Programming Microsoft Visual Basic .NET/Programming Microsoft Visual Basic .NET (Microsoft Press)(X08-78517)(2002).bin/03 control flow and error handling/errorhandlingdemo/mainmodule1.vb

Text File | 2002-03-16 | 13.6 KB | 425 lines

Module MainModule

    Sub Main()
        ' Run one of the Textxxxx procedures below by uncommenting only one statement

        'TestException()
        'TestException2()
        'TestMultipleExitPointFunction()
        'TestCatchWhen()
        'TestCatchWhen2()
        'TestCatchWhen3()
        'TestCustomException()
        'TestGetExceptionMethod()
        'TestInnerException()
        'TestInnerException2()
        'TestExceptionOverhead()
        'TestOverflowExceptionOverhead()
        'TestOnErrorPerformance()

        ' These statements are usuful when running inside Visual Studio.NET
        Console.WriteLine("")
        Console.WriteLine(">>> press Enter to terminate the program <<<")
        Console.ReadLine()
    End Sub

    ' this procedure test Exception properties

    Sub TestException()
        ' display the ToString property of an exception object
        ' NOTE: you get different results, depending on whether you compile this code
        '       with or without debug information
        Try
            Throw New DivideByZeroException()
        Catch ex As Exception
            Console.WriteLine(ex.ToString)
            ' Uncomment next line to see the stack trace
            ' Console.WriteLine(ex.StackTrace)
        End Try
    End Sub

    ' this procedure tests the Finally block

    Sub TestException2()
        Dim cdir As String
        Try
            ' Remember the current directory.
            cdir = FileSystem.CurDir()
            ' Change to another directory
            FileSystem.ChDir("c:\docs")
            ' ....
        Catch ex As Exception
            ' Deal here with errors.
            ' ...
        Finally
            ' In all cases, restore the current directory.
            FileSystem.ChDir(cdir)
        End Try
    End Sub

    ' this procedure demonstrates using Try blocks for functions with multiple exit points

    Function TestMultipleExitPointFunction() As Integer
        Dim x, y, z As Integer
        Try
            ' ...
            If x > 0 Then Return 1
            ' ...
            If y = 0 Then Return 2
            ' ...
            If z > 0 Then Return 3
            ' ...
            Return 4
        Finally
            ' This code runs whatever exit path the code takes.
            ' ...
            Console.Write("This function is returning the value ")
            Console.WriteLine(TestMultipleExitPointFunction)
        End Try
    End Function

    Sub TestCatchWhen()
        Dim x, y, z, res As Integer    ' All variables are zero

        Try
            ' You can see different behaviors by commenting out or changing
            ' the order of the following statements

            ' ...
            res = y \ x
            ' ...
            res = x \ y
            ' ...
            res = x \ z
        Catch ex As DivideByZeroException When (x = 0)
            Console.WriteLine("Division error: x is null.")
        Catch ex As DivideByZeroException When (y = 0)
            Console.WriteLine("Division error: y is null.")
        Catch ex As DivideByZeroException
            Console.WriteLine("Division error: no information on variables")
        Catch
            Console.WriteLine("An error has occurred.")
        End Try
    End Sub

    ' this procedure shows the When clause used with properties of the exception object

    Sub TestCatchWhen2()
        Try
            ' comment out next statement to see the behavior when another file is missing
            FileOpen(1, "c:\myapp.ini", OpenMode.Input)
            FileOpen(2, "c:\xxxxx.dat", OpenMode.Input)

        Catch ex As System.IO.FileNotFoundException When InStr(ex.Message, """c:\myapp.ini""") > 0
            ' the ini file is missing.
            Console.WriteLine("Can't initializa: MyApp.Ini file not found")
        Catch ex As System.IO.FileNotFoundException
            ' Another file wasn't found - extract the file name from the Message property.
            Dim filename As String = Split(ex.Message, """")(1)
            Console.WriteLine("The following file is missing: " & filename)
            Debug.WriteLine(ex.message)
        End Try
    End Sub

    ' this procedure shows how to use the When clause to track where the error occurred

    Sub TestCatchWhen3()
        Dim currentStep As Integer      ' You might also use an Enum value.

        Try
            ' Initialize the program.
            currentStep = 1

            ' comment and uncomment the Throw statements, so that you can simulate
            ' an exception in different places of this procedure

            Throw New System.Exception()

            ' Open the data file.
            currentStep = 2
            ' ...
            Throw New System.IO.FileNotFoundException()

            ' ...
            Throw New System.Exception()

            ' Process the file contents.
            currentStep = 3
            ' ...
            Throw New System.Exception()

            ' ...and so on ...

        Catch ex As Exception When currentStep = 1
            Console.WriteLine("An error occurred in the initialization step.")

        Catch ex As System.IO.FileNotFoundException When currentStep = 2
            Console.WriteLine("The data file wasn't found.")

        Catch ex As Exception When currentStep = 2
            Console.WriteLine("An error occurred while opening the data file.")

        Catch ex As Exception When currentStep = 3
            Console.WriteLine("An error occurred while processing data.")
            ' Add here other Catch blocks
            ' ...
        End Try
    End Sub

    ' this procedure tests custom exception objects

    Sub TestCustomException()
        Try
            LoadIniFile()
        Catch ex As UnableToLoadIniFileException
            ' Deal with the most specific error here.
            ' Next statement displays custom message "Unable to load..."
            Console.WriteLine(ex.Message)
        Catch ex As Exception
            ' Deal with other errors here.
        End Try
    End Sub

    ' The routine that opens the INI file.
    Sub LoadIniFile()
        Try
            ' Try to open the INI file. (cause an error in this demo)
            FileOpen(1, "c:\missingfile.ini", OpenMode.Input)

        Catch ex As Exception
            ' Whatever caused the error, throw a more specific exception.
            Throw New UnableToLoadIniFileException()
        End Try
    End Sub

    ' test the Err.GetException method

    Sub TestGetExceptionMethod()
        Try
            Call OldStyleErrorHandlerProc()
        Catch ex As DivideByZeroException
            Console.WriteLine("A DivideByZeroException has been caught.")
        End Try
    End Sub

    ' this procedure traps an error using an old-style On Error Goto
    ' and return it to the caller as an exception

    Sub OldStyleErrorHandlerProc()
        On Error GoTo ErrorHandler

        ' cause a division by zero error
        Dim x, y As Integer
        y = 1 \ x

        Exit Sub

ErrorHandler:
        ' add here clean up code as necessary
        ' ...
        ' report it to the caller as an Exception object
        Throw Err.GetException
    End Sub

    ' this procedure tests inner exceptions

    Sub TestInnerException()
        Dim x, y As Integer

        Try
            MathIntensiveProc(x, y)
        Catch ex As ArgumentException
            Console.WriteLine("An argument exception has occurred.")
            ' Print a more detailed message about the actual cause of the error.
            Console.WriteLine(ex.InnerException.Message)
        Catch ex As Exception
            ' Deal with other errors.
            Console.WriteLine(ex.Message)
        End Try
    End Sub

    Sub MathIntensiveProc(ByVal x As Integer, ByVal y As Integer)
        Try
            ' Process arguments here.
            ' ...
            ' (cause a division by zero error to test this code)
            x = 1 \ y
        Catch ex As Exception
            ' Map any exception to the ArgumentException object,
            ' but remember the inner (actual) exception object.
            Throw New ArgumentException("Wrong arguments", ex)
            Console.WriteLine(ex.GetBaseException.Message())
        End Try
    End Sub

    Sub TestInnerException2()
        Try
            InitializeApplication()
        Catch ex As InitializationFailedException
            ' Deal with the most specific error here.
            ' Next statement displays custom message "Unable to load..."
            Console.WriteLine(ex.Message)
            ' also display a message from the inner exception
            Console.WriteLine("Inner exception: " & ex.InnerException.Message)
        Catch ex As Exception
            ' Deal with other errors here.
        End Try
    End Sub

    ' The routine that opens the INI file.
    Sub InitializeApplication()
        Try
            ' Try to open the INI file. (cause an error in this demo)
            FileOpen(1, "c:\missingfile.ini", OpenMode.Input)

        Catch ex As Exception
            ' Whatever caused the error, throw a more specific exception.
            Throw New InitializationFailedException(ex)
        End Try
    End Sub

    ' this procedure tests exception overheads

    Dim rand As New Random()    ' random number generator

    Sub TestExceptionOverhead()
        ' This is the probability that the called method raises an error
        Const Probability As Double = 0.001
        ' This is the number of repetitions
        Const Repetitions As Integer = 1000000

        Dim i As Integer
        Dim startTime As Date
        Dim errorCount As Integer

        ' test the method that notifies an error condition by throwing an exception.
        startTime = Now
        errorCount = 0
        For i = 1 To Repetitions
            Try
                CanThrowException(Probability)
            Catch
                errorCount += 1
            End Try
        Next
        Console.WriteLine("Exceptions: {0} secs. ", Now.Subtract(startTime))

        ' test the method that notifies an error condition by returning False.
        startTime = Now
        errorCount = 0
        For i = 1 To Repetitions
            If CanReturnFalse(Probability) = False Then
                errorCount += 1
            End If
        Next
        Console.WriteLine("Return value:  {0} secs.", Now.Subtract(startTime))
    End Sub

    ' this method can throw an exception with a given probability.
    Sub CanThrowException(ByVal probability As Double)
        If rand.NextDouble <= probability Then
            Throw New System.Exception()
        End If
    End Sub

    ' This method can signal error condition by returning False
    Function CanReturnFalse(ByVal probability As Double) As Boolean
        If rand.NextDouble < probability Then
            ' this simulates an error.
            Return False
        Else
            ' this means a successful operation
            Return True
        End If
    End Function

    ' this procedure benchmarks a simple integer addition.
    ' you should compile this code with and without the Remove integer overflow check option

    Sub TestOverflowExceptionOverhead()
        Dim start As Date
        Dim i As Integer
        Dim intVar As Integer
        Dim lngVar As Long

        start = Now
        For i = 1 To 1000000000
            intVar = intVar + 1
        Next
        Console.WriteLine("Incrementing an Integer variable: {0} secs.", Now.Subtract(start))

        start = Now
        For i = 1 To 1000000000
            lngVar = lngVar + 1
        Next
        Console.WriteLine("Incrementing a Long variable: {0} secs.", Now.Subtract(start))
    End Sub

    ' this procedure tests the overhead of On Error vs. Try-Catch

    Sub TestOnErrorPerformance()
        Dim start As Date
        Dim i As Integer
        Dim res As Integer

        start = Now
        For i = 1 To 10000000
            res = TestOnErrorGotoOverhead()
        Next
        Console.WriteLine("TestOnErrorGotoOverhead: {0} secs.", Now.Subtract(start))

        start = Now
        For i = 1 To 10000000
            res = TestOnErrorResumeNextOverhead()
        Next
        Console.WriteLine("TestOnErrorResumeNextOverhead: {0} secs.", Now.Subtract(start))

        start = Now
        For i = 1 To 10000000
            res = TestTryCatchOverhead()
        Next
        Console.WriteLine("TestTryCatchOverhead: {0} secs.", Now.Subtract(start))

        start = Now
        For i = 1 To 10000000
            res = TestNoErrorTrappingOverhead()
        Next
        Console.WriteLine("TestNoErrorTrappingOverhead: {0} secs.", Now.Subtract(start))
    End Sub

    ' auxiliary functions

    Function TestOnErrorGotoOverhead() As Integer
        On Error GoTo ErrorHandler
        Dim x As Integer = 123
        x = x * x
ErrorHandler:
        TestOnErrorGotoOverhead = x \ 10
    End Function

    Function TestOnErrorResumeNextOverhead() As Integer
        On Error Resume Next
        Dim x As Integer = 123
        x = x * x
        TestOnErrorResumeNextOverhead = x \ 10
    End Function

    Function TestTryCatchOverhead() As Integer
        Dim x As Integer = 123
        Try
            x = x * x
        Finally
            TestTryCatchOverhead = x \ 10
        End Try
    End Function

    Function TestNoErrorTrappingOverhead() As Integer
        Dim x As Integer = 123
        x = x * x
        TestNoErrorTrappingOverhead = x \ 10
    End Function
End Module