how to retrieve data to a textbox depending on the value selected from a dropdownlist

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    switch (comboBox1.Text)
    {
        case "Transaction":
            textBox1.Text = "0";
            break;
        case "Not Transaction":
            textBox1.Text = "";
            break;
    }
}

<asp:DropDownList ID="DropDownList1" runat="server"
    onselectedindexchanged="DropDownList1_SelectedIndexChanged" >
<asp:ListItem valueForTextBox="0">Transaction</asp:ListItem>
<asp:ListItem valueForTextBox="100">Non Transaction</asp:ListItem>
</asp:DropDownList>

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
   {
      Textbox1.Text = DropDownList1.SelectedItem.valueForTextBox;

   }

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
   {
      Textbox1.Text = DropDownList1.SelectedIndex;
   }