“对象引用未设置到对象的实例”添加到列表时

Question

我的程序需要一些帮助。当我使用自定义 DayView 控件。

**************** 异常文本 ************** System.NullReferenceException：未将对象引用设置为对象的实例。 在 C:\Users\Daniel\My Programs\Visual Basic\SeaCow\SeaCow\SeaCow\Main.vb 中的 SeaCow.Main.DayView1_ResolveAppointments(Object sender, ResolveAppointmentsEventArgs args)：第 120 行 在 Calendar.DayView.OnResolveAppointments（ResolveAppointmentsEventArgs args） 在 Calendar.DayView.OnPaint(PaintEventArgs e) 在System.Windows.Forms.Control.PaintWithErrorHandling（PaintEventArgs e，Int16层） 在 System.Windows.Forms.Control.WmPaint(Message&m) 在 System.Windows.Forms.Control.WndProc(Message&m) 在System.Windows.Forms.NativeWindow.Callback（IntPtr hWnd，Int32 msg，IntPtr wparam，IntPtr lparam）

根据错误代码，下面的“foreach”循环导致 NullReferenceException 错误。默认情况下，“约会”列表没有分配任何内容，我找不到调用 ResolveAppointments 函数的位置。

    Private Sub DayView1_ResolveAppointments(ByVal sender As Object, ByVal args As Calendar.ResolveAppointmentsEventArgs) Handles DayView1.ResolveAppointments
    Dim m_Apps As New List(Of Calendar.Appointment)

    For Each m_App As Calendar.Appointment In appointments
        If (m_App.StartDate >= args.StartDate) AndAlso (m_App.StartDate <= args.EndDate) Then
            m_Apps.Add(m_App)
        End If
    Next

    args.Appointments = m_Apps
End Sub

以下是 DayView.cs 控制文件中的 OnResolveAppointments 和 ResolveAppointment 函数。

public event EventHandler<ResolveAppointmentsEventArgs> OnResolveAppointments;



protected virtual void ResolveAppointments(ResolveAppointmentsEventArgs args)
    {
        System.Diagnostics.Debug.WriteLine("Resolve app");

        if (OnResolveAppointments != null)
            OnResolveAppointments(this, args);

        this.allDayEventsHeaderHeight = 0;

        // cache resolved appointments in hashtable by days.
        cachedAppointments.Clear();

        if ((selectedAppointmentIsNew) && (selectedAppointment != null))
        {
            if ((selectedAppointment.StartDate > args.StartDate) && (selectedAppointment.StartDate < args.EndDate))
            {
                args.Appointments.Add(selectedAppointment);
            }
        }

        foreach (Appointment appointment in args.Appointments)
        {
            int key = -1;
            AppointmentList list;

            if (appointment.StartDate.Day == appointment.EndDate.Day && appointment.AllDayEvent == false)
            {
                key = appointment.StartDate.Day;
            }
            else
            {
                key = -1;
            }

            list = (AppointmentList)cachedAppointments[key];

            if (list == null)
            {
                list = new AppointmentList();
                cachedAppointments[key] = list;
            }

            list.Add(appointment);
        }
    }

另外，这是 OnPaint 方法

protected override void OnPaint(PaintEventArgs e)
    {
        e.Graphics.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;

        // resolve appointments on visible date range.
        ResolveAppointmentsEventArgs args = new ResolveAppointmentsEventArgs(this.StartDate, this.StartDate.AddDays(daysToShow));
        ResolveAppointments(args);

        using (SolidBrush backBrush = new SolidBrush(renderer.BackColor))
            e.Graphics.FillRectangle(backBrush, this.ClientRectangle);

        // Visible Rectangle
        Rectangle rectangle = new Rectangle(0, 0, this.Width - VScrollBarWith, this.Height);

        DrawDays(ref e, rectangle);

        DrawHourLabels(ref e, rectangle);

        DrawDayHeaders(ref e, rectangle);
    }

有人有什么建议吗？

Answer 1

DayView1_ResolveAppointments 显然是 DayView1 控件的 ResolveAppointments 事件的事件处理程序。如果 For Each 抛出异常，则意味着 appointments 当时是 Nothing，而不是您期望的空列表就这样吧。添加

If appointments Is Nothing Then
    Return
End If

在 For Each 循环之前。

Answer 2

您的 Calendar.DayView 控件似乎在其对 OnPaint 的重写中调用了 OnResolveAppointments 函数。我建议你检查那里的代码。

同时，如果appointments 为Nothing，您可能可以跳过For Each。