title: ASP.NET Hidden Field Value not modified on callback
body: <p>I'm changing the value of an asp:HiddenField during callback.</p>
  
  <pre><code>&lt;asp:HiddenField runat="server" ID="hiddenField" Value="old value" /&gt;
  
  private void Page_Load(object sender, EventArgs e)
  {
     if (IsCallBack)
     {
        hiddenField.Value = "new value";
     }
  }
  </code></pre>
  
  <p>but after the callback, the value of the hidden field is "old value". When debugging, I can see that the value of the hidden field is "new value". I would like to know why it is retaining the original value. Thanks.</p>