Managing Errors in VB.NET

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Managing Errors
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In addition to simply watching for them and screaming Error! there are a few other things you should know about error management in Visual Basic programs.
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Generating Errors
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Believe it or not, there are times when you might want to generate runtime errors in your code. In fact, many of the runtime errors you encounter in your code occur because Microsoft wrote code in the Framework Class Libraries (FCLs) that specifically generates errors. This is by design. Let s say that you had a class property that was to accept only percentage values from 0 to 100, but as an Integer data type.
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Private StoredPercent As Integer Public Property InEffectPercent( ) As Integer Get Return StoredPercent End Get Set(ByVal value As Integer) StoredPercent = value End Set End Property
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Nothing is grammatically wrong with this code, but it will not stop anyone from setting the stored percent value to either 847 or 847, both outside the desired range. You can add an If statement to the Set accessor to reject invalid data, but properties don t provide a way to return a failed status code. The only way to inform the calling code of a problem is to generate an exception.
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Managing Errors |
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Set(ByVal value As Integer) If (value < 0) Or (value > 100) Then Throw New ArgumentOutOfRangeException("value", _ value, "The allowed range is from 0 to 100.") Else StoredPercent = value End If End Set
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Now, attempts to set the InEffectPercent property to a value outside the 0-to-100 range will generate an error, an error that can be caught by On Error or Try...Catch error handlers. The Throw statement accepts a System.Exception (or derived) object as its argument, and sends that exception object up the call stack on a quest for an error handler. Similar to the Throw statement is the Err.Raise method. It lets you generate errors using a number-based error system more familiar to Visual Basic 6.0 and earlier environments. I recommend that you use the Throw statement, even if you employ unstructured error handling elsewhere in your code.
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Mixing Error-Handling Methods
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You are free to mix both unstructured and structured error-handling methods broadly in your application, but a single procedure or method may use only one of these methods. That is, you may not use both On Error and Try...Catch...Finally in the same routine. A routine that uses On Error may call another routine that uses Try...Catch...Finally with no problems. Now you may be thinking to yourself, Self, I can easily see times when I would want to use unstructured error handling, and other times when I would opt for the more structured approach. It all sounds very reasonable, but let me warn you in advance that there are error-handling zealots out there who will ridicule you for decades if you ever use an On Error statement in your code. For these programmers, objectoriented purity is essential, and any code that uses nonobject methods to achieve what could be done through an OOP approach must be destroyed.
I m about to use a word that I forbid my elementary-school-aged son to use. If you have tender ears, cover them now, though it won t protect you from seeing the word on the printed page.
Rejecting the On Error statement like this is just plain stupid. As you may remember from earlier chapters, everything in your .NET application is object-oriented, since all the code appears in the context of an object. If you are using unstructured error handling, you can still get to the relevant exception object through the Err.GetException( ) method, so it s not really an issue of objects.
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9: Functional Programming
Determining when to use structured or unstructured error handling is no different from deciding to use C# or Visual Basic to write your applications. For most applications, the choice is irrelevant. One language may have some esoteric features that may steer you in that direction (such as optional method arguments in Visual Basic), but the other 99.9% of the features are pretty much identical. The same is true of error-handling methods. There may be times when one is just plain better than the other. For instance, consider the following code that calls three methods, none of which includes its own error handler:
On Error Resume Next RefreshPart1( ) RefreshPart2( ) RefreshPart3( )
Clearly, I don t care whether an error occurs in one of the routines or not. If an error causes an early exit from RefreshPart1, the next routine, RefreshPart2, will still be called, and so on. I often need more diligent error-checking code than this, but in low-impact code, this is sufficient. To accomplish the same thing using structured error handling would be a little more involved.
Try RefreshPart1( ) Catch End Try Try RefreshPart2( ) Catch End Try Try RefreshPart3( ) Catch End Try
That s a lot of extra code for the same functionality. If you re an On Error statement hater, by all means use the second block of code. But if you are a more reasonable programmer, the type of programmer who would read a book such as this, use each method as it fits into your coding design.
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