vb.net code 39 generator open source Canceling Events in .NET framework

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Canceling Events
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The WizardNavigationEventArgs structure also contains a read/write Boolean property named Cancel. If you set this property to True, you just cancel the transition to the page being performed. The following code shows how to prevent the display of the next step if the user is on the start page and types in a last name of Esposito:
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void OnNext(object sender, WizardNavigationEventArgs e) { if (e.CurrentStepIndex == 0 && LastName.Text == Esposito ) { e.Cancel = true; return; } }
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8 Rich Controls and Services
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You can cancel events from within any event handler and not just from the NextButtonClick event handler. This trick is useful to block navigation if the server-side validation of the input data has failed. In this case, though, you re responsible for showing some feedback to the user.
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Finalizing the Wizard
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All wizards have some code to execute to finalize the task. If you use the ASP.NET 2.0 Wizard control, you place this code in the FinishButtonClick event handler. Figure 8-5 shows the final two steps of a wizard that completed successfully.
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void OnFinish(object sender, WizardNavigationEventArgs e) { // Add the employee if (!AddEmployee()) { e.Cancel = true; return; } // Show a final message if you don t have a Completed step FinalMsg.Text = The operation completed successfully."; }
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Figure 8-5 The final two steps of a wizard that has completed successfully
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F08DR05
If the wizard contains a Complete step, that page is displayed after the Finish button is clicked and the final task has completed successfully. If something goes wrong with the update, you should cancel the transition to prevent the Complete page from appearing, and give the user another chance.
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Part III
Application Services
Generating Dynamic Images
In ASP.NET 1.x, image generation and manipulation are possible, but the implementation costs are shifted to the programmer. By writing some code, you can reference dynamically generated images in your ASP.NET pages and display bytes stored in disk files, database fields, and memory. Whatever the source of the bytes, you can link an image to a page only through a URL-based <img> tag. If the image has to be dynamically generated (e.g., fetched from a database), you must reference it through an ad hoc HTTP handler or a page. The following code snippet shows the typical way of retrieving database images in ASP.NET 1.x:
<img src= <%# photo.aspx id= + theID.Text %> >
The image element is bound to the output of a tailor-made ASP.NET page that retrieves and returns the bytes of an image. The image is retrieved based on the input parameters specified on the URL query string. The structure of the photo.aspx page is a sort of boilerplate code:
<script runat="server"> void Page_Load(object sender, EventArgs e) { // Prepare the SQL command // TO DO // Execute the command and get the image bytes SqlConnection conn = new SqlConnection(conn_string); SqlCommand cmd = new SqlCommand(cmdText, conn); conn.Open(); byte[] img = (byte[]) cmd.ExecuteScalar(); conn.Close(); // Output the image bytes Response.ContentType = image/jpeg"; Response.OutputStream.Write(img, 0, img.Length); } </script>
The code is nearly identical in classic ASP, with the obvious changes related to the use of ADO.NET classes. You end up maintaining multiple image manipulation pages one per each logical group of images and image data stores. What s missing in classic ASP and ASP.NET 1.x is an ad hoc image generation service and more powerful image controls. ASP.NET 2.0 fills that gap.
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8 Rich Controls and Services
Note In ASP.NET 1.x, using a made-to-measure HTTP handler
instead of a plain ASPX page results in slightly more efficient code because an HTTP handler with an .ashx extension (or a custom extension) doesn t go through the same long pipeline as a regular Web page. An HTTP handler engages the HTTP runtime less than a regular page does. As you ll see in a moment, the same pattern is maintained in ASP.NET 2.0.
The DynamicImage Control
ASP.NET 1.x has the Image control, which is little more than a wrapper around the <img> HTML tag. The Image control provides a thin layer of abstraction over some of the <img> properties. In the end, it allows you to reference images only by URL and to set some of the image layout attributes. In the real world, images can have a variety of origins. An image can be the content of a file, it can be stored in a database field, or it can be dynamically generated in memory using a graphic API such as the GDI+ classes. As you can see, only the first case when a disk file is used is well served by the <img> tag and the ASP.NET 1.x API. In all other cases, programmers must write their own code. In spite of the many possible sources for image bytes, the Web page still handles the same logical entity an image. Ideally, you should always be able to reference images in the same way, irrespective of the storage medium. But the <img> tag and the ASP.NET Image class are patently inadequate for the job. ASP.NET 2.0 provides one more image-related control that you met back in 1: DynamicImage. It derives from DynamicImageBase, which in turn derives from Image.
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