c# generate upc barcode

c# upc barcode generator

barcode generator c# wpf, generate barcode c#, code 128 rendering c#, c# code 128 font, c# create code 39 barcode, c# create code 39 barcode, c# data matrix barcode generator, data matrix generator c#, c# ean 128, ean 13 check digit calculator c#, c# pdf417 barcode generator, qr code with c#, c# upc barcode generator

Once you ve created your HTTP handler class and made it available to your web application (either by placing it in the App_Code directory or by adding a reference), you re ready to use your extension. The next step is to alter the web.config file for the web application so that it registers your HTTP handler. Here s an example: <httpHandlers> <add verb="*" path="test.simple" type="HttpExtensions.SimpleHandler,HttpExtensions" /> </httpHandlers> When you register an HTTP handler, you specify three important details. The verb attribute indicates whether the request is an HTTP POST or HTTP GET request (use * for all requests). The path attribute indicates the file extension that will invoke the HTTP handler. In this example, the web.config section links the SimpleHandler class to the filename test.simple. Finally, the type attribute identifies the HTTP handler class. This identification consists of two portions. First is the fully qualified class name (in this example, HttpExtensions.SimpleHandler). That portion is followed by a comma and the name of the DLL assembly that contains the class (in this example, HttpExtensions.dll). Note that the .dll extension is always assumed, and you don t include it in the name. If you re using the App_Code approach instead of a separately compiled assembly, you can omit the DLL name entirely, because ASP.NET generates it automatically. <httpHandlers> <add verb="*" path="test.simple" type="HttpExtensions.SimpleHandler" /> </httpHandlers> Visual Studio doesn t allow you to launch your HTTP handler directly. Instead, you need to run your web project and then type in a URL that includes test.simple. For example, if your web application URL is set to http://localhost:19209/05 in the local server, you need to manually change it to http://localhost:19209/05/test.simple. You ll see the HTML shown in Figure 5-12.

it also overwrites every field with the values in Chen s page effectively replacing the address Lucy entered with the old address. To defend against this sort of problem, you can enforce stricter concurrency checking. One way to do this is to create a command that performs the update only if every field matches. Here s what that command would look like: UpdateCommand="UPDATE Employees SET FirstName=@FirstName, LastName=@LastName, Title=@Title, City=@City FROM Employees WHERE EmployeeID=@original_EmployeeID AND FirstName=@original_FirstName AND LastName=@original_LastName AND Title=@original_Title AND City=@original_City"> The problem is that the command doesn t have access to the original values of every field instead, it has only the original value of any key fields. You could define every field in the table as a key field, but this is certain to cause confusion. A better solution is to set the SqlDataSource.ConflictDetection property to ConflictOptions.CompareAllValues instead of ConflictOptions.OverwriteChanges (the default). The data control will then supply the original value of every field, and the command will work as written.

Maintenance in large applications: Every page that accesses the database needs its own set of SqlDataSource controls. This can turn into a maintenance nightmare, particularly if you have several pages using the same query (each of which requires a duplicate instance of the SqlDataSource). In a component-based application, you ll use a higher-level model. The web pages will communicate with a data access library, which will contain all the database details. Lack of flexibility: Every data access task requires a separate SqlDataSource. If you want to provide a user with multiple ways to view or query data, this can swamp your page with data sources objects, one for each command variant. This can get complicated fast. Inapplicability to other data tasks: The SqlDataSource doesn t properly represent some types of tasks. The SqlDataSource is intended for data display and data editing scenarios. However, this model breaks down if you need to connect to the database and perform another task, such as placing a shipment request into an order pipeline or logging an event. In these situations, you ll need custom database code. It will simplify your application if you have a single database library that encapsulates these tasks along with data retrieval and updating operations.

Figure 5-12. Running a custom HTTP handler Using a custom HTTP handler isn t as convenient when you deploy your web application to an IIS web server. The problem is that the .simple extension won t be recognized by IIS. This means that by default it s handled by IIS on its own, not by ASP.NET. IIS simply checks for a file with that name, and if it exists, IIS returns the raw data from the file. If it doesn t, IIS returns an error message. To change this behavior, you need to add an IIS file mapping for your application that explicitly tells IIS to send all .simple requests to ASP.NET. 18 walks you through the process.

Commands that compare values are often inefficient, because they require more data to be sent over the network and mean more comparison work for the database. A better solution is to use a timestamp field. If the row is unchanged, the timestamp will always match. In this case, you would hide the timestamp column from the data control but set the DataKeyFields property to include it, so you could use it when constructing your command.

Instead of using a web.config or machine.config file, ASP.NET provides an alternate approach for registering HTTP handlers you can use the recognized extension .ashx. All requests that end in .ashx are automatically recognized as requests for a custom HTTP handler. To create an .ashx file in Visual Studio, select Website Add New Item and choose Generic Handler (see Figure 5-13).

In fact, in a well-abstracted three-tier application, your web page may call a method such as Business.PlaceOrder() without worrying about whether this operation involves saving an order record in a database, sending a message to a message queue, communicating with a remote component, or using a combination of all these tasks.