crystal reports 2011 qr code VIEWSTATE AND SCRIPTING in C#.NET

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CHAPTER 4 VIEWSTATE AND SCRIPTING
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2.0 Solution: Cross-Page Postbacks
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In ASP.NET 2.0, the ability to post to a different Web Form has been added. However, rather than having ViewState restored by the new target page, there s a new property of the Page type named PreviousPage. This property is also an instance of the Page class, and is an instance of the Page that did the cross-page postback (see Figure 4-6).
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Figure 4-6. Cross-page postback object reference Typically, you can set the action attribute of a Form element in your markup. In ASP .NET, however, the action attribute is always set to the page that s rendering the form. You accomplish cross-page postbacks with a bit of client-side script that s rendered by the Framework. To generate this script you need to set the PostBackUrl attribute of a Button Web Control. Here s a simple example of a page with a Button that posts to a different Web Form: <%@ Page Language="VB" CodeFile="CrossPage.aspx.vb" Inherits="CrossPage" %> <html> <head id="Head1" runat="server"> <title>Cross Poster</title> </head> <body> <form id="form1" runat="server"> Make a page that has:<br /> Background: <asp:TextBox Runat=server ID=txtBgcolor /> <br /> And displays: <asp:TextBox Runat=server ID=txtText /> <br /> In the color of:<asp:TextBox Runat=server ID=txtTextColor /> <br /> <asp:Button Runat=server ID=btn1 Text=Submit PostBackUrl="~/CrossPage2.aspx" /> </form> </body> </html>
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CHAPTER 4 VIEWSTATE AND SCRIPTING
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You must give types careful consideration when one page does a postback to another. The biggest reason for this is that the page that does the posting is exposed on the target page via the PreviousPage attribute. This is an instance of a Page object, which is all well and good, but when it comes time to do something with the information on the Page that did the posting, you must make some assumptions about the that page. You may even find yourself casting the PreviousPage property into the type of the posting page. In doing so, you re creating a tight coupling between these pages, which may minimize reuse of the page that gets posted to, something you should be aware of during design. Let s take a look at a few ways you can get to the information from the posted page. Perhaps the easiest is to use the FindControl method of the Page type. Here s some code you can use from the page that gets posted to (CrossPage2.aspx): string bodyColor = ((TextBox)this.PreviousPage.FindControl("txtBgcolor")).Text; Here a string is declared and set equal to the Text property of a TextBox. To obtain the reference to the TextBox, you pass its ID (as declared on CrossPage.aspx) to the FindControl method. Since FindControl returns a reference to a System.Web.UI.Control object (the base class of TextBox), you must explicitly cast it to the TextBox type in order to drill into the Text property. This coupling doesn t mean that only CrossPage.aspx can use CrossPage2.aspx. The coupling it creates is this: Any page posting to CrossPage2 must contain a TextBox named txtBgcolor. This isn t necessarily a problem, and it doesn t mean CrossPage2 cannot be reused; it s just something you need to take into account during design. This method of accessing the previous page is very late bound. If txtBgcolor doesn t exist, this will not show up as a problem until runtime. To create a much looser coupling, you could check the return value of FindControl for nulls (the value returned when the control isn t found) and deal with this case is a separate branch of code: TextBox txt = (TextBox)this.PreviousPage.FindControl("txtBgcolor"); string bodyColor; if (txt == null) bodyColor = "Lime"; else bodyColor = txt.Text; FindControl does a recursive descent of the page s control tree, which can be expensive if there are a lot of controls to search through and/or you re seeking references to many different controls. You re also doing a run-time type coercion, which could also fail if someone gives a control (like an input of type text) an ID of txtBgcolor. When a stronger-typed reference to the previous page is required, you can use the PreviousPageType directive. Let s take a look at the markup for CrossPage2.aspx: <%@ Page Language="C#" CompileWith="CrossPage2.aspx.cs" ClassName="CrossPage2_aspx" %> <%@ PreviousPageType VirtualPath="~/CrossPage.aspx" %>
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