vb.net - 取消日期时间选择器 ValueChanged 事件

Question

有没有办法通过条件取消事件？我试过 e.cancel但它不起作用。取消事件后dtpAudit_From.Value必须恢复到原来的值。

Private Sub dtpAudit_From_ValueChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles dtpAudit_From.ValueChanged
        'check if two DTPs (Date time pickers) are valid
        If dtpAudit_From.Value > dtpAudit_To.Value Then
            MsgBox("cancel the event")

        End If
End Sub

Answer 1

一种方法是子类化 DateTimePicker并添加一个 ValueChanging事件。下面是一个例子:

Public Class UIDateTimePicker
    Inherits DateTimePicker

    Public Sub New()
        Me.cachedValue = Me.Value
    End Sub

    Public Event ValueChanging As CancelEventHandler

    Protected Overrides Sub OnValueChanged(e As EventArgs)
        If (Not Me.reverting) Then
            Dim evargs As New CancelEventArgs(False)
            Me.OnValueChanging(evargs)
            If ((Not evargs Is Nothing) AndAlso evargs.Cancel) Then
                Dim value As Date = Me.Value
                Me.reverting = True
                Me.Value = Me.cachedValue
            Else
                Me.cachedValue = Value
                MyBase.OnValueChanged(e)
            End If
            Me.reverting = False
        End If
    End Sub

    Protected Overridable Sub OnValueChanging(e As CancelEventArgs)
        RaiseEvent ValueChanging(Me, e)
    End Sub

    Private cachedValue As DateTime
    Private reverting As Boolean

End Class

用法

Private Sub dtpAudit_From_ValueChanging(sender As Object, e As CancelEventArgs) Handles dtpAudit_From.ValueChanging
    e.Cancel = {Condition}
End Sub