private void cbUser_SelectedIndexChanged(object sender, EventArgs e)
        {
            MainGridRefresh();
        }

        private void dtStart_ValueChanged(object sender, EventArgs e)
        {
            MainGridRefresh();
        }

        private void dtEnd_ValueChanged(object sender, EventArgs e)
        {
            MainGridRefresh();
        }
...

  SystemHelper.ShowWarning("Выберите пользователя.");

     private bool flagCbUserFirst = true;
        private void cbUser_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (!flagCbUserFirst)
                MainGridRefresh();
            flagCbUserFirst = false;
        }

    private async void MainForm_Load(object sender, EventArgs e)
        {
            // фильтры период
            dtStart.Value = DateTime.Now.AddDays(-14);
            dtEnd.Value = DateTime.Now;
...