In my application, When I click the submit button on the MainForm, the Please Wait form appears, which shows untill the processing is complete. The submit button redirects to Books.cs. When someone accidentally clicks the Close(X) sign on the main form(MainForm.cs), the form is closed but the execution is not terminated, the please wait form continues to show and completes the process. It is because the processing is done on a different thread. I want to prevent the user from closing the MainForm when the processing is in progress.
My issue is that e.Cancel does not seem to be working. I can still close the mainform. I tried to set the breakpoint on the mainform form closing event, but that breakpoint never hit.

MainForm.cs

private bool isProcessing = false;

private void MainForm_FormClosing(object sender, FormClosingEventArgs e)
{
    if (isProcessing)
    {
        e.Cancel = true; // Cancel the closing event
    }
    else
    { 
     e.Cancel = false;

    }

public void SetProcessingState(bool processing)
{
    isProcessing = processing;
}

Books.cs

private async Task btnClick()
{
    MainForm mainform = new MainForm();
    
try
    {
      mainForm.SetProcessingState(true);

        cts = new CancellationTokenSource();

        PleaseWait pleasewait = new PleaseWait();
        pleasewait.Closed += PleaseWait_Closed;
 
        pleasewait.Show();

try{
        // Perform your long-running task asynchronously
        await Task.Run(() => ProcessBookstoreExcel(cts.Token));
}
catch(OperationCanceledException)
{
       cancel = true;
        pleasewait.Close();
       //showmessage
}
catch(Exception ex)
{
        pleasewait.Close();
       //showmessage
}
        pleasewait.Closed -= PleaseWait_Closed;

        pleasewait.Close();

        if(!cancel)
        MessageBox.Show("Processing Complete!");
    }
    catch(Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
    finally
    {
        cts.Dispose();

        cts = null;
        mainForm.SetProcessingState(false);

    }
}

private void PleaseWait_Closed(object sender, EventArgs e) => cts.Cancel();

private void ProcessBookstoreExcel(CancellationToken cancellationToken)
{

try{

    string path = TxtBox.Text;
    ExcelPackage.LicenseContext = LicenseContext.NonCommercial;
    var records = new List<BookRecord>();

    using (var package = new ExcelPackage(new FileInfo(path)))
    {
        ExcelWorksheet worksheet = package.Workbook.Worksheets[0];
        int rowCount = worksheet.Dimension.Rows;

        for (int row = 2; row <= rowCount; row++) // Skip header row
        {
            cancellationToken.ThrowIfCancellation();

            records.Add(new BookRecord
            {
                LineNumber = row,
                ISBN = worksheet.Cells[row, 1].Text,
                Title = worksheet.Cells[row, 2].Text,
                Stock = int.Parse(worksheet.Cells[row, 3].Text),
                Price = decimal.Parse(worksheet.Cells[row, 4].Text),
                PublisherID = int.Parse(worksheet.Cells[row, 5].Text),
                PublisherName = worksheet.Cells[row, 6].Text
            });
        }
    }

    var discrepancies = new List<Discrepancy>();
    var groupedRecords = new Dictionary<string, Dictionary<string, (decimal Price, int Stock)>>();

    foreach (var record in records)
    {
        cancellationToken.ThrowIfCancellation();
 
        var publisherKey = record.PublisherID.ToString();
        var isbnKey = record.ISBN;

        if (!groupedRecords.ContainsKey(publisherKey))
        {
            groupedRecords[publisherKey] = new Dictionary<string, (decimal Price, int Stock)>();
        }

        if (!groupedRecords[publisherKey].ContainsKey(isbnKey))
        {
            groupedRecords[publisherKey][isbnKey] = (record.Price, record.Stock);
        }
        else
        {
            var existing = groupedRecords[publisherKey][isbnKey];
            if (existing.Price != record.Price || existing.Stock != record.Stock)
            {
                discrepancies.Add(new Discrepancy
                {
                    LineNumber = record.LineNumber,
                    Publisher = record.PublisherName,
                    Title = record.Title
                });
            }
        }
    }

    WriteDiscrepancies(discrepancies);
}
catch(OperationCanceledException)
{
throw;
}
catch(Exception ex)
{
throw;
}
}

private void WriteDiscrepancies(List<Discrepancy> discrepancies)
{
    try{
    var outputPath = Path.Combine(Path.GetDirectoryName(filePath), "discrepancies.xlsx");

    using (var package = new ExcelPackage())
    {
        var worksheet = package.Workbook.Worksheets.Add("Discrepancies");
        worksheet.Cells[1, 1].Value = "LineNumber";
        worksheet.Cells[1, 2].Value = "Publisher";
        worksheet.Cells[1, 3].Value = "Title";

        for (int i = 0; i < discrepancies.Count; i++)
        {
            worksheet.Cells[i + 2, 1].Value = discrepancies[i].LineNumber;
            worksheet.Cells[i + 2, 2].Value = discrepancies[i].Publisher;
            worksheet.Cells[i + 2, 3].Value = discrepancies[i].Title;
        }

        package.SaveAs(new FileInfo(outputPath));
    }
    }
    catch
    {
        throw;
    }
}