private void startCalculcationButton_Click(object sender, EventArgs e) {
    int number;
    if (int.TryParse(this.numberTextBox.Text, out number)) {
    this.calculationResultLabel.Text = "(computing)";
        Task.Factory.StartNew(() => {
            int result = LongCalculation(number);
            this.calculationResultLabel.BeginInvoke(new ThreadStart(() => {
                this.calculationResultLabel.Text = result.ToString();
            }));
        });
    }
}