ViewState overwrites changes on dynamically added UserControl (in Placeholder) on PostBack

Question

In my ASP.NET application - having ViewState enabled - I can reset inputs by simply setting them again. Here is a basic simple sample that works:

_TEST.aspx

<asp:ScriptManager runat="server" />        
<asp:DropDownList ID="ddl" runat="server" DataSource='<%# new string[] { "A", "B", "C" } %>' />
<asp:Button ID="b" runat="server" UseSubmitBehavior="false" OnClick="b_Click" Text="Test 1" />

_TEST.aspx.cs

protected void Page_Load(object sender, EventArgs e)
{
  this.DataBind();
  this.ddl.SelectedValue = "B";
}

protected void b_Click(object sender, EventArgs e)
{
  Console.WriteLine("Updated");
}

In this example ddl will be bound with "A", "B" and "C" on Page_Load and it will always select "B". So when you change the selection and submit, it will be reset. Imagine this is used to reset some controls on specific conditions.

---

In my case I need this kind of behavior on a dynamically loaded UserControl that is added to a Placeholder. Because a Placeholder does not persist the content I need to recreate the controls on every postback. The ViewState seems to be okay with it. Here is a sample:

_TEST.aspx

<asp:ScriptManager runat="server" />
<asp:PlaceHolder ID="ph" runat="server" />

_TEST.aspx.cs

protected void Page_Load(object sender, EventArgs e)
{
  _TESTUC uc = (_TESTUC)this.Page.LoadControl("~/Controls/_TESTUC.ascx");
  this.ph.Controls.Add(uc);
}

_TESTUC.ascx

<asp:DropDownList ID="ddl" runat="server" DataSource='<%# new string[] { "A", "B", "C" } %>' />
<asp:Button ID="b" runat="server" UseSubmitBehavior="false" OnClick="b_Click" Text="Test 2" />

_TESTUC.ascx.cs

protected void Page_Load(object sender, EventArgs e)
{
  this.DataBind();
  this.ddl.SelectedValue = "B";
}

protected void b_Click(object sender, EventArgs e)
{
  Console.WriteLine("Updated");
}

Again, on every postback I want to reset the selection of the DropDownList for test purposes, but interestingly it remembers my selection.

What can I do to fix this?

Answer 1

While writing this question I came to the solution but decided to post it for other peoples, anyways.

It seems ASP.NET will load the ViewState between Page_Init and Page_Load in controls that are defined in ASPX, while it loads the ViewState between Page_Load and Page_LoadComplete in controls that are loaded in code-behind.

Setting the values in Page_LoadComplete solves the issue:

protected void Page_Init(object sender, EventArgs e)
{
  this.Page.LoadComplete += new EventHandler(this.Page_LoadComplete);
}

protected void Page_Load(object sender, EventArgs e)
{
  this.DataBind();
}

protected void Page_LoadComplete(object sender, EventArgs e)
{
  this.ddl.SelectedValue = "B";
}

protected void b_Click(object sender, EventArgs e)
{
  Console.WriteLine("Updated");
}