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CHAPTER 4 VIEWSTATE AND SCRIPTING
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This makes the need for possibly checking the IsPostback property in the page load obvious enough, but when will the other Boolean, IsCrossPagePostback, ever be true In order to see the answer, you have to think about what happens on CrossPage2 when CrossPage posts to it. Under the hood, a new instance of CrossPage is created. Even though there has not been a request for this page, and even though the page won t be expected to render any output, the page s life cycle is still kicked off; and so the page Load event still fires. You can clearly see this behavior by putting a break point in the page Load trap of CrossPage. The debugger stops there the first time the page is requested, and stops there again when the user clicks the button, even though you re requesting a different page. This is important to realize for a couple of reasons. Here you re short-circuiting logic that you would rather not see get executed on the postback. In this case, there would be no harm in letting the code execute, but if you were doing a read from the database, this would be a much bigger deal. You could also take some specific action to prepare for CrossPage2 s processing. Here s the complete code-behind for CrossPage2.aspx, which references CrossPage using the different techniques we ve discussed: protected void Page_Load(object sender, EventArgs e) { //Straight reference to textbox that would break if //posting page does not have a textbox named txtText string output = string.Format("<h1>{0}</h1>", ((TextBox)this.PreviousPage.FindControl ("txtText")).Text); //This is safer code, that accounts for txtBgcolor //not being present and sets a default value TextBox txt = (TextBox)this.PreviousPage.FindControl("txtBgcolor"); string bodyColor; if (txt == null) bodyColor = "Lime"; else bodyColor = txt.Text; body.Attributes.Add("bgcolor", bodyColor); //Strong typed reference to previous page type //can leverage a public property lblOutput.ForeColor = PreviousPage.outputColor; lblOutput.Text = output; //ViewState is exposed via strong type property as well lblOutput.Text += "<BR>Value from ViewState: " + this.PreviousPage.SomeValue; }
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CHAPTER 4 VIEWSTATE AND SCRIPTING
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In summary, this code is using the late-bound FindControl method to access a couple of field values, and then using the early-bound, property-based method to access a couple of others. The late-bound method creates a looser coupling between the page types, but could lead to more run-time errors and will perform more slowly. The early-bound method creates a tight coupling between the pages, but provides compile-time type checking, better encapsulation (and so a better object-oriented design), and improved performance.
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1.x Problem: Fat Serialization
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The Framework Machine.config file encodes and encrypts the ViewState value before packing it away as the value of the hidden input and sending it to the client. This is nice because it keeps the value secure, but the Base64 encoding can really cause the size of the field to bloat. Let s examine a simple example of binding the authors table to a DataGrid using version 1.1 of ASP .NET. The authors table contains about 4K of data. You can see this opening a connection to the pubs database with Query Analyzer, selecting star from the authors table, and viewing the results as text (see Figure 4-7). When you write the results out to disk, the resulting file is 4K in size. In order to avoid any overhead of metadata for the report file, copy the data from the Query Analyzer window and paste it into Notepad. Write this out to disk as a file named au_data.txt.
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Figure 4-7. The data from the authors table in Query Analyzer
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CHAPTER 4 VIEWSTATE AND SCRIPTING
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You ll marshal all of this data to the web server and transform it into HTML using the following code from the Page_Load of a Web Form: private void Page_Load(object sender, System.EventArgs e) { SqlConnection cn = new SqlCnnection("server=.;database=pubs;uid=sa;pwd="); SqlCommand cm = new SqlCommand("select * from authors", cn); DataGrid DataGrid1 = new DataGrid(); this.FindControl("form1").Controls.Add(DataGrid1); cn.Open(); DataGrid1.DataSource = cm.ExecuteReader(); DataGrid1.DataBind(); cn.Close(); }
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Note In Version 2.0 of the Framework, Microsoft has shipped the GridView control. This Web Control is
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the replacement for and enhancement of the existing DataGrid. Because GridView has non-backwards compatible changes, and because Microsoft is making an effort to maintain backwards compatibility, it created a new control rather than enhancing the functionality of DataGrid. We use DataGrid here, since this example demonstrates the ViewState size in version 1.x. GridView would, however, work exactly the same for this example in 2.0.
You ll now compare the relative sizes of the pieces and parts of the output you ve generated. Choose View Source from the browser. Choose File Save As from Notepad and write the entire page out to disk as a file named au_page. Highlight all of the markup for the authors table, starting with <table> and ending with </table>. Open a new text document and copy and paste the table markup into it. Write this out to disk as a file named au_markup.txt. Repeat the process, this time copying the value of the hidden input named __VIEWSTATE. Write this out to disk as a file named au_viewstate.txt. Then, just for fun, add this line of code to the method above: DataGrid1.EnableViewState = false; Refresh the browser, view the source again, and save this one as au_page_noviewstate.txt. What should the relative sizes of these files be You have a result set from the database saved as au_data.txt. You ve transformed it into markup, saved as au_markup.txt. This has also been squirreled away in ViewState by DataGrid, so it can restore its DataGridItem collection should a postback occur. This is saved as au_ViewState.txt. Since you re dealing with three representations (data, markup, hashed values) of the same basic data, they should be about the same size, right Not quite (see Figure 4-8).
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