DateTimePicker 的 MonthCalendar 的日期未更新

如何解决DateTimePicker 的 MonthCalendar 的日期未更新

在启动时，我将 DateTimePicker Value 设置为最小日期值。
当用户使用 MouseDown 事件单击 DateTimePicker 时，日期将更改为当前日期。
一个 TextBox 更改为当前日期，但弹出的日历没有更新，仍然显示原始日期。

Private Sub frm1_Load(sender As Object,e As EventArgs) Handles Me.Load        
        dtpReceiveDate.Value = clsCommon.NULL_DATE     
End Sub

 Private Sub dtpReceiveDate_MouseDown(sender As Object,e As MouseEventArgs) Handles dtpReceiveDate.MouseDown       
        dtpReceiveDate.Value = Today       
End Sub

解决方法

MouseDown 事件在 DateTimePicker 的 MonthCalendar 控件更新为当前 DateTime 值后引发。
如果在打开 MonthCalendar 的按钮上生成 MouseDown 事件。
单击控件的日期部分不会打开 MonthCalendar。

我建议不要使用 MouseDown 事件来更新 DateTimePicker 的值，因为这会导致每次用户单击控件上的任何位置时重置日期。这可能会令人不安（这取决于你）。
您可以改用 DropDown 事件：仅当用户点击向下箭头按钮时才会引发该事件。

▶ 您不能将 ValueChanged 事件与此处显示的代码一起使用：这可能会导致死锁（考虑到所涉及的 Win32 控件的性质和 SendMessage 的使用，死锁的类型可能会传播到整个系统，所以，不要）。

此处的 DtpUpdateMonthCalendardDate() 方法向作为参数传递的 DateTimePicker 发送 DTM_GETMONTHCAL 消息。
如果成功获取了 MonthCalendar 的 Handle，则向 MonthCalendar 控件发送 MCM_SETCURSEL 消息，将其 Date 与父 DateTimePicker 的 Date 对齐。
使用 SYSTEMTIME 结构传递新日期，该结构使用 DateTimePicker 的值进行初始化。

Private Sub dtpReceiveDate_DropDown(sender As Object,e As EventArgs) Handles dtpReceiveDate.DropDown
    dtpReceiveDate.Value = Date.Today
    ' If result is False,something went wrong and the Date is not set
    Dim result = DtpUpdateMonthCalendardDate(dtpReceiveDate)
End Sub

Private Function DtpUpdateMonthCalendardDate(dtp As DateTimePicker) As Boolean
    If Not dtp.IsHandleCreated OrElse dtp.ShowUpDown Then Return False

    Dim hWndCal = SendMessage(dtp.Handle,DTM_GETMONTHCAL,0)
    If hWndCal = IntPtr.Zero Then Return False

    Dim sysTime = New SYSTEMTIME(dtp.Value)
    Return SendMessage(hWndCal,MCM_SETCURSEL,sysTime) <> 0
End Function

Win32 声明：

Friend Const DTM_FIRST As Integer = &H1000
Friend Const DTM_GETMONTHCAL As Integer = DTM_FIRST + 8

Friend Const MCM_FIRST As Integer = &H1000
Friend Const MCM_GETCURSEL As Integer = MCM_FIRST + 1
Friend Const MCM_SETCURSEL As Integer = MCM_FIRST + 2

<StructLayout(LayoutKind.Sequential)>
Friend Structure SYSTEMTIME
    Public Year As Short
    Public Month As Short
    Public DayOfWeek As Short
    Public Day As Short
    Public Hour As Short
    Public Minute As Short
    Public Second As Short
    Public Milliseconds As Short

    Public Sub New(dt As Date)
        Year = CShort(dt.Year)
        Month = CShort(dt.Month)
        DayOfWeek = CShort(dt.DayOfWeek)
        Day = CShort(dt.Day)
        Hour = CShort(dt.Hour)
        Minute = CShort(dt.Minute)
        Second = CShort(dt.Second)
        Milliseconds = CShort(dt.Millisecond)
    End Sub

    Public Function ToDateTime() As Date
        Return New DateTime(Year,Month,Day,Hour,Minute,Second,Milliseconds)
    End Function
End Structure

<DllImport("user32.dll",SetLastError:=True,CharSet:=CharSet.Auto)>
Friend Shared Function SendMessage(hWnd As IntPtr,Msg As Integer,wParam As Integer,lParam As Integer) As IntPtr
End Function

<DllImport("User32",msg As Integer,lParam As SYSTEMTIME) As Integer
End Function