.NET Process.Kill () in a safe way
I control a creaky old FORTRAN simulator from the VB.NET GUI using redirected I / O to communicate with the simulator executable. The GUI displays a status window with a progress bar, estimated time, and a STOP button ( Button_Stop
).
Now I want to Button_Stop
terminate the simulator process immediately. The obvious way to do this is to call Kill()
on a Child
Process object . This throws an exception if it was executed after the process exited, but I can check if the process exited before trying to kill it, right?
OK, so when the button is clicked, I do the following:
If Not Child.HasExited Then
Child.Kill()
Button_Stop.Enabled = False
End If
However, what happens if the process ends with a test and a call to Kill()
? In this case, I am getting an exception.
The next thing that had to happen was what I can execute Button_Stop.Enabled = False
in the event handler Process.Exited
and thus prevent the call Child.Kill()
in the handler Button_Stop.Clicked
. But since the handler Process.Exited
is called on a different thread, that still leaves the following possible interleaving:
- The exit of the child process.
-
Process.Exited
fires, callsInvoke
to scheduleButton_Stop.Enabled = False
- User clicks
Button_Stop
, launchesChild.Kill()
-
Button_Stop.Enabled = False
actually happens.
In step 3, an exception will be thrown.
How can I kill a process without any race conditions? Am I thinking about this completely wrong?
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Just catch the exception and disable the button in finally
:
Try
Child.Kill()
Catch ex As Exception
MsgBox(ex.ToString())
Finally
Button_Stop.Enabled = False
End Try
Instead of catching all types of exceptions, of course, it would be better to only catch InvalidOperationException
and Win32Exception
, since they are thrown if the process exits or has already terminated.
You are probably thinking that it is "bad" for exceptions to occur in a program and that you must design your program to eliminate exceptions altogether. However, there are different types of exceptions and exception handling, some of which are bad design decisions and others - like this one - are mandatory because the reason for the exception (i.e. terminating another process) is beyond control.
If you want to read further, I recommend Eric Lipperts' posts on different kinds of exceptions:
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You can P / Invoke in TerminateProcess
, which won't throw if the process has already exited:
Sub Main()
Dim p = Process.Start("C:\Windows\system32\notepad.exe")
Thread.Sleep(1000)
TerminateProcess(p.Handle, 0)
TerminateProcess(p.Handle, 0) ''# This call won't throw, it will just fail silently.
End Sub
<DllImport("kernel32.dll", SetLastError:=True)>
Private Function TerminateProcess(ByVal hProcess As IntPtr, ByVal uExitCode As UInteger) As <MarshalAs(UnmanagedType.Bool)> Boolean
End Function
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