Остановить выполнение приложения при закрытии формы C#

В общем, с вашим кодом есть некоторые проблемы, однако я остановлюсь на вашем вопросе.

Чтобы решить проблему, вам необходимо реализовать логику удаления задачи, если так решит пользователь, и для этого вы можете использовать CancellationToken.

Итак, вы объявляете CancellationTokenSource в классе Home.cs, который вы будете использовать в случае, если форма PleaseWait закроется до завершения задачи, чтобы отменить ее.

Имейте в виду, что я использовал метод ThrowIfCancellationRequested, который генерирует исключение, хотя вы также можете использовать свойство IsCancellationRequested.

Home.cs

CancellationTokenSource cts;

private async Task btnClick()
{
    try
    {
        cts = new CancellationTokenSource();

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

        // Perform your long-running task asynchronously
        await Task.Run(() => ProcessBookstoreExcel(cts.Token));

        pleasewait.Closed -= PleaseWait_Closed;

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

        cts = null;
    }
}

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

private void ProcessBookstoreExcel(CancellationToken cancellationToken)
{
    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);
    MessageBox.Show("Processing complete. Check the output file for discrepancies.");
}

private void WriteDiscrepancies(List<Discrepancy> discrepancies)
{
    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));
    }
}

private void PleaseWait_FormClosing(object sender, FormClosingEventArgs e)
{
    var x = MessageBox.Show("Are you sure you want to really exit ? ", 
                            "Exit", MessageBoxButtons.YesNo, MessageBoxIcon.Question);

    if (x == DialogResult.No) 
    {
        e.Cancel = true;
    }
    else
    {
        cts.Cancel();

        e.Cancel = false;
    }
}