解决方法@H_404_4@

您可以根据验证逻辑在 OnServerValidate方法中设置错误消息：

protected void customValidator1_Validate(object sender,ServerValidateEventArgs e)
{
    if (e.Value.Length < 5)
    {
        e.IsValid = true;
    }
    else
    {
        customValidator1.ErrorMessage = "Length must be less than 5.";
        e.IsValid = false;
    }
}