Functions Language: Details on File Processing

Exception Handling in File Processing

Finally

So far, to handle exceptions, we were using the try, catch, and throw keywords. These allowed us to perform normal assignments in a try section and then handle an exception, if any, in a catch block. We also mentioned that, when you create a stream, the operating system must allocate resources and dedicate them to the file processing operations. Additional resources may be provided for the object that is in charge of writing to, or reading from, the stream. We also saw that, when the streaming was over, we should free the resources and give them back to the operating system. To do this, we called the Close() method of the variable that was using resources.

More than any other assignment, file processing is in prime need of exception handling. During file processing, there are many things that can go wrong. For this reason, the creation and/or management of streams should be performed in a try block to get ready to handle exceptions that would occur. Besides actually handling exceptions, you can use the finally keyword to free resources.

The finally keyword is used to create a section of an exception. Like catch, a finally block cannot exist by itself. It can be created following a try section. The formula used would be:

try
{
}
finally
{
}

Based on this:

The finally section has a body of its own, delimited by its curly brackets
Like catch, the finally section is created after the try section
Unlike catch, finally never has parentheses and never takes arguments
Unlike catch, the finally section is always executed

Because the finally clause always gets executed, you can include any type of code in it but it is usually appropriate to free the resources that were allocated such as those used during streaming. Here is an example:

using namespace System;
using namespace System::IO;

int main()
{
    String ^ NameOfFile = L"Members.clb";
	
    FileStream ^ fstPersons = gcnew FileStream(NameOfFile, FileMode::Create);
    BinaryWriter ^ wrtPersons = gcnew BinaryWriter(fstPersons);
        
    try 
    {
        wrtPersons->Write(L"James Bloch");
        wrtPersons->Write(L"Catherina Wallace");
        wrtPersons->Write(L"Bruce Lamont");
        wrtPersons->Write(L"Douglas Truth");
    }
    finally
    {
        wrtPersons->Close();
        fstPersons->Close();
    }

    return 0;
}

In the same way, you can use a finally section to free resources used when reading from a stream:

using namespace System;
using namespace System::IO;

int main()
{
/*    String ^ NameOfFile = L"Members.clb";
	
    FileStream ^ fstPersons = gcnew FileStream(NameOfFile, FileMode::Create);
    BinaryWriter ^ wrtPersons = gcnew BinaryWriter(fstPersons);
        
    try 
    {
        wrtPersons->Write(L"James Bloch");
        wrtPersons->Write(L"Catherina Wallace");
        wrtPersons->Write(L"Bruce Lamont");
        wrtPersons->Write(L"Douglas Truth");
    }
    finally
    {
        wrtPersons->Close();
        fstPersons->Close();
    }*/

	String ^ NameOfFile = L"Members.clb";
    String ^ strLine = nullptr;
        
    FileStream ^ fstMembers = gcnew FileStream(NameOfFile, FileMode::Open);
    BinaryReader ^ rdrMembers = gcnew BinaryReader(fstMembers);

    try
    {
        strLine = rdrMembers->ReadString();
        Console::WriteLine(strLine);
        strLine = rdrMembers->ReadString();
        Console::WriteLine(strLine);
        strLine = rdrMembers->ReadString();
        Console::WriteLine(strLine);
        strLine = rdrMembers->ReadString();
        Console::WriteLine(strLine);
    }
    finally
    {
        rdrMembers->Close();
        fstMembers->Close();
    }

    return 0;
}

Of course, since the whole block of code starts with a try section, it is used for exception handling. This means that you can add the necessary and appropriate catch section(s) but you don't have to.

Practical Learning: Finally Releasing Resources

Start Microsoft Visual C++ and create a CLR Console Application named IceCream4
To create a new class, on the main menu, click Project -> Add Class...
In the Add Class dialog box, in the Templates list, click C++ Class and click Add
Set the Name to CIceCream
Make sure the Managed check box is selected and click Finish

Change the file as follows:

#pragma once

using namespace System;
using namespace System::IO;

public ref class CIceCream sealed
{
    // This is the base price of an ice scream
    // Optional values may be added to it
public:
    static const double BasePrice = 1.55;

    // These arrays are used to build the components 
    // of various ice creams
private:
    array<String ^> ^ Flavor;
    array<String ^> ^ Container;
    array<String ^> ^ Ingredient;

    // Additional factor used to process an ice scream order
    int Scoops;
    double TotalPrice;

    // Variables that will hold the user's choice
    // These are declared "globally" so they can be 
    // shared among methods
    int ChoiceFlavor;
    int ChoiceContainer;
    int ChoiceIngredient;

    // This default constructor is the best place for 
    // us to initialize the array
public:
    CIceCream(void);
};

Access the IceCream.cpp file and change the constructor as follows:

#include "StdAfx.h"
#include "IceScream.h"

CIceCream::CIceCream(void)
{
        Flavor = gcnew array<String ^>(10);
        Flavor[0] = L"Vanilla";
        Flavor[1] = L"Cream of Cocoa";
        Flavor[2] = L"Chocolate Chip";
        Flavor[3] = L"Organic Strawberry";
        Flavor[4] = L"Butter Pecan";
        Flavor[5] = L"Cherry Coke";
        Flavor[6] = L"Chocolate Brownies";
        Flavor[7] = L"Caramel Au Lait";
        Flavor[8] = L"Chunky Butter";
        Flavor[9] = L"Chocolate Cookie";

        Ingredient = gcnew array<String^>(4);
        Ingredient[0] = L"No Ingredient";
        Ingredient[1] = L"Peanuts";
        Ingredient[2] = L"M & M";
        Ingredient[3] = L"Cookies";

        Container = gcnew array<String^>(3);
        Container[0] = L"Cone";
        Container[1] = L"Cup";
        Container[2] = L"Bowl";
}

In the Class View, right-click CIceCream -> Add -> Add Function
Set the Return Type to void
Set the Function Name to ChooseFlavor and click Finish

In the IceCream.cpp source file, implement the ChooseFlavor method as follows:

// This method requests a flavor from the user and
// returns the choice
void CIceCream::ChooseFlavor(void)
{
    // Make sure the user selects a valid number 
    //that represents a flavor...
    do {
        // In case the user types a symbol that 
        // is not a number
        try {
            Console::WriteLine(L"What type of flavor do you want?");
            for (int i = 0; i < Flavor->Length; i++)
                Console::WriteLine(L"{0} - {1}", i + 1, Flavor[i]);
            Console::Write(L"Your Choice? ");
	    ChoiceFlavor = int::Parse(Console::ReadLine());
        }
        catch(FormatException ^) // display an appropriate message
        {
            Console::WriteLine(L"You must enter a valid number "
                               L"and no other character!");
        }

        // If the user typed an invalid number out of the 
        // allowed range
        // let him or her know and provide another chance
        if( (ChoiceFlavor < 1) || (ChoiceFlavor > Flavor->Length) )
            Console::WriteLine(L"Invalid Choice - Try Again!\n");
    } while (ChoiceFlavor < 1 || ChoiceFlavor > Flavor->Length);
}

In the Class View, right-click CIceCream -> Add -> Add Function
Set the Return Type to void
Set the Function Name to ChooseContainer and click Finish

In the IceCream.cpp source file, implement the ChooseContainer method as follows:

void CIceCream::ChooseContainer(void)
{
    // Make sure the user selects a valid number that 
    // represents a container
    do {
        // If the user types a symbol that is not a number
        try {
            Console::WriteLine(L"What type of container do you want?");
            for (int i = 0; i < Container->Length; i++)
                 Console::WriteLine(L"{0} - {1}", i + 1, Container[i]);
            Console::Write(L"Your Choice? ");
	    ChoiceContainer = int::Parse(Console::ReadLine());
        }
        catch(FormatException ^) // display an appropriate message
        {
            Console::WriteLine(L"You must enter a valid "
                               L"number and no other character!");
        }

        // If the user typed an invalid number out of the 
        // allowed range
        // let him or her know and provide another chance
        if( (ChoiceContainer < 1) ||
            (ChoiceContainer > Container->Length) )
            Console::WriteLine(L"Invalid Choice - Try Again!");
    } while( (ChoiceContainer < 1) ||
             (ChoiceContainer > Container->Length) );
}

In the Class View, right-click CIceCream -> Add -> Add Function
Set the Return Type to void
Set the Function Name to ChooseIngredient and click Finish

In the IceCream.cpp source file, implement the ChooseIngredient method as follows:

void CIceCream::ChooseIngredient(void)
{
    do {
        try {
            Console::WriteLine(L"Do you want an ingredient or not");
            for (int i = 0; i < Ingredient->Length; i++)
                Console::WriteLine(L"{0} - {1}",
                                   i + 1,
                                   Ingredient[i]);
            Console::Write(L"Your Choice? ");
            ChoiceIngredient = int::Parse(Console::ReadLine());
        }
        catch(FormatException ^)
        {
            Console::WriteLine(L"You must enter a valid "
                               L"number and no other character!");
        }

        if( (ChoiceIngredient < 1) ||
            (ChoiceIngredient > Ingredient->Length) )
            Console::WriteLine(L"Invalid Choice - Try Again!");
    } while( (ChoiceIngredient < 1) ||
             (ChoiceIngredient > Ingredient->Length) );
}

In the Class View, right-click CIceCream -> Add -> Add Function
Set the Return Type to void
Set the Function Name to SpecifyNumberOfScoops and click Finish

In the IceCream.cpp source file, implement the SpecifyNumberOfScoops method as follows:

void CIceCream::SpecifyNumberOfScoops(void)
{
    do {
        try {
            Console::Write(L"How many scoops(1, 2, or 3)? ");
            Scoops = int::Parse(Console::ReadLine());
        }
        catch(FormatException ^)
        {
            Console::WriteLine(L"You must enter a valid number "
                               L"and no other character!");
        }

        if( (Scoops < 1) || (Scoops > 3) )
            Console::WriteLine(L"Invalid Choice - Try Again!");
    } while (Scoops < 1 || Scoops > 3);
}

In the Class View, right-click CIceCream -> Add -> Add Function
Set the Return Type to void
Set the Function Name to ProcessAnOrder and click Finish

In the IceCream.cpp source file, implement the ProcessAnOrder method as follows:

void CIceCream::ProcessAnOrder(void)
{
    double PriceIngredient, PriceScoop;

    // Let the user know that this is a vending machine
    Console::WriteLine(L"=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=");
    Console::WriteLine(L"Ice Scream Vending Machine");
    Console::WriteLine(L"-----------------------------------");

    // Let the user select the components of the ice scream
    ChooseFlavor();
    Console::WriteLine(L"-----------------------------------");
    ChooseContainer();
    Console::WriteLine(L"-----------------------------------");
    ChooseIngredient();
    Console::WriteLine(L"-----------------------------------");
    SpecifyNumberOfScoops();
    Console::WriteLine(L"-----------------------------------");

    // If the user selects an ingredient instead of "No Ingredient",
    // add $0.50 to the order
    if( (ChoiceIngredient == 2) ||
        (ChoiceIngredient == 3) ||
        (ChoiceIngredient == 4) )
        PriceIngredient = 0.50;
    else
        PriceIngredient = 0.00;

    // Instead of multiplying a number scoops to a value,
    // We will use an incremental value depending on 
    // the number of scoops
    if( Scoops == 1 )
        PriceScoop = 0.65;
    else if( Scoops == 2 )
        PriceScoop = 1.05;
    else
        PriceScoop = 1.55;

    // Calculate the total price of the ice scream
    TotalPrice = BasePrice + PriceScoop + PriceIngredient;

    // Create the ice scream...

    // And display a receipt to the user
    DisplayReceipt();
}

In the Class View, right-click CIceCream -> Add -> Add Function
Set the Return Type to void
Set the Function Name to DisplayReceipt and click Finish

In the IceCream.cpp source file, implement the DisplayReceipt method as follows:

void CIceCream::DisplayReceipt(void)
{
    Console::WriteLine(L"\n=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=");
    Console::WriteLine(L"Ice Scream Order");
    Console::WriteLine(L"-----------------------------------");
    Console::WriteLine(L"Flavor:      {0}",
                       Flavor[ChoiceFlavor - 1]);
    Console::WriteLine(L"Container:   {0}",
                       Container[ChoiceContainer - 1]);
    Console::WriteLine(L"Ingredient:  {0}",
                       Ingredient[ChoiceIngredient - 1]);
    Console::WriteLine(L"Scoops:      {0}", Scoops);
    Console::WriteLine(L"Total Price: {0:C}", TotalPrice);
    Console::WriteLine(L"=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=\n");
}

In the Class View, right-click CIceCream -> Add -> Add Function
Set the Return Type to void
Set the Function Name to SaveOrder and click Finish

In the IceCream.cpp source file, implement the SaveOrder method as follows:

void CIceCream::SaveOrder(void)
{
    String ^ strNameOfFile;

    Console::Write(L"Please enter your initials or the name "
                   L"we will use to remember your order: ");
    strNameOfFile = Console::ReadLine();
    strNameOfFile = strNameOfFile + L".icr";

    // Find out if the user entered a name of a file 
    // that is already in the machine
	if( File::Exists(strNameOfFile) )
    {
        String ^ answer;

        FileStream ^ stmIceScream =
                    gcnew FileStream(strNameOfFile, FileMode::Create);
        BinaryWriter ^ bnwIceScream =
                    gcnew BinaryWriter(stmIceScream);

        try {
            // If so, find out if the user wants to 
            // replace the old file
            Console::WriteLine(L"The file you entered exists already.");
            Console::Write(L"Do you want to replace it(y/n)?");
			answer = Console::ReadLine();

            // If the customer wants to replace it...
            if( (answer == L"y") || (answer == L"Y") )
            {
                // ... do so
                Console::WriteLine(L"The former order with the same "
                                   L"name will be replaced");

                Console::WriteLine(L"\n=-= Ice Scream Vending Machine =-=");
                Console::WriteLine(L" Saving Order: {0}", strNameOfFile);
                bnwIceScream::Write(Flavor[ChoiceFlavor - 1]);
                bnwIceScream::Write(Container[ChoiceContainer - 1]);
                bnwIceScream::Write(Ingredient[ChoiceIngredient - 1]);
                bnwIceScream::Write(Scoops);
                bnwIceScream::Write(TotalPrice);
            }
            // If the customer wants to save the new order with 
            // a different name
            else if( (answer == L"n") || answer == L"N") )
            {
                // Ask the user to enter a name to remember the order
                Console::Write(L"Please enter a name we will use "
                               L"to remember this order: ");
                strNameOfFile = Console::ReadLine();
                strNameOfFile = strNameOfFile + L".icr";

                stmIceScream = gcnew FileStream(strNameOfFile, FileMode::Create);
                bnwIceScream = gcnew BinaryWriter(stmIceScream);

                Console::WriteLine(L"\n=-= Ice Scream Vending Machine =-=");
                Console::WriteLine(L" Saving Order: {0}", strNameOfFile);
                bnwIceScream::Write(Flavor[ChoiceFlavor - 1]);
                bnwIceScream::Write(Container[ChoiceContainer - 1]);
                bnwIceScream::Write(Ingredient[ChoiceIngredient - 1]);
                bnwIceScream::Write(Scoops);
                bnwIceScream::Write(TotalPrice);
            }
            else
                Console::WriteLine(L"Invalid Answer - We will close");
        }
        finally
        {
            bnwIceScream->Close();
            stmIceScream->Close();
        }
    }
    else
    {
        FileStream ^ stmIceScream =
                    gcnew FileStream(strNameOfFile, FileMode::Create);
        BinaryWriter ^ bnwIceScream =
                    gcnew BinaryWriter(stmIceScream);

        try {
            Console::WriteLine(L"\n=-= Ice Scream Vending Machine =-=");
            Console::WriteLine(L" Saving Order: {0}", strNameOfFile);
            bnwIceScream::Write(Flavor[ChoiceFlavor - 1]);
            bnwIceScream::Write(Container[ChoiceContainer - 1]);
            bnwIceScream::Write(Ingredient[ChoiceIngredient - 1]);
            bnwIceScream::Write(Scoops);
            bnwIceScream::Write(TotalPrice);
        }
        finally
        {
            bnwIceScream->Close();
            stmIceScream->Close();
        }
    }
}

In the Class View, right-click CIceCream -> Add -> Add Function
Set the Return Type to void
Set the Function Name to OpenOrder and click Finish

In the IceCream.cpp source file, implement the OpenOrder method as follows:

void CIceCream::OpenOrder(void)
{
    String ^ strNameOfFile;
    String ^ SelectedFlavor;
    String ^ SelectedContainer;
    String ^ SelectedIngredient;

    // Ask the user to enter a name of a previously saved order
    Console::Write(L"Please enter the name you previously "
                   L"gave to remember your order: ");
    strNameOfFile = Console::ReadLine();
    strNameOfFile = strNameOfFile + L".icr";

    FileStream ^ stmIceScream =
                gcnew FileStream(strNameOfFile, FileMode::Open);
    BinaryReader ^ bnrIceScream =
                gcnew BinaryReader(stmIceScream);

    try {
        // Find out if this order was previously saved in the machine
		if( File::Exists(strNameOfFile) )
        {
            // If so, open it
            SelectedFlavor = bnrIceScream->ReadString();
            SelectedContainer = bnrIceScream->ReadString();
            SelectedIngredient = bnrIceScream->ReadString();
            Scoops = bnrIceScream->ReadInt32();
            TotalPrice = bnrIceScream->ReadDouble();

            // And display it to the user
            Console::WriteLine(L"\n=-= Ice Scream Vending Machine =-=");
            Console::WriteLine(L" Previous Order: {0}", strNameOfFile);
            Console::WriteLine(L"Flavor:      {0}", SelectedFlavor);
            Console::WriteLine(L"Container:   {0}", SelectedContainer);
            Console::WriteLine(L"Ingredient:  {0}", SelectedIngredient);
            Console::WriteLine(L"Scoops:      {0}", Scoops);
            Console::WriteLine(L"Total Price: {0:C}\n", TotalPrice);
        }
        else
            Console::WriteLine(L"The name you entered is not "
                               L"registered in our previous orders");
    }
    finally
    {
        bnrIceScream->Close();
        stmIceScream->Close();
    }  
}

Access the IceCream4.cpp source file and change it as follows:

// IceCream4.cpp : main project file.

#include "stdafx.h"
#include "IceCream.h"

using namespace System;

int main()
{
    String ^ answer = L"n";
    CIceCream ^ ic = gcnew CIceCream();

    Console::Write(L"Do you want to re-order a previously "
                   L"saved order(y/n)? ");
    answer = Console::ReadLine();

    if( (answer == L"y") || (answer == L"Y") )
        ic->OpenOrder();
    else
    {
        ic->ProcessAnOrder();
        Console::Write(L"Do you want us to remember this "
                       L"order the next time you come to "
                       L"get your ice scream (y/n)? ");
        answer = Console::ReadLine();
        if( (answer == L"y") || (answer == L"Y") )
            ic->SaveOrder();
        else
            Console::WriteLine(L"\nIt was nice serving you."
                               L"\nCome Again!!!\n");
    }

    return 0;
}

Execute the application and test it. Here is an example:

Do you want to re-order a previously saved order(y/n)? n
=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=
Ice Scream Vending Machine
-----------------------------------
What type of flavor do you want?
1 - Vanilla
2 - Cream of Cocoa
3 - Chocolate Chip
4 - Organic Strawberry
5 - Butter Pecan
6 - Cherry Coke
7 - Chocolate Brownies
8 - Caramel Au Lait
9 - Chunky Butter
10 - Chocolate Cookie
Your Choice? 6
-----------------------------------
What type of container do you want?
1 - Cone
2 - Cup
3 - Bowl
Your Choice? 2
-----------------------------------
Do you want an ingredient or not
1 - No Ingredient
2 - Peanuts
3 - M & M
4 - Cookies
Your Choice? 1
-----------------------------------
How many scoops(1, 2, or 3)? 2
-----------------------------------

=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=
Ice Scream Order
-----------------------------------
Flavor:      Cherry Coke
Container:   Cup
Ingredient:  No Ingredient
Scoops:      2
Total Price: $2.60
=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=

Do you want us to remember this order the next time you 
come to get your ice scream (y/n)? n

It was nice serving you.
Come Again!!!

Press any key to continue . . .

Close the DOS window

Execute the application and test it. Enter a wrong file name. Here is an example:

Do you want to re-order a previously saved order(y/n)? y
Please enter the name you previously gave to remember your order: dtr

Unhandled Exception: System::IO.FileNotFoundException: Could not find file 'C:\Do
cuments and Settings\Administrator\My Documents\Visual Studio 2005\Projects\Vend
ingMachine4\IceCream4\bin\Release\dtr.icr'.
File name: 'C:\Documents and Settings\Administrator\My Documents\Visual Studio 2
005\Projects\VendingMachine4\IceCream4\bin\Release\dtr.icr'
   at System::IO.__Error.WinIOError(Int32 errorCode, String ^ maybeFullPath)
   at System::IO.FileStream.Init(String ^ path, FileMode mode, FileAccess access, I
nt32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions o
ptions, SECURITY_ATTRIBUTES secAttrs, String ^ msgPath, Boolean bFromProxy)
   at System::IO.FileStream..ctor(String ^ path, FileMode mode)
   at IceCream4.IceScream.OpenOrder() in C:\Documents and Settings\Administrator
\My Documents\Visual Studio 2005\Projects\VendingMachine4\IceCream4\IceScream.cs
:line 360
   at IceCream4.Program.Main(String[] args) in C:\Documents and Settings\Adminis
trator\My Documents\Visual Studio 2005\Projects\VendingMachine4\IceCream4\Progra
m.cs:line 18
Press any key to continue . . .

Close the DOS window

.NET Framework Exception Handling for File Processing

In the previous lesson of our introduction to file processing, we behaved as if everything was alright. Unfortunately, file processing can be very strict in its assignments. Fortunately, the .NET Framework provides various Exception-oriented classes to deal with almost any type of exception you can think of.

One of the most important aspects of file processing is the name of the file that will be dealt with. In some cases you can provide this name to the application or document. In some other cases, you would let the user specify the name of the path. Regardless of how the name of the file would be provided to the operating system, when this name is acted upon, the compiler will be asked to work on the file. If the file doesn't exist, the operation cannot be carried. Furthermore, the compiler would throw an error. There are many other exceptions that can be thrown as a result of something going bad during file processing:

FileNotFoundException: The exception thrown when a file has not been found is of type FileNotFoundException. Here is an example of handling it:

using namespace System;
using namespace System::IO;

int main()
{
/*    String ^ NameOfFile = L"Members.clb";
	
    FileStream ^ fstPersons = gcnew FileStream(NameOfFile, FileMode::Create);
    BinaryWriter ^ wrtPersons = gcnew BinaryWriter(fstPersons);
        
    try 
    {
        wrtPersons->Write(L"James Bloch");
        wrtPersons->Write(L"Catherina Wallace");
        wrtPersons->Write(L"Bruce Lamont");
        wrtPersons->Write(L"Douglas Truth");
    }
    finally
    {
        wrtPersons->Close();
        fstPersons->Close();
    }*/
    
    String ^ NameOfFile = L"Members.clc";
    String ^ strLine = L"";

    try {
        FileStream ^ fstMembers =
                gcnew FileStream(NameOfFile, FileMode::Open);
        BinaryReader ^ rdrMembers = gcnew BinaryReader(fstMembers);

        try {
            strLine = rdrMembers->ReadString();
            Console::WriteLine(strLine);
			strLine = rdrMembers->ReadString();
            Console::WriteLine(strLine);
            strLine = rdrMembers->ReadString();
            Console::WriteLine(strLine);
            strLine = rdrMembers->ReadString();
            Console::WriteLine(strLine);
        }
        finally
        {
            rdrMembers->Close();
            fstMembers->Close();
        }
    }
    catch (FileNotFoundException ^ ex)
    {
        Console::Write(L"Error: " + ex->Message);
        Console::WriteLine(L" May be the file doesn't exist " +
                           L"or you typed it wrong!");
    }

    return 0;
}

Here is an example of what this would produce:

Error: Could not find file 'c:\Documents and Settings\Administrator
\My Documents\Visual Studio 2005\Projects\FileProcessing2
\FileProcessing2\Members.clc'. May be the file doesn't exist 
or you typed it wrong!
Press any key to continue . . .

IOException: As mentioned already, during file processing, anything could go wrong. If you don't know what caused an error, you can throw the IOException exception.

Practical Learning: Handling File Processing Exceptions

In the Scopes combo box, select CIceCream
In the Functions combo box, select SaveOrder

To throw exceptions, change the method as follows:

void CIceCream::SaveOrder(void)
{
    String ^ strNameOfFile;

    Console::Write(L"Please enter your initials or the name "
                   L"we will use to remember your order: ");
    strNameOfFile = Console::ReadLine();
    strNameOfFile = strNameOfFile + L".icr";

    try {
        // Find out if the user entered a name of a file 
        // that is already in the machine
        if( File::Exists(strNameOfFile) )
        {
            String ^ answer;
            FileStream ^ stmIceScream =
                    gcnew FileStream(strNameOfFile, FileMode::Create);
            BinaryWriter ^ bnwIceScream =
                    gcnew BinaryWriter(stmIceScream);

            try {
                // If so, find out if the user wants to 
                // replace the old file
                Console::WriteLine(L"The file you entered exists already.");
                Console::Write(L"Do you want to replace it(y/n)?");
		        answer = Console::ReadLine();

                // If the customer wants to replace it...
                if( (answer == "y") || (answer == "Y") )
                {
                    // ... do so
                    Console::WriteLine(L"The former order with the same "
                                       L"name will be replaced");

                    Console::WriteLine(L"\n=-= Ice Scream Vending Machine =-=");
                    Console::WriteLine(L" Saving Order: {0}", strNameOfFile);
                    bnwIceScream->Write(Flavor[ChoiceFlavor - 1]);
                    bnwIceScream->Write(Container[ChoiceContainer - 1]);
                    bnwIceScream->Write(Ingredient[ChoiceIngredient - 1]);
                    bnwIceScream->Write(Scoops);
                    bnwIceScream->Write(TotalPrice);
                }
                // If the customer wants to save the new order with 
                // a different name
                else if( (answer == "n") || (answer == "N") )
                {
                    // Ask the user to enter a name to remember the order
                    Console::Write(L"Please enter a name we will use "
                                   L"to remember this order: ");
                    strNameOfFile = Console::ReadLine();
                    strNameOfFile = strNameOfFile + L".icr";

                   try {
                        stmIceScream = gcnew FileStream(strNameOfFile, FileMode::Create);
                        bnwIceScream = gcnew BinaryWriter(stmIceScream);

                        Console::WriteLine(L"\n=-= Ice Scream Vending Machine =-=");
                        Console::WriteLine(L" Saving Order: {0}", strNameOfFile);
                        bnwIceScream->Write(Flavor[ChoiceFlavor - 1]);
                        bnwIceScream->Write(Container[ChoiceContainer - 1]);
                        bnwIceScream->Write(Ingredient[ChoiceIngredient - 1]);
                        bnwIceScream->Write(Scoops);
                        bnwIceScream->Write(TotalPrice);
                    }
                    catch(IOException ^)
                    {
                        Console::Write(L"\nThe file you wanted us to "
				       L"create exists already. ");
                        Console::WriteLine(L"In case it was registered "
                  			   L"by a different customer, "
				           L"we will not delete it.");
                    }
                }
                else
                    Console::WriteLine(L"Invalid Answer - We will close");
            }
            catch(IOException ^)
            {
                Console::Write(L"\nThat file exists already. ");
                Console::WriteLine(L"We need to preserve it just in "
				   L"case another customer will require it.");
            }
            finally
            {
                bnwIceScream->Close();
                stmIceScream->Close();
            }
        }
        else
        {
            FileStream ^ stmIceScream =
                    gcnew FileStream(strNameOfFile, FileMode::Create);
            BinaryWriter ^ bnwIceScream =
                    gcnew BinaryWriter(stmIceScream);

            try {
                Console::WriteLine(L"\n=-= Ice Scream Vending Machine =-=");
                Console::WriteLine(L" Saving Order: {0}", strNameOfFile);
                bnwIceScream->Write(Flavor[ChoiceFlavor - 1]);
                bnwIceScream->Write(Container[ChoiceContainer - 1]);
                bnwIceScream->Write(Ingredient[ChoiceIngredient - 1]);
                bnwIceScream->Write(Scoops);
                bnwIceScream->Write(TotalPrice);
            }
            finally
            {
                bnwIceScream->Close();
                stmIceScream->Close();
            }
        }
    }
    catch(IOException ^ ex)
    {
        Console::WriteLine(L"\nError: " + ex->Message);
        Console::WriteLine(L"Operation Canceled: The file you want "
                           L"to create exists already.");
    }
}

In the Functions combo box, select OpenOrder

To throw exceptions, change the method as follows:

void CIceCream::OpenOrder(void)
{
    String ^ strNameOfFile;
    String ^ SelectedFlavor;
    String ^ SelectedContainer;
    String ^ SelectedIngredient;

    // Ask the user to enter a name of a previously saved order
    Console::Write(L"Please enter the name you previously "
                   L"gave to remember your order: ");
    strNameOfFile = Console::ReadLine();
    strNameOfFile = strNameOfFile + ".icr";

    try {
        FileStream ^ stmIceScream =
                gcnew FileStream(strNameOfFile, FileMode::Open);
        BinaryReader ^ bnrIceScream =
                gcnew BinaryReader(stmIceScream);

        try {
            // Find out if this order was previously saved in the machine
            if( File::Exists(strNameOfFile) )
            {
                // If so, open it
                SelectedFlavor = bnrIceScream->ReadString();
                SelectedContainer = bnrIceScream->ReadString();
                SelectedIngredient = bnrIceScream->ReadString();
                Scoops = bnrIceScream->ReadInt32();
                TotalPrice = bnrIceScream->ReadDouble();

                // And display it to the user
                Console::WriteLine(L"\n=-= Ice Scream Vending Machine =-=");
                Console::WriteLine(L" Previous Order: {0}", strNameOfFile);
                Console::WriteLine(L"Flavor:      {0}", SelectedFlavor);
                Console::WriteLine(L"Container:   {0}", SelectedContainer);
                Console::WriteLine(L"Ingredient:  {0}", SelectedIngredient);
                Console::WriteLine(L"Scoops:      {0}", Scoops);
                Console::WriteLine(L"Total Price: {0:C}\n", TotalPrice);
            }
            else
                Console::WriteLine(L"The name you entered is not "
                                   L"registered in our previous orders");
        }
        finally
        {
            bnrIceScream->Close();
            stmIceScream->Close();
        }
    } 
    catch(FileNotFoundException ^ ex)
    {
        Console::Write(L"\nError: " + ex->Message);
        Console::WriteLine(L"It is unlikely that the file exists in "
                  		   L"our machine's register.");
    }
}

Exercise the application and test it
Close the DOS window

File Information

Introduction

In its high level of support for file processing, the .NET Framework provides the FileInfo class. This class is equipped to handle all types of file-related operations including creating, copying, moving, renaming, or deleting a file. FileInfo is based on the FileSystemInfo class that provides information on characteristics of a file.

Practical Learning: Introducing File Information

Start a new CLR Console Application named WattsALoan2
To take advantage of the Visual Basic rich library, in the Solution Explorer, right-click WattsALoan2 and click References...
In the Property Pages, click the Add New Reference button
In the .NET tab, click Microsoft.VisualBasic
Click OK and click OK

Change the source file file as follows:

// WattsALoan2.cpp : main project file.

#include "stdafx.h"

using namespace System;

int main(array<System::String ^> ^args)
{
    String ^ EmployerName, ^ ApplicantName;
    String ^ HomePhone, ^ WorkPhone;
    double LoanAmount = 0.00, InterestRate = 0.0;
    double MonthlyPayment = 0.00, Periods = 0;

    Console::WriteLine(L" -=- Car Loan Application -=-");
    Console::WriteLine(L"Enter the following pieces "
                       L"of information\n");
    Console::WriteLine(L"Applicant Information");
    Console::Write(L"Full Name:     ");
    ApplicantName = Console::ReadLine();
    Console::Write(L"Employer Name: ");
    EmployerName = Console::ReadLine();
    Console::Write(L"Home Phone:    ");
    HomePhone = Console::ReadLine();
    Console::Write(L"Work Phone:    ");
    WorkPhone = Console::ReadLine();
    Console::WriteLine(L"Loan Estimation");

    try {
        Console::Write(L"Amount of Loan: ");
		LoanAmount = double::Parse(Console::ReadLine());
    }
    catch(FormatException ^)
    {
        Console::WriteLine(L"Invalid Loan Amount");
    }

    try {
        Console::Write(L"Interest Rate(0 to 100): ");
		InterestRate = double::Parse(Console::ReadLine()) / 100;
    }
    catch(FormatException ^)
    {
        Console::WriteLine(L"Invalid Interest Rate");
    }

    try {
        Console::Write(L"Number of Months: ");
		Periods = double::Parse(Console::ReadLine());
    }
    catch(FormatException ^)
    {
        Console::WriteLine(L"Invalid Number of Months");
    }

	try {
        MonthlyPayment =
	    Microsoft::VisualBasic::Financial::Pmt(InterestRate / 12,
                Periods, -LoanAmount,
		0, Microsoft::VisualBasic::DueDate::BegOfPeriod);
    }
    catch(ArgumentException ^)
    {
        Console::WriteLine(L"Some invalid values were provided\n");
    }

    Console::WriteLine();
    Console::WriteLine(L" -=-=-=-=-=-=-=-=-=-=-=-=-=-=");
    Console::WriteLine(L" -=- Car Loan Application -=-");
    Console::WriteLine(L" -=-=-=-=-=-=-=-=-=-=-=-=-=-=");
    Console::WriteLine(L"Applicant Information");
    Console::WriteLine(L"Full Name:     {0}", ApplicantName);
    Console::WriteLine(L"Employer Name: {0}", EmployerName);
    Console::WriteLine(L"Home Phone:    {0}", HomePhone);
    Console::WriteLine(L"Work Phone:    {0}", WorkPhone);
    Console::WriteLine(L"Loan Estimation");
    Console::WriteLine(L"Loan Amount:      {0:C}", LoanAmount);
    Console::WriteLine(L"Interest Rate:    {0:P}", InterestRate);
    Console::WriteLine(L"Number of Months: {0:F}", Periods);
    Console::WriteLine(L"Monthly Payment:  {0:C}\n", MonthlyPayment);

    return 0;
}

Execute the application and test it. Here is an example:

 -=- Car Loan Application -=-
Enter the following pieces of information

Applicant Information
Full Name:     James Watts
Employer Name: Wattson Enterprises
Home Phone:    (202) 374-4738
Work Phone:    (301) 894-4789
Loan Estimation
Amount of Loan: 12500
Interest Rate(0 to 100): 10.25
Number of Months: 48

 -=-=-=-=-=-=-=-=-=-=-=-=-=-=
 -=- Car Loan Application -=-
 -=-=-=-=-=-=-=-=-=-=-=-=-=-=
Applicant Information
Full Name:     James Watts
Employer Name: Wattson Enterprises
Home Phone:    (202) 374-4738
Work Phone:    (301) 894-4789
Loan Estimation
Loan Amount:      $12,500.00
Interest Rate:    10.25 %
Number of Months: 48.00
Monthly Payment:  $315.84

Press any key to continue...

Close the DOS window

File Initialization

The FileInfo class is equipped with one constructor whose syntax is:

public:
    FileInfo(String ^ fileName);

This constructor takes as argument the name of a file or its complete path. If you provide only the name of the file, the compiler would consider the same directory of its project. Here is an example:

FileInfo ^ fleMembers = gcnew FileInfo(L"First.txt");

Alternatively, if you want, you can provide any valid directory you have access to. In this case, you should provide the complete path.

File Creation

The FileInfo constructor is mostly meant only to indicate that you want to use a file, whether it exists already or it would be created. Based on this, if you execute an application that has only a FileInfo object created using the constructor as done above, nothing would happen.

To create a file, you have various alternatives. If you want to create one without writing anything in it, which implies creating an empty file, you can call the FileInfo::Create() method. Its syntax is:

public:
    FileStream ^ Create();

This method simply creates an empty file. Here is an example of calling it:

FileInfo ^ fleMembers = gcnew FileInfo(L"First.txt");
fleMembers->Create();

The FileInfo::Create() method returns a FileStream object. You can use this returned value to write any type of value into the file, including text. If you want to create a file that contains text, an alternative is to call the FileInfo::CreateText() method. Its syntax is:

public:
    StreamWriter CreateText();

This method directly returns a StreamWriter object. You can use this returned object to write text to the file.

File Existence

When you call the FileInfo::Create() or the FileInfo::CreateText() method, if the file passed as argument, or as the file in the path of the argument, exists already, it would be deleted and a new one would be created with the same name. This can cause the right file to be deleted. Therefore, before creating a file, you may need to check whether it exists already. To do this, you can check the value of the Boolean FileInfo::Exists property. This property holds a true value if the file exists already and it holds a false value if the file doesn't exist or it doesn't exist in the path.

Here is an example of checking the existence of a file:

FileInfo ^ fleMembers = gcnew FileInfo(L"First.txt");
fleMembers->Create();

if( fleMembers.Exists == true )
	return;

Writing to a File

As mentioned earlier, the FileInfo::Create() method returns a FileStream object. You can use this to specify the type of operation that would be allowed on the file.

To write normal text to a file, you can first call the FileInfo::CreateText() method. This method returns a StreamWriter object. The StreamWriter class is based on the TextWriter class that is equipped with the Write() and the WriteLine() methods used to write values to a file. The Write() method writes text on a line and keeps the caret on the same line. The WriteLine() method writes a line of text and moves the caret to the next line.

After writing to a file, you should close the StreamWriter object to free the resources it was using during its operation(s).

Practical Learning: Writing to a File

To allow the user to create a new employee, change the file as follows:

// WattsALoan2.cpp : main project file.

#include "stdafx.h"

using namespace System;
using namespace System::IO;

void CreateNewEmployee()
{
    String ^ employeeNumber, ^ employeeName;
    FileInfo ^ fleEmployees = gcnew FileInfo(L"Employees.txt");
    StreamWriter ^ swrEmployees = fleEmployees->CreateText();

    try {
        Console::WriteLine(L"Hiring New Employee");
        Console::Write(L"Enter Employee Number as 00-000: ");
        employeeNumber = Console::ReadLine();
        Console::Write(L"Enter Employee Name: ");
        employeeName = Console::ReadLine();

        swrEmployees->WriteLine(employeeNumber);
        swrEmployees->WriteLine(employeeName);

   }
   finally
   {
       swrEmployees->Flush();
       swrEmployees->Close();
   }
}

int main(array<System::String ^> ^args)
{
    try {
        Console::Write(L"Do you want to hire a new "
                       L"employee(0=No/1=Yes)? ");
        int answer = int::Parse(Console::ReadLine());

        if( answer == 1 )
            CreateNewEmployee();
    }
    catch(FormatException ^)
    {
        Console::WriteLine(L"That was an invalid answer!\n");
    }

    String ^ EmployerName, ^ ApplicantName;
    String ^ HomePhone, ^ WorkPhone;
    double LoanAmount = 0.00, InterestRate = 0.0;
    double MonthlyPayment = 0.00, Periods = 0;

    . . . No Change

    return 0;
}

Execute the application. Here is an example:

Do you want to hire a new employee(0=No/1=Yes)? 1
Hiring New Employee
Enter Employee Number as 00-000: 44-228
Enter Employee Name: Johnny Olney

Close the DOS window

Appending to a File

You may have created a text-based file and written to it. If you open such a file and find out that a piece of information is missing, you can add that information to the end of the file. To do this, you can call the FileInfo::AppenText() method. Its syntax is:

public:
    StreamWriter ^ AppendText();

When calling this method, you can retrieve the StreamWriter object that it returns, then use that object to add new information to the file.

Practical Learning: Appending to a File

To allow the user to add more employees to the file that holds their names, change the file as follows:

// WattsALoan2.cpp : main project file.

#include "stdafx.h"

using namespace System;
using namespace System::IO;

void CreateNewEmployee()
{
    String ^ employeeNumber, ^ employeeName;
    FileInfo ^ fleEmployees = gcnew FileInfo(L"Employees.txt");
    StreamWriter ^ swrEmployees = nullptr;

    // If the file exists already, then add the new employee to it
    if( fleEmployees->Exists == true)
        swrEmployees = fleEmployees->AppendText();
    else // Otherwise, create a new file
        swrEmployees = fleEmployees->CreateText();

    try {
        . . . No Change

   }
   finally
   {
       swrEmployees->Flush();
       swrEmployees->Close();
   }
}

int main(array<System::String ^> ^args)
{
    . . . No Change

    return 0;
}

Execute the application

Type y to create an employee. Here is an example:

Do you want to hire a new employee(0=No/1=Yes)? 1
Hiring New Employee
Enter Employee Number as 00-000: 72-604
Enter Employee Name: Ernest Barzan

Close the DOS window

Functions Language

Pages

Details on File Processing

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