In the aspx page write
<asp:textbox id="TextBox1" runat="server" autopostback="true" ontextchanged="TextBox1_TextChanged" xmlns:asp="#unknown" />
<asp:textbox id="TextBox2" runat="server" autopostback="true" ontextchanged="TextBox1_TextChanged" xmlns:asp="#unknown" />
<asp:textbox id="TextBox3" runat="server" xmlns:asp="#unknown" />
In Code behind page
protected void TextBox1_TextChanged(object sender, EventArgs e)
{
if(TextBox1.Text.Length != 0 && TextBox2.Text.Length != 0)
{
TextBox3.Text = (Convert.ToInt32(TextBox1.Text) + Convert.ToInt32(TextBox2.Text)).ToString();
}
}
Try above code
I have added a event name OnTextChanged which will fire when u change the text of the textbox.