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thrown if you have not allocated an array and then attempt to
reference an element； or if you attempt to reference a nega
tive index of the array。
InsufficientMemoryException Thrown if not enough memory is available。 Although this
exception is not generated often； it can be generated if you
attempt to allocate an array when you specify something
along the lines of 5 trillion elements （due to an improperly
assigned array size variable）。
InvalidCastException Thrown if you attempt to cast one type to another type that is
not supported。 This exception is very mon when you use
inheritance and attempt a cast。
NotImplementedException Thrown when using methods or properties without an imple
mentation。 Often； you don’t have time to implement all of the
code at once。 For those properties or methods that have not
been implemented； don’t leave an empty property or method
implementation。 Instead； throw an exception。 Then you will
know if you have forgotten to implement something。
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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 129
Table 5…1。 mon Exception Types
Exception Description
NotSupportedException Thrown when you attempt to use an interface instance and a
method that cannot work。 For example； if you open a read…write
buffer to a read…only CD…ROM； you will get this exception
when writing to the CD…ROM。 If you attempt to read from the
interface instance； an exception will not be thrown。
NullReferenceException Thrown when you attempt to call a method or property of a
variable that has not been assigned with a valid type instance。
OutOfMemoryException Similar to InsufficientMemoryException。
OverflowException Thrown when you attempt to perform numeric operations
that are not supported； such as adding 2 billion to 2 billion
using a 32…bit integer。
SystemException Thrown by the operating system。 Do not attempt to derive
from this class。
Writing Exception…Safe Code
Now that you’ve seen how to implement exception handlers； we’ll look at an even better approach
to exceptions： not generating them。 We’ll focus on how you can make your code safer and less
likely to generate exceptions。
Writing Defensive Code
All too often； developers get exceptions such as NullReferenceException because they didn’t
make sure that the state in a piece of code was valid。 And if the state is not valid； an exception
will occur。 In fact； in this chapter； one of the previous examples has just such a situation； which
you may have noticed。 Here is the code that has a dumb little exception possibility：
Sub TestCallingExample（）
Dim cls As CallingExample = Nothing
Try
cls = New CallingExample（）
cls。Method（）
Catch ex As Exception
End Try
Console。WriteLine（〃Depth is （〃 & cls。GetDepth（） & 〃）〃）
End Sub
The problem is in the bolded line； which assumes that cls will always reference a valid
instance of CallingExample。 This is an assumption that you cannot afford to make。 If an excep
tion occurred while instantiating CallingExample； cls will still be Nothing； and the Catch block
will catch the instantiation exception； saving a program crash。 However； the use of cls。GetDepth（）
immediately after that throws your protection out the window； as cls is Nothing and will
generate a NullReferenceException。 Here’s a better way to write this code：
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130 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
Sub TestCallingExample（）
Dim cls As CallingExample = Nothing
Try
cls = New CallingExample（）
cls。Method（）
Catch ex As Exception
End Try
If cls IsNot Nothing Then
Console。WriteLine（〃Depth is （〃 & cls。GetDepth（） & 〃）〃）
End If
End Sub
The bolded code illustrates defensive coding that tests if cls is not Nothing； and if so； then
allows the referencing of cls。GetDepth（）。 Now the code is exception…safe。 It does not mean
exceptions cannot occur； because calling GetDepth（） could still generate an exception inter
nally； but with respect to the TestCallingExample（） method； you have made everything as safe
as possible； and have assumed GetDepth（） is a low exception…risk method。
Missing from the TestCalli