try this
<asp:ListBox ID="lstSelect" runat="server" SelectionMode="Multiple" AutoPostBack="true" OnSelectedIndexChanged="lstSelect_SelectedIndexChanged">
<asp:ListItem Text="Mango" Value="1" />
<asp:ListItem Text="Apple" Value="2" />
<asp:ListItem Text="Banana" Value="3" />
</asp:ListBox>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
protected void lstSelect_SelectedIndexChanged(object sender, EventArgs e)
{
string items = "";
foreach (ListItem item in lstSelect.Items)
if (item.Selected)
items += item.Text + ",";
TextBox1.Text = items.Trim().TrimEnd(','); ;
}