gistfile1.txt

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

    private async void button1_Click(object sender, EventArgs e)
    {
        await Task.Run(() =>
        {
            for (int i = 0; i < 1000000; i++)
            {
                textBox1.Text = i.ToString();
            }
        });
    }
}