在赋值之前使用变量'details'。

Question

变量details在赋值之前使用。 details有什么问题？

Option Explicit On
Imports System.Text
Imports System.IO
Public Class Main
    Private SelectedItem As ListViewItem
    Dim data As String

    Dim strpriority As String
    Dim task As String
    Dim createdate As String
    Dim duedate As String

    Private Sub btnAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAdd.Click
        AddTask.Show()
        Me.Hide()
    End Sub

    Private Sub HistoryToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles HistoryToolStripMenuItem.Click
        History.Show()
        Me.Hide()

    End Sub

    Private Sub Main_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim fpath As String
        Dim splitdata
        fpath = AppDomain.CurrentDomain.BaseDirectory
        Dim filepath As String
        filepath = fpath & "task.txt"
        Dim details As String
        details = My.Computer.FileSystem.ReadAllText(filepath)
        splitdata = Split(details, vbCrLf)
        Dim i As Integer
        For i = 0 To UBound(splitdata)
            lblTaskName.Items.Add(splitdata(i))
        Next

        lblTime.Enabled = True
        Timer1.Interval = 10
        Timer1.Enabled = True
        lblDate.Text = DateTime.Now.ToString("dd MMMM yyyy")
    End Sub

    Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
        lblTime.Text = TimeOfDay
    End Sub

    Private Sub btnExit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnExit.Click
        End
    End Sub

    Private Sub btnRemove_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnRemove.Click
        If lblTaskName.SelectedItem = "" Then
            MsgBox("Please select a record")

        Else

            If lblTaskName.Items.Count > 0 Then
                If MessageBox.Show("Do you really want to delete this record?", "Delete", MessageBoxButtons.YesNo) = MsgBoxResult.Yes Then
                    lblTaskName.Items.Remove(lblTaskName.SelectedItem.ToString())

                Else
                    MessageBox.Show("Operation Cancelled")

                End If
            End If

        End If

        Try

            Dim fpath As String
            fpath = AppDomain.CurrentDomain.BaseDirectory
            Dim filepath As String
            filepath = fpath & "task.txt"
            Dim details As String
            If lblTaskName.Items.Count > 0 Then
                details = lblTaskName.Items(0)
                Dim i As Integer
                For i = 1 To lblTaskName.Items.Count - 1

                    details = details & vbCrLf & lblTaskName.Items(i)

                Next
            End If
            My.Computer.FileSystem.WriteAllText(filepath, details, False)
        Catch ex As Exception
            MsgBox("Values Can't be inserted this time")
        End Try
    End Sub

    Private Function filepaths() As String
        Throw New NotImplementedException
    End Function
End Class

Answer 1

问题出在本部分的btnRemove_Click方法中：

Dim details As String
If lblTaskName.Items.Count > 0 Then
    details = lblTaskName.Items(0)

如果条件的计算结果为false，则在初始化之前使用details变量，因为它仅在if块中设置到目前为止。

我想你想将以下行移动到if块来解决问题：

My.Computer.FileSystem.WriteAllText(filepath, details, False)

或者，您可以为details提供默认值，以便在任何情况下都设置它。出于性能原因，您可以在String.Empty分支中设置默认值（例如text或else）：

Dim details As String
If lblTaskName.Items.Count > 0 Then
     ' ...
Else
    details = "Default Value"
End If

Answer 2

你需要考虑你的程序流程。考虑以下代码：

Dim details As String
If lblTaskName.Items.Count > 0 Then
    details = lblTaskName.Items(0)
    Dim i As Integer
    For i = 1 To lblTaskName.Items.Count - 1
        details = details & vbCrLf & lblTaskName.Items(i)
    Next
End If
My.Computer.FileSystem.WriteAllText(filepath, details, False)

您在顶部声明了details变量。然后你检查lblTaskName控件中至少有一个项目。如果该测试通过，则将第一个项目分配给details。但是如果那个测试没有通过呢？如果lblTaskName控件中有0个项目怎么办？在那种情况下，If区块的内部永远不会运行，并且没有任何东西存储在details中。然后在最后一行中，您尝试使用details块之外的If变量*的值。这是非法的，因为它可能没有分配值。

也许你的意思是WriteAllText线在If区域内？否则，您需要在Else语句中添加If子句来处理lblTaskName中有0项的情况。

---

除此之外，从风格上讲，您应该尽可能在声明时初始化变量。例如，而不是写：

    Dim fpath As String
    Dim splitdata
    fpath = AppDomain.CurrentDomain.BaseDirectory
    Dim filepath As String
    filepath = fpath & "task.txt"
    Dim details As String
    details = My.Computer.FileSystem.ReadAllText(filepath)
    splitdata = Split(details, vbCrLf)

把它写成：

    Dim fpath As String       = AppDomain.CurrentDomain.BaseDirectory
    Dim filepath As String    = fpath & "task.txt"
    Dim details As String     = My.Computer.FileSystem.ReadAllText(filepath)
    Dim splitdata() As String = Split(details, vbCrLf)

（我是强迫症，所以我排列了我的等号。那部分是完全可选的。）

它不会使代码运行得更快，但它确实使它更容易阅读！更重要的是，它减少了错误的可能性。