vb.net 2008 barcode generator Coding that handles exceptions in C#.NET

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Coding that handles exceptions
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Figure 20-26 Unhandled exception
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Although we have done nothing exciting within the catch block in this example, in a real-life scenario you would provide code that would either prompt the user for additional credentials, or prevent the user from accessing the database, or do whatever is necessary to gracefully exit the error condition Figures 20-27, 20-28, and 20-29 show the sequence of events when an SqlException occurs in the code shown in Figure 20-25 Notice that the finally block is executed too In Figure 20-29, you can see that we used the exception object that is created (SqlException s), and then we displayed its Message property to provide a user-friendly message regarding the status of the exception EXAM TIP You can create your own data-access exceptions be sure that they inherit from SystemDataDataException Some of the properties of the exception object are listed in Table 20-4 From this point on in our discussions about data, we will be following this convention of catching data exceptions and providing the coding to handle them
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Accessing Data Programmatically
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As promised, we will now move our discussion from Visual Studio NET data controls and wizards to programmatic coding for data access As mentioned before, accessing data using specialized and prebuilt functions is very valuable, but it may be impractical in some circumstances For example, you may find that the limitations of a particular control stop you from providing a desired service In that case, you will need to be able to create your own connections, commands, data adapters, and other objects
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20: Data in Windows Forms
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Figure 20-27 The try block the first message box
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Figure 20-28 The try block using the exception object
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PART III
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Figure 20-29 The finally block
Description
HelpLink InnerException Message Source StackTrace Table 20-4
Provides a link to a help file giving more information about the exception Contains the exception object that caused the code to enter the catch block This is used if the exception happened inside a catch block Provides a text message to give more information about the exception to the user Provides the name of the object that caused the exception Provides a means of viewing the call stack (the list of methods in execution order)
Properties of the Exception Object
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Creating a Connection Object
The connection object allows us to connect and authenticate against a data source As you know from 10, there are two different connection classes associated with ADONET SqlConnection and OleDbConnection In this chapter, we will deal with the SqlConnection object, since it is the source of many more questions on the Microsoft exams EXAM TIP The SqlConnection class is used for accessing Microsoft SQL Server 7 (and higher) To access an earlier version of SQL Server, you will need to use a different data source provider class When you instantiate an SqlConnection object, you must provide the connection parameters to the constructor The best way to handle this is to create a string object first, and then pass the string object to the SqlConnection constructor, like this:
string connString = "data source=mrlaptop;initial catalog=College;user id=sa;password;"
The connection string is very similar to those for the ADO or OLE-DB providers in version 6 of Visual C++ or Visual Basic It consists of three components: data source the server location of the database If the server was located somewhere other than on the client, the entry would be something like this: "data source=\\SQLServer" initial catalog the name of the database user id and password the log in credentials You could specify integrated security instead of providing the userid and password within the connection string In that case, you would code "integrated security" The connection object can be thought of as the bridge between your application and a data provider Connection strings vary slightly depending on the data provider (OLE-DB, ODBC, and so on) Once the string is created, you then pass it to the connection constructor as follows:
SqlConnection myConnection = new SQLConnection (connString);
By using this method, you can modify the connection string at any time without touching the actual creation of the connection object All that s left to do now is to open the connection, which, essentially, provides us with a link to the data source No command is being run against the data in the following code, no data is transferred this simply creates a pipeline to the data
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