（调用）无法访问已处置的对象

Question

I was trying the FileTransferManager library, which is used to transfer files/folders. When I close my form while a copy is in progress sometimes I get this error:

Cannot access a disposed object. Object name: 'formMain'.

And the weird part as I mentioned earlier, I don't always get this error. Sometimes, the form closes without any issues.

I tried to check if formMain is actually disposed using this.IsDisposed at the time when the error occurs, but it always returns false . I even tried to put that check in the delegate but it didn't help.

Here's the StackTrace:

   at System.Windows.Forms.Control.MarshaledInvoke(Control caller, Delegate method, Object[] args, Boolean synchronous)
   at System.Windows.Forms.Control.Invoke(Delegate method, Object[] args)
   at DirCopyTest.formMain.SetProgressBarValue(ProgressBar progressBar, Int32 val) in C:\Users\me\Documents\Visual Studio 2019\Projects\FileTransferManagerDemo\formMain.cs:line 73
   at DirCopyTest.formMain.TransferProgressChanged(TransferProgress tp) in C:\Users\me\Documents\Visual Studio 2019\Projects\FileTransferManagerDemo\formMain.cs:line 60
   at IOExtensions.FileTransferManager.<>c__DisplayClass7_0.<CopyFileWithProgress>b__0(Int64 size, Int64 transferred, Int64 streamSize, Int64 bytesTransferred, UInt32 number, CopyProgressCallbackReason reason, IntPtr file, IntPtr destinationFile, IntPtr data)

This is the specific line that causes the error:

Invoke(new SetProgressBarCallback(SetProgressBarValue), new object[] { progressBar, val });

And here is the full code:

    CancellationTokenSource canceller;

    private async void buttonCopy_Click(object sender, EventArgs e)
    {
        canceller = new CancellationTokenSource();
        Action<TransferProgress> progress = new Action<TransferProgress>(TransferProgressChanged);

        await FileTransferManager.CopyWithProgressAsync(source, destination, progress, canceller.Token, false, true);
    }

    void TransferProgressChanged(TransferProgress tp)
    {
        if (!canceller.IsCancellationRequested)
           SetProgressBarValue(progressBar1, (int)tp.Percentage);
    }

    delegate void SetProgressBarCallback(ProgressBar progressBar, int val);

    private void SetProgressBarValue(ProgressBar progressBar, int val)
    {
        if (progressBar.InvokeRequired)
            Invoke(new SetProgressBarCallback(SetProgressBarValue), new object[] { progressBar, val });
        else
            progressBar.Value = val;  
    }

    private void formMain_FormClosing(object sender, FormClosingEventArgs e)
    {
        canceller.Cancel();
    }

Answer 1

What you have here is a good old-fashioned race condition between

if (!canceller.IsCancellationRequested)

and

progressBar.Value = val; 

You could do a final check just before the later, it will shorten the window but not mitigate it.

if (!canceller.IsCancellationRequested)
   progressBar.Value = val;  

And / or you could just wrap the Value = val update in a try and catch on ObjectDisposedException and write-it-off