prevent writing all the letters except (the digits and (.))

Hi Iamast Iamasat,

In your question have no defined which application are used. I hope are understand what i say.
if you are working on web application you need the javascript or you are using the window application create the key_press event.

What you think about this code

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{ if (!char.IsControl(e.KeyChar) && !char.IsDigit(e.KeyChar) && e.KeyChar != '.')
{ e.Handled = true; }   // only allow one decimal point
 if (e.KeyChar == '.' && (sender as TextBox).Text.IndexOf('.') > -1)
 { e.Handled = true; } }