检查VB.Net中多个下拉列表的值的值

Question

我有4个字段：

    •   calStartDate = Control for the start Date Calendar (drop downs for Day, Month and Year) 
    •   ddlTime = Start time drop down
    •   calEndDate = Control for the End Date Calendar (drop downs for Day, Month and Year) 
    •   ddlTime2 = End time drop down

现在我已设法将以下值保存在DB中;

•   STARTDATETIME = “2/24/2012 6:30:00 AM” 
•   ENDDATETIME = “2/24/2013 6:30:00 AM”

如下面的代码;

 Dim EndDateTime As DateTime
    Dim StartDateTime As DateTime
    If chkbxAccAppBasedOnTimeInterval.Checked = True Then
       Dim StartDate As System.DateTime? = calStartDate.CurrentDate
       If StartDate.HasValue Then
          StartDateTime = StartDate.Value.AddMinutes(CType(ddlTime.SelectedValue, Double))
          EditRelTbl.STARTDATETIME = StartDateTime
       End If

       Dim EndDate As System.DateTime? = calEndDate.CurrentDate
       If EndDate.HasValue Then
          EndDateTime = EndDate.Value.AddMinutes(CType(ddlTime2.SelectedValue, Double))
          EditRelTbl.ENDDATETIME = EndDateTime
       End If

       If EndDateTime > StartDateTime Then
          DisplayAlert("End date time, must be less than Start Date and time", "Warning")
          EndDateTime = StartDateTime
       End If
Else
    EditRelTbl.STARTDATETIME = Nothing
    EditRelTbl.ENDDATETIME = Nothing
End If

问题是我必须检查STARTDATETIME是否必须大于ENDDATETIME，如果End dateTime大于Start DateTime，那么填写End datetime作为Start DateTime ，因为我试图在代码

If EndDateTime > StartDateTime Then
   DisplayAlert("End time must be greater than Start time", "Warning")
   EndDateTime = StartDateTime
End If

但这不是检查我想要的支票。任何人都可以指导我如何进行检查，或者我错了。

注意：'EditRelTbl'是表，我将保存结束/开始日期时间的值。