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C++ and C# are case-sensitive languages; VBNET is not, in the sense that no matter what is typed in, the editor automatically corrects the capitalization In C# and C++, the statement is trycatch; in VBNET, it is TryCatch
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A1 Why Exception Handling
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If you've been writing software for very long, you probably already know why you want to handle exceptions Have you ever seen a program crash and display a weird message that doesn't make any sense This is what happens when developers do not handle exceptions properly Let's look at a simple example Listing A1 opens a file namedc:\abctxt
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Listing A1 Opening a file
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using System; using SystemIO; namespace ListingA1andA2 { class Class1 { static void Main(string[] args) { FileOpen("c:\\abctxt", FileModeOpen); } } } What if the file does not exist We get the error message shown in Figure A1 We are fortunate that CLR handles so much for us because otherwise this error message could have been a lot worse
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Figure A1 An error generated from Listing A1
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Now let's make a small modification to our program The new code is shown in Listing A2 This time we use a simpletrycatch block to handle the exception
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Listing A2 A simple exception handling block
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using System; using SystemIO; namespace ListingA1andA2 { class Class1 { static void Main(string[] args) { try { FileOpen("c:\\abctxt", FileModeOpen); } catch (Exception exp) { ConsoleWriteLine(expMessage); } } } } Figure A2 shows the output from the modified program Not only is the exception handled, but also the cause of the exception is reported
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Figure A2 An exception-handled error message
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A2 Understanding the trycatch Block
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If you come from a Visual Basic background, we recommend that you just forget about unstructured exception handling and learn this new approach using the trycatch statement After you finish reading this appendix, you'll find the structured approach much better!
A21 The trycatch Statement
Using the trycatch statement is very straightforward First we decide which code we want the error handler to monitor by placing that code inside the try block When an exception occurs in the encapsulated code, a control goes to the catch block that handles the exception A simple template for a trycatch block is shown in Listing A3
Listing A3 A simple trycatch block
try { // Place the code that may generate // an exception in this block } catch ( exception type) { // This code executes when the try block fails and // the filter on the catch statement is true // Here you can write your own custom error message // or get the message description or other details // from the exception class }
A22 The trycatchfinally Statement
The trycatchfinally statement is an extended version of the trycatch statement If an error occurs during execution of any of the code inside the try section, the control moves to thecatch block when the filter condition is true The finally block always executes last, just before the error handling block loses scope, regardless of whether an exception has occurred The finally block is the perfect place to close files and dispose of objects A simple trycatchfinally statement is shown in Listing A4
Listing A4 A simple trycatchfinally statement
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try { // Place the code that may generate // an exception in this block } catch ( exception type) { // This code executes when the try block fails and // the filter on the catch statement is true // Here you can write your own custom error message // or get the message description or other details // from the exception class } finally { // Release and dispose of objects and // other resources here } Listing A5 allocates resources at the beginning of the method and releases them inside thefinally block Regardless of whether an exception occurs, execution control will pass to the finally block and release the resources