ViewState在PostBack上覆盖动态添加的UserControl（占位符）上的更改(ViewState overwrites changes on dynamically added Use

ViewState在PostBack上覆盖动态添加的UserControl（占位符）上的更改(ViewState overwrites changes on dynamically added UserControl (in Placeholder) on PostBack)

在我的ASP.NET应用程序中 - 启用了ViewState - 我可以通过简单地设置它们来重置输入。 这是一个基本的简单示例：

_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"); }

在这个例子中， ddl将在Page_Load上与“A”，“B”和“C”绑定，并且它将始终选择“B”。 因此，当您更改选择并提交时，它将被重置。 想象一下，这用于重置某些特定条件的控件。

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在我的情况下，我需要在动态加载的UserControl上添加一个Placeholder这种行为。 由于Placeholder不会持续保留内容，因此我需要在每次回发中重新创建控件。 ViewState似乎没有问题。 这是一个示例：

_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"); }

同样，在每次回发时，我想重置DropDownList的选择以进行测试，但有趣的是它记住了我的选择。

我能做些什么来解决这个问题？

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.

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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?

最满意答案

在写这个问题时，我找到了解决方案，但决定将它发布给其他人，无论如何。

看起来ASP.NET会在Page_Init和Page_Load之间加载在ASPX中定义的ViewState之间的ViewState ，而在加载代码隐藏的控件中的Page_Load和Page_LoadComplete之间加载ViewState 。

设置Page_LoadComplete的值可以解决问题：

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"); }

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"); }