Part V in Visual C#

Drawer QR Code in Visual C# Part V

Note What s the difference between RegisterXXX methods in the ScriptManager control and the page s ClientScript object, which is an instance of the ClientScriptManager class The registration methods of ClientScriptManager and ScriptManager serve the same purpose but in radically different scenarios.

You need to use the ScriptManager s methods only if you need to emit script code during an AJAX partial rendering postback operation. An AJAX partial-rendering postback operation is processed by the run time as usual, except for the rendering stage. At this time, the markup is generated and any registered script is emitted. Because during AJAX postbacks the ScriptManager is responsible for the markup rendering, it s the ScriptManager that needs to know about registered scripts to emit. If you stick to using ClientScriptMananager s methods in an AJAX page, you risk the possibility that no script will be emitted during the refresh of an updatable panel. As a result, a portion of your page might display strange behaviors.

TABLE 20-3

Occurs when an exception goes unhandled on the server during an asynchronous postback. Occurs when the user clicks the browser s Back or Forward button. Occurs when the ScriptManager control is going to resolve a composite script reference. Occurs when the ScriptManager control is going to resolve a script reference.

These events are much more than mere notifications of something that has happened on the server. Both give you good chances to intervene effectively in the course of the application. For example, by handling the ResolveScriptReference event, you can change the location from where the script is going to be downloaded on the client:

protected void ResolveScript(object sender, ScriptReferenceEventArgs e) { // Check Path or Name on the e.Script object based on what you ve put in Scripts. // Next, you specify the real file to load if (String.Equals(e.Script.Path, "personal.js", StringComparison.OrdinalIgnoreCase)) e.Script.Path = "person.js"; }

By handling the AsyncPostBackError event, you can edit the error message being returned to the client during a partial rendering operation. Here s an example:

protected void AsyncPostBackError(object sender, AsyncPostBackErrorEventArgs e) { ScriptManager sm = sender as ScriptManager; if (Request.UserHostAddress == "127.0.0.1") { sm.AsyncPostBackErrorMessage = String.Format( "<b>An error occurred. <br/>{0}<b>", e.Exception.Message); } else { sm.AsyncPostBackErrorMessage = String.Format( "<b>An error occurred. <br/>{0}<b>", "Please contact your Web master."); } }

What if you want to redirect the user to an error page instead In this case, you configure the page to use the traditional error-handling mechanism for ASP.NET pages. You configure the <customErrors> section in the web.config file and indicate HTML error pages to reach in case of specific errors. This behavior can be disabled by setting to false the value of the AllowCustomErrorRedirects property of the ScriptManager object. Note When an exception is thrown during a partial rendering operation, the HTTP request

returns a regular HTTP 200 status code, but instead of including the updated markup, it includes a message in which a flag indicates the success or failure of the operation. In addition, the message includes the full description of the error or the updated markup. In case of error, the client-side default error handler throws a JavaScript exception passing the error message as an argument.

Only one instance of the ScriptManager control can be added to an ASP.NET AJAX page. However, there are two ways in which you can do this. You can add it directly on the page using the <asp:ScriptManager> tag or indirectly by importing a component that already contains a script manager. Typically, you can accomplish the second alternative by importing a user control, creating a content page for a master page, or authoring a nested master page. What if a content page needs to add a new script reference to the manager In this case, you need a reference to the script manager. Although it s defined in the master page (or in a user