为什么Page.PreviousPage始终为null？

Question

I am experimenting with cross-page posting by following this MSDN article . I have this code:

CrossPagePosting1.aspx

<form id="form1" runat="server">
    <h1>Page 1</h1>
    <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox><br />
    <asp:Button ID="Button1" runat="server" Text="Button" PostBackUrl="CrossPagePosting2.aspx"/>
</form>

CrossPagePosting2.aspx

<form id="form1" runat="server">
    <h1>Page 2</h1>
    <asp:Label ID="Label1" runat="server" Text=""></asp:Label>
</form>

CrossPagePosting2.aspx.cs

protected void Page_Load(object sender, EventArgs e)
{
    TextBox TextBox1 = (TextBox)Page.PreviousPage.FindControl("TextBox1");
    Label1.Text = TextBox1.Text;
}

This code above produces a NullReferenceException at Page.PreviousPage . Why?

This is an ASP.Net 4.0 application.

It uses FriendlyUrls, which is the default.

NOTE: I do NOT want the previous page to be strongly-typed, eg using the PreviousPageType directive. According to the referenced article, this shouldn't be necessary.

Answer 1

I found that Friendly URLS might get you this problem. By default, the Web Forms template includes ASP.NET Friendly URLs.

When you use the default WebForm from visual Studio, the AutoRedirectMode is set to Permanent. This makes you request into a "GET" and since you are using Friendly URLS, you can't evaluate the PreviousPage.

Workarounds:

• If you want a "POST" action then set the AutoRedirectMode = RedirectMode.Off (this will give you PreviousPage info but only from non-Friendly-Url pages [ex: www.you.com/mypage.aspx], however this will get you an error if you try to access the Friendly-Url page [ex: www.you.com/mypage] << no .aspx).

• If you want the PreviousPage information you will have to set the previous post directive in you webpage <%@ PreviousPageType VirtualPath="~/Page1.aspx" %> OR maybe use the Server.Transfer in a OnClick Method.

Answer 2

这里的问题是由FriendlyUrls引起的，它默认安装在我正在使用的测试站点上。我禁用了FriendlyUrls，它运行良好。

Answer 3

I think following article will helps you.

http://csharpdotnetfreak.blogspot.com/2009/08/cross-page-posting-in-aspnet.html

there are two method how to use Cross Page Posting PostBack

Answer 4

The reason this is happening is simply because you did not set the postbackurl property correctly.

If CrossPagePosting2.aspx is at root of your program change postbackurl to ~/CrossPagePosting1.aspx

You do not need to add the <%@ PreviousPageType %> directive when using postbackurl property. using PreviousPage.FindControl(id) will search the form elements that are posted using postbackurl property

Answer 5

Try this

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack && PreviousPage != null)
        {
            Page page = PreviousPage;
            Label1.Text = ((TextBox)page.FindControl("TextBox1")).Text;
        }
    }