.net 在Visual Basic中遇到时间跨度问题

在VB.net中，我一直在学习Murach的Visual Basic 2010。在学习如何处理时间和字符串时，我遇到了时间跨度的问题

我需要采取2个日期，并找到他们之间的天数

所以我声明了我的变量

Dim currentDay As Date
Dim futureDate As Date
Dim timespan As TimeSpan = currentDay.Subtract(futureDate)
Dim strMsgText As String
Dim daysUntilDue = timespan.Days

然后我为currentDay和futureDate设置格式

currentDay = Convert.ToDateTime(Now)
futureDate = Convert.ToDateTime(txtFutureDate.Text) 'input from user

然后我设置需要显示的数据

 strMsgText = "Current Date: " & currentDay.ToShortDateString() _
            & "Future Date: " & futureDate.ToShortDateString() _
            & "Days Util Due " & daysUntilDue

接下来，我提供了数据验证

If IsDate(txtFutureDate.Text) Then
   futureDate = CDate(txtFutureDate.Text)
End If

最后我显示数据

MessageBox.Show(strMsgText)

我没有收到语法错误或vb ide错误 但是，当它计算日期时，它会在消息框中为我提供此信息

前

我试图在计算中改变日期

例如，我将其设置为

futureDate.Subtract（当前日期）

，而不是

currentDay.Subtract（当前日期）

，只是想看看它是否会给我一个不同的结果。但是，唉，结果仍然是0

---

我知道我做了一些错误的事情，但我无法发现IDE/编译器没有给我任何错误，这本书也没有给我任何建议，也不知道如何让它正常工作。

问题是，在您设置timespan值时，

currentDay

和

futureDate

尚未初始化，并且具有

DateTime

的默认值。这些值的减法将始终为0

TimeSpan

设置这两个日期后，设置时间间隔

Dim currentDay As Date
Dim futureDate As Date
Dim strMsgText As String

currentDay = Convert.ToDateTime(Now)
futureDate = Convert.ToDateTime(txtFutureDate.Text) 'input from user

Dim timespan As TimeSpan = currentDay.Subtract(futureDate)
Dim daysUntilDue = timespan.Days

---

由于需要检查用户是否输入了有效日期，因此可以使用DateTime.TryParse确定：

Dim currentDay As Date = DateTime.Now
Dim futureDate As Date
Dim strMsgText As String
Dim daysUntilDue As Integer

currentDay = DateTime.Now

Dim ci As New Globalization.CultureInfo("en-US")

' check if a parseable date has been entered before doing the calculation
If DateTime.TryParse(txtFutureDate.Text, ci, Globalization.DateTimeStyles.AllowWhiteSpaces, futureDate) Then
    daysUntilDue = (futureDate - currentDay).Days
    strMsgText = "Current Date: " & currentDay.ToShortDateString() & " Future Date: " & futureDate.ToShortDateString() & " Days Until Due " & daysUntilDue.ToString
Else
    strMsgText = "I could not understand the future date as entered."
End If

MessageBox.Show(strMsgText)

---

好的，谢谢。在currentDay和futureDate被赋值后，将代码与timespan和daysUntilDue一起放入，并将我的公式反转为futureDate.Subtrace（currentDay）。卡巴姆：效果很好。我知道这一定是逻辑错误我不是世界上最聪明的蜡笔box@thewaytonever-没问题。希望你明白你错在哪里，以及为什么这是答案。@thewaytonever-我们都从某个地方开始。当谈到编程时，事情并不明显或自然。VisualBasic绝对需要按执行顺序安排一切，以便逐行运行程序。

Dim currentDay As Date = DateTime.Now
Dim futureDate As Date
Dim strMsgText As String
Dim daysUntilDue As Integer

currentDay = DateTime.Now

Dim ci As New Globalization.CultureInfo("en-US")

' check if a parseable date has been entered before doing the calculation
If DateTime.TryParse(txtFutureDate.Text, ci, Globalization.DateTimeStyles.AllowWhiteSpaces, futureDate) Then
    daysUntilDue = (futureDate - currentDay).Days
    strMsgText = "Current Date: " & currentDay.ToShortDateString() & " Future Date: " & futureDate.ToShortDateString() & " Days Until Due " & daysUntilDue.ToString
Else
    strMsgText = "I could not understand the future date as entered."
End If

MessageBox.Show(strMsgText)