Beginning Visual Basic 2005 Databases
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Try-Catch-Finally structured error handling is a part of. NET languages. Here is a relatively minimalist example of using the Try-Catch-Finally syntax:.
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Text cmd. Read Then Label1.
Show ex. ToString Finally ' In a real application, put cleanup code here. Text; cmd. GetOracleString 0. For example, look at the following message displayed when the database is unavailable:. A better solution is to add an additional Catch statement to trap for the most common database errors and provide user-friendly messages. Notice the two Catch statements in the code sample above. If there aren't any Oracle errors to catch, the first Catch statement branch is skipped, leaving any other non-Oracle error to be caught by the second Catch statement.
Catch statements must be ordered in the code from most specific to most general. After implementing the user-friendly exception handling code, the ORA error message appears as follows:. The Finally code block is always executed regardless of whether or not an error occurred. It is where cleanup code belongs. If you don't use Using or using, you should dispose your connection and other objects in the Finally code block. Thus far our examples have only showed how to retrieve a single value. An OracleDataReader can retrieve values for multiple columns and multiple rows.
First consider a multiple column, single row query:. To keep things simple for this article, we are using a single table, the departments table. To obtain the values of the columns, either zero-based ordinals or column names can be used. Ordinals are relative to the order in the query. Item 2 or dr. To process multiple rows returned from an OracleDataReader, some type of looping construct is needed.
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Furthermore, a control that can display multiple rows is desirable. An OracleDataReader is a forward-only, read-only cursor, so it can't be bound to an updateable or fully scrollable control such as a Windows Forms DataGrid control. An OracleDataReader is compatible with a ListBox control, as the following code snippet illustrates:. This article has introduced you to the process of accessing Oracle databases using.
NET programming languages. You should now have the capability to connect to the database and retrieve multiple columns and rows. We begin with the requirement that you've created a project and added a reference as shown previously in this article. Continue by adding a button control and a label control to the Windows form. Be sure to leave room above the controls to allow additions that will be made in Lab 2. Add code to retrieve data from the Oracle database and display the results on the form.
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Put the code in a click event handler for the button. The easiest way to get started with this task is to double click the button because it will create a stub for the event handler. Add Visual Basic Imports statements before the Public Class declaration or C using statements before the namespace declaration. Now that the basics of database access are implemented in the code, the next step is to add interactivity to the application.
Instead of running the hard coded query, a textbox control can be added to accept a user input for the department number i. Add a textbox control and another label control to the form as shown below. Run the application. The application will abort. Recall that the ExecuteReader method actually returns an object.
Replace the line containing dr. Read with all of the following statements.
Build a .NET Application on the Oracle Database with Microsoft Visual Studio 2010
Now the application no longer aborts. Enter the letter A instead of a number and click the button. The application aborts. Clearly, our application needs a better approach to handling errors. Although it could be argued that the application should not allow the user to make invalid inputs that would cause an error, ultimately the application must have robust error handling added.
Not all errors are preventable, so error handling must be implemented.
Now that a single value has been retrieved, the next step is to retrieve multiple columns and rows with an OracleDataReader. A ListBox control is added to the form to display the results. Add a ListBox control to the form. Resize the control to fill most of the width of the form as shown below. The query results will be read in a while loop and will populate the ListBox control. More skills for object-oriented programming. An introduction to database programming. How to develop a database application with ADO. How to work with bound controls and parameterized queries.
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How to develop a Web Forms application. How to create and use web services. An introduction to ADO. How to develop a simple database application. How to work with typed datasets and bound controls. How to work with typed datasets and unbound controls. How to work with untyped datasets. How to work with data views, parameterized queries, and relationships. How to work with data commands. How to work with dataset schemas. How to develop and use database classes. A complete order entry application. An introduction to database programming with ASP.
Programming techniques for ASP. NET database applications. How to use the DataList and DataGrid controls. How to work with XML data. How to use Crystal Reports to develop reports. How to use the Server Explorer. Plus, hands-on examples and try-it-out exercises help you put your reading into practice so that with each chapter, you'll gradually build the pieces of a single application.
Some experience with Visual Basic is helpful but not required. Wrox Beginning guides are crafted to make learning programming languages and technologies easier than you think, providing a structured, tutorial format that will guide you through all the techniques involved. Ihr Name:. Folgen Sie uns beck-shop. Zustellung durch.