As you learn about the Exception
in my previous article now learn what if you want to change this behavior? This
leads us to the next important concept, checked and unchecked statements in
VB.NET.
The different ways of controlling overflow behavior are listed below:
-
Set the compiler option to checked + or
checked -
-
Add the checked statement and unchecked
statement to your code Compiler options apply to the application level
whereas checked and unchecked statements can apply to specific blocks of
code. Checked and unchecked statements always override the compiler options,
so if you have compiled with the checked+ option and have a block of code
surrounded by an unchecked statement, the overflow exception will not be
raised for any overflow conditions within the unchecked block. Similarly, if
you have compiled with the checked– option and have a block of code
surrounded by the checked statement, the overflow exception would be raised
for any overflow conditions within the checked block of code. The next
example shows what the code looks like after adding the checked statement.
Exception3.VB,
Checked-Unchecked Example
Imports
System
Imports
System.Text
Public
Class
LearnAboutTryCatch1
Public
Shared
Sub
GenerateException(ByVal
arg1 As
Int32,
ByVal
arg2 As
Int32)
Dim result1
As
Int32
= 0
Dim result2
As
Int32
= 0
Try
'BLOCK OF CODE WHERE ERRORS ARE EXPECTED
'division
result1 = arg1 / arg2
'multiplication
result2 = arg1 * arg2
Console.WriteLine(result1)
Console.WriteLine(result2)
Catch e
As
System.DivideByZeroException
'HANDLE THE ERROR
'StackTrace gives us the location of error
' as string
'e.Message: gives the error message in this
' case, Attempted to divide by zero.
'e.Stacktrace: gives the location of the error
' in text form. In this case at
' LearnAboutTryCatch1.GenerateDivideByZeroError(
' Int32 Numerator,Int32 Denominator)
'in this case the output would be
'Attempted to divide by zero. at
' LearnAboutTryCatch1.GenerateDivideByZeroError(
' Int32 Numerator,Int32 Denominator)
Console.WriteLine([String].Concat(e.Message,
e.StackTrace))
Catch e
As
System.OverflowException
'HANDLE THE ERROR
'StackTrace gives us the location of error as
'string.
'e.Message: gives the error message in this
' case,
'e.Stacktrace: gives the location of the error
' in text form.
Console.WriteLine([String].Concat(e.Message,
e.StackTrace))
End
Try
End
Sub
Public
Shared
Sub
Main()
GenerateException(6, 0)
GenerateException(999999999, 999999999)
Console.ReadLine()
End
Sub
End
Class
Output
The code above would raise both exceptions: the divide-by-zero
exception and the overflow exception. We leave it to you as an exercise to play
with different combinations of compiler options, as well as checked and
unchecked statements.
Now, let's say we want to capture all other exceptions other than the specific
exceptions described here (DivideByZeroException and OverflowException). To do
that we have to add another catch statement with the argument of type
System.Exception. By adding the System.Exception, the code now will look like
next example. Pay specific attention to the place where we insert the catch
statement.
Exception4.VB, General Exception Example
Imports
System
Imports
System.Text
Public
Class
LearnAboutTryCatch1
Public
Shared
Sub
GenerateException(ByVal
arg1 As
Int32,
ByVal
arg2 As
Int32)
Dim result1
As
Int32
= 0
Dim result2
As
Int32
= 0
Try
'BLOCK OF CODE WHERE ERRORS ARE EXPECTED
'division
result1 = arg1 / arg2
'multiplication
result2 = arg1 * arg2
Console.WriteLine(result1)
Console.WriteLine(result2)
Catch e
As
System.DivideByZeroException
'HANDLE THE ERROR
'StackTrace gives us the location of error as string
'e.Message: gives the error message, in this
' case, Attempted to divide by zero.
'e.Stacktrace: gives the location of the error in text form. In
' this case at LearnAboutTryCatch1.GenerateDivideByZeroError(
' Int32 Numerator,Int32 Denominator)
'in this case the output would be
'Attempted to divide by zero. at
' LearnAboutTryCatch1.GenerateDivideByZeroError(
' Int32 Numerator,Int32 Denominator)
Console.WriteLine([String].Concat(e.Message,
e.StackTrace))
Catch e
As
System.OverflowException
'HANDLE THE ERROR
'stackTrace gives us the location of error as string
'e.Message: gives the error message in this case,
'e.Stacktrace: gives the location of the error in text
' form.
Console.WriteLine([String].Concat(e.Message,
e.StackTrace))
End
Try
End
Sub
Public
Shared
Sub
Main()
GenerateException(6, 0)
GenerateException(999999999, 999999999)
Console.ReadLine()
End
Sub
End
Class
Output of above example:
The compiler would not let you compile the code
above. Instead, it
would give you the following error message: "A previous catch clause already
catches all exceptions of this or a super type ('System.Exception')." The lesson
to learn is you have to first catch specific exceptions and then catch the
general exception (System.Exception). We leave it as an exercise for you to
change the code above to fix the order of catching exceptions.
As shown in next example, there are two ways to catch a general System.Exception.
In the first case you would capture the exception and get all the information
about it from the Exception parameter. However, in the second code block you
would not have access to that information.
Try-Catch Syntax
Try
Catch
End
Try
Conclusion
Hope this
article would have helped you in understanding Exception Statements in VB.NET.