//IDBConnectInstance.cs
    class IDBConnectInstance
    {
        public static int count;

        public static int SelectRows()
        {
            count = 0;
            for (int i = 0; i < 1000; i++)
            {
                count += i;
                Thread.Sleep(100);
            }
            return count;
        }
    }

    //Form1.cs
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        CancellationTokenSource _tokenSource;
        Task task;

        private void button1_Click(object sender, EventArgs e)
        {
            _tokenSource = new CancellationTokenSource();

            var rows = 0;
            task = new Task(() =>
            {
                rows = IDBConnectInstance.SelectRows();
            }, _tokenSource.Token);
            task.ContinueWith(t =>
            {
                textBox1.Text += "Значение rows:" + rows + Environment.NewLine;
            });

            task.Start();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            _tokenSource.Cancel();
        }

        private void button3_Click(object sender, EventArgs e)
        {
            textBox1.Text += String.Format("Task status:{0}; Counter: {1}{2}", task.Status, IDBConnectInstance.count, Environment.NewLine);
        }
    }