Demo Thousand Separator

DemoThousandSeparator.cs

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

    private void Form1_Load(object sender, EventArgs e)
    {
        txtThousandSeparator.TextAlign = HorizontalAlignment.Right;
    }

    private void SetThousandSeparator(TextBox txt)
    {
        if (txt.Text.Length > 0)
        {
            try
            {
                string s = txt.Text.Replace(",", "");

                int i = int.Parse(s);
                txt.Text = string.Format("{0:###,###,###}", i);

                txt.SelectionStart = txt.Text.Length;
            }
            catch
            {
            }
        }
    }

    private bool NumericOnly(System.Windows.Forms.KeyPressEventArgs e)
    {
        string strValid = "0123456789.";

        if (strValid.IndexOf(e.KeyChar) < 0 && !(e.KeyChar == Convert.ToChar(Keys.Back)))
        {
            return true; // not valid
        }
        else
        {
            return false; // valid
        }
    }

    private void txtThousandSeparator_TextChanged(object sender, EventArgs e)
    {
        SetThousandSeparator((TextBox)sender);
    }

    private void txtThousandSeparator_KeyPress(object sender, KeyPressEventArgs e)
    {
        e.Handled = NumericOnly(e);
    }

}