VALIDATE event handler Form D365 FormDataSourceEventType::ValidatingWrite

How to write a FormDataSourceEventType validate write data event handler code for a Form

/// <summary>
    /// When saving a new rental, prevent setting the start mileage on the FMRental form to a value that is equal to 1
    /// </summary>
    [FormDataSourceEventHandler(formDataSourceStr(FMRental, FMRental), FormDataSourceEventType::ValidatingWrite)]
    public static void FMRental_OnValidatingWrite(FormDataSource sender, FormDataSourceEventArgs e)
    {
        var datasource = sender as FormDataSource;
        var args = e as FormDataSourceCancelEventArgs;
        if (args != null && datasource != null)
        {
            FMRental record = datasource.cursor() as FMRental;
            if (record.recId == 0)
            {
                if(record.startmileage == 1)
                {
                    boolean doCancel = !checkFailed("Start Mileage = 1 is not allowed");
                    args.cancel(doCancel);
                }
            }
        }
    }

Below is the code at the table level for table FMVehicle

Class FMVehicleEventHandlers //write any name of the class
{
    /// <summary>
    ///
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
[DataEventHandler(tableStr(FMVehicle), DataEventType::ValidatedWrite)]
    public static void FMVehicle_onValidatedWrite(Common sender, DataEventArgs e)
    {
        ValidateEventArgs validateArgs = e as ValidateEventArgs;
        FMVehicle vehicle = sender as FMVehicle;
        boolean result = validateArgs.parmValidateResult();

        if (vehicle.NumberOfCylinders > 8)
        {
            result = checkFailed("Invalid number of cylinders.");
            validateArgs.parmValidateResult(result);
        }
    }
}