cannot get the selected value of a dropdownlist which is the result of a selection of another dropdownlist

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ASP.NET

Hello it's urgent pls,

I have 2 dropdown lists:
1- ddlCity
2-ddlBranch

the purpose is to display all branches per city. so once i select a city, related branches should be selected.

now once i select a city , the related branches are uploaded into the ddlBranch dropdownlist but the problem is that i cannot recupere the SelectedValue of the selected branch.

Here is the code

1-asp.net form:

  <asp:DropDownList ID="ddlCity" runat="server" Width="120px" 
        DataSourceID="tblCity" DataTextField="city" DataValueField="cityID" AutoPostBack="true"
        onselectedindexchanged="ddlCity_SelectedIndexChanged">
    </asp:DropDownList>

<asp:DropDownList ID="ddlBranch" runat="server" Width="120px"  
        onselectedindexchanged="ddlBranch_SelectedIndexChanged"  >
    </asp:DropDownList>

<asp:TextBox ID="TextBox1" runat="server"  AutoPostBack="True" Visible="true"></asp:TextBox>

<asp:SqlDataSource ID="tblCity" runat="server" 
              ConnectionString="<%$ ConnectionStrings:SmartBookingEngineConn %>" 
              SelectCommand="SELECT [cityID],[city] FROM [tblCity]"></asp:SqlDataSource>
          <asp:SqlDataSource ID="tblBranch" runat="server" 
              ConnectionString="<%$ ConnectionStrings:SmartBookingEngineConn %>" 
              SelectCommand="SELECT [BranchID],[Branch] FROM [tblBranch]">
          </asp:SqlDataSource>

2-C# code:

 protected void Page_Load(object sender, EventArgs e)
 {
    if (!IsPostBack)
            {
                

                //dropdown branches per city
                ddlCity.DataBind();
                dtbranch = null;
                string selectedIDCity = ddlCity.SelectedValue;
                string mysqlSelectBranch = "select Branch,BranchID from     viewBranchPerCity where CityID=" + selectedIDCity;
                dtbranch = db.sqlServer.SelectRecord(mysqlSelectBranch);
                ddlBranch.DataSource = dtbranch;
                ddlBranch.DataValueField = "BranchID";
                ddlBranch.DataTextField = "Branch";
                ddlBranch.DataBind();
              

               TextBox1.Text = ddlBranch.SelectedValue;
            }

 }


 protected void ddlCity_SelectedIndexChanged(object sender, EventArgs e)
        {
           
               
                //loading branches per city
                ddlBranch.DataBind();
                dtbranch = null;
                string selectedIDCity = ddlCity.SelectedValue;
                string mysqlSelectBranch = "select Branch,BranchID from viewBranchPerCity where CityID=" + selectedIDCity;
                dtbranch = db.sqlServer.SelectRecord(mysqlSelectBranch);
                ddlBranch.DataSource = dtbranch;
                ddlBranch.DataValueField = "BranchID";
                ddlBranch.DataTextField = "Branch";
                ddlBranch.DataBind();
                

        }

protected void ddlBranch_SelectedIndexChanged(object sender, EventArgs e)

        {
         TextBox1.Text = ddlBranch.SelectedValue;          
        }

the problem is that Textbox1 diplays the right branch id only while loading the page ; after that this value is not changeable.

thanks alot for your help.