如何解决从字符串“转换”的转换输入'Double'无效。 [英] how to solve Conversion from string "" to type 'Double' is not valid.

问题描述

Private Sub Timer30_Tick(sender As Object, e As EventArgs) Handles Timer30.Tick
Second = Second + 1
        If Second >= 30 Then
            Timer30.[Stop]()
            MessageBox.Show("Time Up ")
        End If
            Dim ScreenSize As Size = New Size(My.Computer.Screen.Bounds.Width, My.Computer.Screen.Bounds.Height)
            Dim BMP As New Bitmap(My.Computer.Screen.Bounds.Width, My.Computer.Screen.Bounds.Height)
            Dim g As System.Drawing.Graphics = System.Drawing.Graphics.FromImage(BMP)
            g.CopyFromScreen(New Point(0, 0), New Point(0, 0), ScreenSize)
        Dim DirectoryA As String = FileDirectory.Text
        Dim Frame1 As String = FileNumber.Text
        Dim img1 As String = FileType.Text
        FileNumber.Text = FileNumber.Text + 1 'i am getting errors at this point
                          'Conversion from string "" to type 'Double' is not valid.

BMP.Save(DirectoryA & Frame1 & img1)

推荐答案

您正在尝试将文本转换为数值：

You're trying to convert text to numeric value:

FileNumber.Text = FileNumber.Text + 1

而不是它，请使用：

Instead of it, use this:

'on the beginning of the function
Dim counter As Integer = 0
'later
If Int32.TryParse(FileNumber.Text, counter) Then
    counter+=1
    FileNumber.Text = counter.ToString()
End If

如需了解更多信息，请参阅：< a href =https://msdn.microsoft.com/en-us/library/f02979c7%28v=vs.110%29.aspx?cs-save-lang=1&cs-lang=vb#code-snippet- 1> Int32.TryParse方法（字符串，Int32） [ ^ ]

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