运行时错误执行WaitForExit（）

Question

我试图执行下面的代码，但调试失败在myProcess.WaitForExit()

Sub BtnNowClick(sender As Object, e As EventArgs) 

    Dim myProcess As Process 
    Dim processFile As String = dlgFolder.SelectedPath 
    Dim pyLocationDel As String = Path.Combine(dlgFolder.SelectedPath, "pdfmerge.py") 

    Directory.SetCurrentDirectory(dlgFolder.SelectedPath) 

    myProcess.Start("pdfmerge.py") 
    myProcess.WaitForExit()  
    System.IO.File.Delete(pyLocationDel) 


End Sub 

基本上，我想要的东西，是检测一个过程的结束是和如果过程完成，请删除特定的文件。

我得到试图执行代码时此错误：

System.NullReferenceException: Object reference not set to an instance of an object. 
    at lgaPDF.MainForm.BtnNowClick(Object sender, EventArgs e) in C:\Documents and Settings\student3\My Documents\SharpDevelop Projects\lgaPDF\lgaPDF\MainForm.vb:line 184 
    at System.Windows.Forms.Control.OnClick(EventArgs e) 
    at System.Windows.Forms.Button.OnClick(EventArgs e) 
    at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent) 
    at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks) 
    at System.Windows.Forms.Control.WndProc(Message& m) 
    at System.Windows.Forms.ButtonBase.WndProc(Message& m) 
    at System.Windows.Forms.Button.WndProc(Message& m) 
    at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m) 
    at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m) 
    at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam) 
    at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg) 
    at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData) 
    at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context) 
    at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context) 
    at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.OnRun() 
    at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.DoApplicationModel() 
    at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.Run(String[] commandLine) 
    at lgaPDF.My.MyApplication.Main(String[] Args) in 17d14f5c-a337-4978-8281-53493378c1071.vb:line 81 

Answer 1

试试这个：

Sub BtnNowClick(sender As Object, e As EventArgs) 

    Dim processFile As String = dlgFolder.SelectedPath 
    Dim pyLocationDel As String = Path.Combine(dlgFolder.SelectedPath, "pdfmerge.py") 

    Directory.SetCurrentDirectory(dlgFolder.SelectedPath) 

    Dim myProcess As Process = Process.Start("pdfmerge.py") 
    myProcess.WaitForExit()  
    System.IO.File.Delete(pyLocationDel) 


End Sub 

Answer 2

Dim myProcess As NEW Process 

myprocess只宣布，从不实例