free qr code library vb.net Part IV in VS .NET

Printer QR Code in VS .NET Part IV

Part IV
Painting QR-Code In Visual Studio .NET
Using Barcode drawer for ASP.NET Control to generate, create QR Code 2d barcode image in ASP.NET applications.
www.OnBarcode.com
Bar Code Generator In Visual Studio .NET
Using Barcode printer for ASP.NET Control to generate, create barcode image in ASP.NET applications.
www.OnBarcode.com
Core Facilities
QR Code Drawer In C#
Using Barcode drawer for Visual Studio .NET Control to generate, create Denso QR Bar Code image in Visual Studio .NET applications.
www.OnBarcode.com
Print QR Code JIS X 0510 In .NET Framework
Using Barcode creation for .NET Control to generate, create QR Code image in .NET framework applications.
www.OnBarcode.com
Querying Stream s Position property is interesting . First, it is a virtual property and so my OneStatement method has no idea what code will actually execute which could throw any exception at all . Second, Stream is derived from MarshalByRefObject and so the stream argument could actually refer to a proxy object which itself refers to an object in another AppDomain . The other AppDomain could be unloaded and so an AppDomainUnloadedException could also be thrown here . Of course, all the methods that are being called are methods that I personally have no control over since they are produced by Microsoft . And it s entirely possible that Microsoft might change how these methods are implemented in the future, so they could throw new exception types that I could not possibly know about on the day I wrote the OneStatement method . How can I possibly write my OneStatement method to be completely robust against all possible failures By the way, the opposite is also a problem: a catch block could catch an exception type derived from the specified exception type and now I m executing recovery code for a different kind of failure . So now that you have a sense of all the possible failures, you can probably see why it has become culturally acceptable to not write truly robust and reliable code: it is simply impractical . Moreover, one could argue that it is actually impossible . The fact that errors do not occur frequently is another reason why it has become culturally acceptable . Since errors (like OutOfMemoryException) occur very infrequently, the community has decided to trade truly reliable code for programmer productivity . One of the nice things about exceptions is that an unhandled one causes your application to terminate . This is nice because during testing, you will discover problems quickly and the information you get with an unhandled exception (error message and stack trace) are usually enough to allow you to fix your code . Of course, a lot of companies don t want their application to just terminate after it has been tested and deployed and so a lot of developers insert code to catch System.Exception, the base class of all exception types . However, the problem with catching System.Exception and allowing the application to continue running is that state may be corrupted . Earlier in this chapter, I showed an Account class that defines a Transfer method whose job is to transfer money from one account to another account . What if, when this Transfer method is called, it successfully subtracts money from the from account and then throws an exception before it adds money to the to account If calling code catches System. Exception and continues running, then the state of the application is corrupted: both the from and to accounts have less money in them then they should . Since we are talking about money here, this state corruption wouldn t just be considered a simple bug, it would definitely be considered a security bug . If the application continues running, it will attempt to perform more transfers to and from various accounts and now state corruption is running rampant within the application .
QR-Code Creator In VB.NET
Using Barcode generator for Visual Studio .NET Control to generate, create QR Code image in .NET applications.
www.OnBarcode.com
DataMatrix Encoder In .NET Framework
Using Barcode creator for ASP.NET Control to generate, create Data Matrix image in ASP.NET applications.
www.OnBarcode.com
20 Exceptions and State Management
Printing Code 39 Extended In VS .NET
Using Barcode generator for ASP.NET Control to generate, create Code 3 of 9 image in ASP.NET applications.
www.OnBarcode.com
Printing GS1-128 In Visual Studio .NET
Using Barcode maker for ASP.NET Control to generate, create GS1-128 image in ASP.NET applications.
www.OnBarcode.com
One could say that the Transfer method itself should catch System.Exception and restore money back into the from account . And this might actually work out OK if the Transfer method is simple enough . But if the Transfer method produces an audit record of the withdrawn money or if other threads are manipulating the same account at the same time, then attempting to undo the operation could fail as well, producing yet another thrown exception . And now, state corruption is getting worse, not better . Note One could argue that knowing where something went wrong is more useful than knowing what error occurred . For example, it might be more useful to know that transferring money out of an account failed instead of knowing that Transfer failed due to a SecurityException or OutOfMemoryException, etc . In fact, the Win32 error model works this way: methods return true/false to indicate success/failure so you know which method failed . Then, if your program cares about why it failed, it calls the Win32 GetLastError method . System.Exception does have a Source property that tells you the method that failed . But this property is a String that you d have to parse, and if two methods internally call the same method, you can t tell from the Source property alone which method your code called that failed . Instead, you d have to parse the String returned from Exception s StackTrace property to get this information . Since this is so difficult, I ve never seen anyone actually write code to do it .
Barcode Generation In .NET Framework
Using Barcode generator for ASP.NET Control to generate, create barcode image in ASP.NET applications.
www.OnBarcode.com
Painting QR In Visual Studio .NET
Using Barcode generation for ASP.NET Control to generate, create QR Code JIS X 0510 image in ASP.NET applications.
www.OnBarcode.com
There are several things you can do to help mitigate state corruption:
Printing EAN / UCC - 13 In .NET
Using Barcode drawer for ASP.NET Control to generate, create UPC - 13 image in ASP.NET applications.
www.OnBarcode.com
Creating Delivery Point Barcode (DPBC) In VS .NET
Using Barcode printer for ASP.NET Control to generate, create Delivery Point Barcode (DPBC) image in ASP.NET applications.
www.OnBarcode.com
The CLR doesn t allow a thread to be aborted when executing code inside a catch or finally block . So, we could make the Transfer method more robust simply by doing this:
Bar Code Encoder In Java
Using Barcode drawer for Android Control to generate, create bar code image in Android applications.
www.OnBarcode.com
Code 128C Scanner In Java
Using Barcode scanner for Java Control to read, scan read, scan image in Java applications.
www.OnBarcode.com
public static void Transfer(Account from, Account to, Decimal amount) { try { /* do nothing in here */ } finally { from -= amount; // Now, a thread abort (due to Thread.Abort/AppDomain.Unload) can t happen here to += amount; } }
Decoding GS1 - 13 In Visual Studio .NET
Using Barcode reader for VS .NET Control to read, scan read, scan image in .NET applications.
www.OnBarcode.com
GS1 RSS Generator In Java
Using Barcode printer for Java Control to generate, create GS1 DataBar image in Java applications.
www.OnBarcode.com
However, it is absolutely not recommended that you write all your code in finally blocks! You should only use this technique for modifying extremely sensitive state .
Make Barcode In .NET Framework
Using Barcode generation for VS .NET Control to generate, create barcode image in .NET framework applications.
www.OnBarcode.com
Decode Code 128A In .NET
Using Barcode scanner for .NET Control to read, scan read, scan image in .NET applications.
www.OnBarcode.com
You can use the System.Diagnostics.Contracts.Contract class to apply code contracts to your methods . Code contracts give you a way to validate arguments and other variables before you attempt to modify state using these arguments/variables . If the arguments/variables meet the contract, then the chance of corrupted state is minimized (not completely eliminated) . If a contract fails, then an exception is thrown before any state has been modified . I will talk about code contracts later in this chapter . You can use constrained execution regions (CERs), which give you a way to take some CLR uncertainty out of the picture . For example, before entering a try block, you can have the CLR load any assemblies needed by code in any associated catch
USS Code 128 Generation In Objective-C
Using Barcode creator for iPad Control to generate, create Code 128 Code Set A image in iPad applications.
www.OnBarcode.com
Printing Barcode In Java
Using Barcode creation for BIRT Control to generate, create bar code image in Eclipse BIRT applications.
www.OnBarcode.com
Copyright © OnBarcode.com . All rights reserved.