Which kind of logic to do?

Question

Hi!


I need to understand one logic.
I have four txtboxes (txtbx1, txtbx2, txtbx3 and txtbx4) that should enter any decimal. But I have two caracters "-" and ".."
What I need? I need to see appearing in the textbox4 the number "0" (ONLY when this two numbers "-" and ".." are inserted).

see what I try to do but its doesnt make sense:

 private void txtDec1_1_mensal_TextChanged(object sender, EventArgs e)

        {

            if txtbx1.Text 'and ' txtbx2.Text 'and' txtbx3.Text  = '-' then txtbx4.Text = '0';

        }