VB2008从入门到精通(PDF格式英文版)-第37部分
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Try
Throw New Exception（〃Exception in action 2。4。〃）
Catch thrown As Exception
Throw New Exception（〃Exception in action 2 has been caught。〃； thrown）
End Try
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122 CH AP T E R 5 ■ L E A R N IN G AB OU T V I SU A L B AS IC E X C E P TI ON H AN D L IN G
The first variation； Exception（〃Exception in action 2。4。〃）； uses the string description
constructor parameter that passes text describing what went wrong。 The text is meant for human
understanding； so don’t use text like “Error 168： something went wrong。” The second variation；
Exception（〃Exception in action 2 has been caught。〃； thrown）； includes the original excep
tion as an additional constructor parameter in a newly thrown exception。 This allows you to
pass on even more information。
The generated output of this code looks like this：
Unhandled Exception： System。Exception： Exception in action 2 has been caught。
………》 System。Exception： Exception in action 2。4。
The generated exception tells you clearly where the exception occurred and where it was
processed。 You have a plete flow of the actions。
■Note An experienced developer may say that the flow is also presented by the stack that is dumped by
the program if the exception is not caught。 Yes； you could see the program flow based on this stack dump；
but deciphering a stack dump where you have a stack of say 10 or 15 is not much fun to figure out or read。
Consider the following amendment to the code； which reduces the amount of information。
Try
Throw New Exception（〃Exception in action 2。4。〃）
Catch exception1 As Exception
Throw New Exception（〃Exception in action 2 has been caught〃）
End Try
The results are not as enlightening：
Unhandled Exception： System。Exception： Exception in action 2 has been caught。
If you want to gain access to the error string； you can use the Message property of an exception。
Try
Throw New Exception（〃Exception in action 2。4。〃）
Catch thrown As Exception
Console。WriteLine（thrown。Message）
Throw New Exception（〃Exception in action 2 has been caught。〃）
End Try
You still have the more specific message； but not as part of the flow of exceptions：
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CH AP T E R 5 ■ L E AR N IN G AB O U T V I SU A L B AS IC E X CE PT I ON HA N D L IN G 123
Exception in action 2。4。
Unhandled Exception： System。Exception： Exception in action 2 has been caught。
DON’T REPEAT ERROR MESSAGES
When you throw exceptions； make sure that you don’t use the same error message twice。 Imagine the situa
tion where you deliver a program into production and the error message “File not found” is generated。 If this
text is used in multiple places； when the user calls tech support； the support staff will have no idea which file
was not found。 Instead； in the error message； tell the user which file was not found and why。 The more details
you deliver； the easier it is for the support desk to help users get around the problem。
If you do need to use the same text in multiple places； add a context identifier。 For example； you could
generate a load error fault in the context of loading a file using a dialog box。 Or you could generate a load error in
the context of loading a file based on a mand…line argument。 Specify each context as an additional piece
of information appended or prefixed to the exception text； much like the previous code illustrated when catching an
exception in action 2。
Safeguarding Against Stack Unwinding
Exception handling makes it simple for you to stop your program from crashing； but it does not
help you from ensuring that the state of your application is still intact。 Consider the example
shown in Figure 5…5； which illustrates how program state can be corrupted due to an exception
that is caught and swallowed。
Class MyType
Public DataMember As Integer
End Class
Class Tests LocalDataMember is assigned
Public LocalDataMember As Integer
Public Sub GeneratesException（）
Me。LocalDataMember = 10 DataMember should also be assigned；
Dim cls As MyType = Nothing but an exception is raised； and the
cls。DataMember = 10
End Sub assignment doesn’t happen
Public Sub RunAll（）
Console。WriteLine（〃LocalDataMember=〃 & LocalDataMember）
Try At this point； neither
GeneratesException（） member is assigned
Catch exception1 As Exception
End Try
Console。WriteLine（〃LocalDataMember=〃 & LocalDataMember） At this point； DataMember is not
End Sub assigned； but LocalDataMember is
End Class
assigned； indicating a corrupt state
Figure 5…5。 Exceptions can corrupt the state of a program。
When an exception is caught； the stack is unwound。 Consider the example shown in
Figure 5…6； where the unwinding of the stack can have the side effect of jumping over a method call。
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124 CH AP T E R 5 ■ L E A R N IN G AB OU T V I SU A L B AS IC E X C E P TI ON H AN D L IN G
Figure 5…6。 Unwinding the stack can cause a method call to be skipped。
In the example in Figure 5…6； the methods are called sequentially； starting with RunAll（）；
and then when a single exception is thrown； the code in the RunAll（） method’s Catch block is
executed immediately。 So； depth has a value of 2 instead of a value of 0 （the value you would
expect if no exception were thrown） when execution finishes。 You can see that the stack has
unwound too fast； leaving the program with unpredictable results。
The stack unwinding problem causes corruption and can wreak havoc on your program。
A seemingly working program might corrupt itself and slowly fail or generate incorrect data。
Fortunately； there are a couple ways to handle stack unwinding correctly。
Using Finally to Process Unfinished Tasks
The simplest solution to the stack unwinding problem is to use the Finally keyword； which
indicates that you want to guarantee a piece of code executes； whether or not an exception is
thrown。 The following shows how you could rewrite the example in Figure 5…6 using Finally。
When executed； this code will set the depth data member to the correct value。
Class CallingExample
Dim depth As Integer
Public Sub CalledCalledMethod（）
Me。depth = 2
Throw New Exception（）
End Sub
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CH AP T E R 5 ■ L E AR N IN G AB O U T V I SU A L B AS IC E X CE PT I ON HA N D L IN G 125
Public Sub CalledMethod（）
Me。depth = 1
Try
CalledCalledMethod（）
Finally
Me。depth = 1
End Try
End Sub
Public Sub Method（）
Me。depth = 0
Try
CalledMethod（）
Finally
Me。depth = 0
End Try
End Sub
Public Function GetDepth（） As Integer
Return Me。depth
End Function
End Class
Class Tests
Sub TestCallingExample（）
Dim cls As CallingExample = Nothing
Try
cls = new CallingExample（）
cls。Method（）
Catch exception1 As Exception
End Try
Console。WriteLine（〃Depth is （〃 & cls。GetDepth（） & 〃）〃）
End Sub
Public Sub RunAll（）
TestCallingExample（）
End Sub
End Class
In this example； each Finally keyword is associated with a Try block。 You don’t need to
associate a Catch block with a Try block if you are using Finally。 If the Try block is entered；
when the code leaves the Try block； regardless of whether it ran successfully or threw an excep
tion； the code within Finally is executed。 Thus； if stack unwinding occurs； it is possible to reset
or assign state to a consistent value before the exception is processed elsewhere。
■Note When the Finally block is called； you don’t know if it is called due to an exception or successful
code execution。 Thus； you shouldn’t assume that Finally is called because an exception was thrown。
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126 CH AP T E R 5 ■ L E A R N IN G AB OU T V I SU A L B AS IC E X C E P TI ON H AN D L IN G
Sandboxing Your Code
The sandboxing technique is similar to playing with an Etch A Sketch； in that you attempt to
build a state； and if you make a mistake； you can throw the state away。 This requires that you
separate your code in
