How to change the value of input hidden on server-side - asp.net

Question

I have a form with hidden fields:
<form id="Form1" runat="server" style="width: 100%; height: 100%; overflow: hidden" onsubmit="return false;">
<div>
<input type="hidden" runat="server" id="TrackColors" value=""/>
<input type="hidden" runat="server" id="Relogin" value=""/>
</div>
</form>
After Page_Load() on the server-side is called function:
protected void SomeFunction()
{
Dictionary<int, int> trackColors = new Dictionary<int, int>();
if (!String.IsNullOrEmpty(TrackColors.Value))
trackColors = ReadValues(TrackColors.Value);
//if value is null or empty it's assigned to a different
TrackColors.Attributes["value"] = FormValues(trackColors); //FormValues() return string
//change is visible
}
string FormValues(Dictionary<int, int> values)
{
string result = "";
if (values == null || values.Count == 0)
return result;
foreach (KeyValuePair<int, int> p in values)
result += p.Key + "##" + p.Value + "^^";
result = result.TrimEnd('^');
return result;
}
If I change the selected field of ComboBox, the function is called:
<dx:ASPxTextBox ID="ColorTrackCarID" Visible="false" Text='<%# Eval("CarId") %>' />
<dx:ASPxComboBox ID="ASPxComboBox1" runat="server" SelectedIndex='<%# Eval("TrackColor") %>'
ValueType="System.String" Width="30" ShowImageInEditBox="true"
ondatabinding="ASPxComboBox1_DataBinding">
<ClientSideEvents SelectedIndexChanged="function (s,e) {
if (window.TrackColorChanged != null)TrackColorChanged(s,e); }" />
</dx:ASPxComboBox>
function TrackColorChanged(s, e) {
var TrackColors = document.getElementById('TrackColors');
if (TrackColors == null || TrackColors.value == "")
return values;
//values is always emply
}
I understand the value of the form fields are not passed back to the client-side. The question is: How to pass these values ​​back?
And if I change the value on the server-side in Page_Load (), then the client can see everything, that is,
protected void Page_Load(object sender, EventArgs e)
{
TrackColors.Attributes["value"] = "bla-bla-bla";
//All changes are visible on the client-side
}
Thank you for your attention.

Answer 1

To make it even easier, replace your hidden fields with the control:
<asp:HiddenField id="X" runat="server" />
Which you can set the value on it directly:
X.Value = "XYZ";
This value can be passed from client to server, and vice versa, and works very easily. Not that you can't use a server-side input, but HiddenField handles a lot of that for you.
EDIT: Also, are you sure you're not overwriting the value? If you are doing this:
protected void Page_Load(object sender, EventArgs e)
{
TrackColors.Attributes["value"] = "bla-bla-bla";
//All changes are visible on the client-side
}
This will always change the value to "bla-bla-bla". You would want to wrap it in if (!Page.IsPostback) if you initialize it on page load.