Use C# to declare HiddenField's value & Javascript Variable

Question

I need to declare the 'text value' of the dropdownlist 'dropCallbackReason' into the 'ValueHiddenField' ID of the so that I can then use it as a javascript variable.

I need to be able to declare the HiddenField through C# as aswell as declaring the Javascript Variable 'callBackReason' through c# as well, any ideas how to do this through C#?

.cs page.

protected void Page_Load(object sender, EventArgs e)
    {
        HiddenField hiddenField = new HiddenField { ID = "ValueHiddenField", Value = "test" };
        theForm.Controls.Add(hiddenField);
        string script = @"function updateCallBackReason() {
                callBackReason = document.getElementById('<%=ValueHiddenField.ClientID %>').value;
                return callBackReason;
            }";
        this.Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "callBackReasonScript", script, true);

.aspx

<asp:label runat="server" ID="lblCallbackReason" AssociatedControlID="dropCallbackReason" CssClass="textLabel">Reason for callback:</asp:label>
            <asp:DropDownList runat="server" ID="dropCallbackReason" onChange="updateCallBackReason" ClientIDMode="Static" >
                <asp:ListItem Text="-- Select Reason --" Value="1"></asp:ListItem>
                <asp:ListItem Text="Booking a Test Drive" Value="6"></asp:ListItem>
                <asp:ListItem Text="Discussing a Purchase" Value="11"></asp:ListItem>
                <asp:ListItem Text="Contract Hire Quotation" Value="45"></asp:ListItem>
            </asp:DropDownList>

Answer 1

Here is how to add a HiddenField control programmatically. Note that controls cannot be added into Page.Controls directly - they should be placed into some container, like ContentPlaceholder or Panel :

HiddenField hiddenField = new HiddenField {ID = "ValueHiddenField", Value = "test"};
SomePanel.Controls.Add(hiddenField);

And here is how to register a script block:

string script = @"function updateCallBackReason() {
                    callBackReason = document.getElementById('<%=ValueHiddenField.ClientID %>').value;
                    return callBackReason;
                }";
this.Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "callBackReasonScript", script, true);

Good places to do this are either Page_Load or Page_PreRender .