.Show(e.ErrorText)
End Sub