Home

Introduction to File Processing

 

Overview of File Processing and Definitions

 

Introduction

A piece of information used in an application is primarily represented as a group of bits. So far, if we requested information from the user, when the application exited, we lost all information that the user had entered. This is because such information was only temporarily stored in the random access memory (RAM). In some cases, you will want to "keep" information that the user has entered so you can make the information available the next time the user opens the application. In some other cases, whether you request information from the user or inherently provide it to the user, you may want different people working from different computers to use or share the same data. In these and other scenarios, you must store the information somewhere and retrieve it when necessary. This is the basis of file processing.

 

Files

A file is a series of bytes of data that are arranged in a particular manner to produce a usable document. For easy storage, location, and management, the bytes are stored on a medium such as a hard disc, a floppy disc, a compact disc, or any valid and supported type of storage. When these bytes belong to a single but common entity and hold values that are stored on a medium, the group is referred to as a file.

For greater management, files can be stored in a parent object called a directory or a folder. Since a file is a unit of storage and it stores information, it has a size, which is the number of bits it uses to store its values. To manage it, a file has a location also called a path that specifies where and/or how the file can be retrieved. Also, for better management, a file has attributes (characteristics) that indicate what can be done on the file or that provide specific information that the programmer or the operating system can use when dealing with the file.

Streams

File processing consists of creating, storing, and/or retrieving the contents of a file from a recognizable medium. For example, it is used to save word-processed files to a hard drive, to store a presentation on floppy disk, or to open a file from a CD-ROM. A stream is the technique or means of performing file processing. In order to manage files stored in a computer, each file must be able to provide basic pieces of information about itself. This basic information is specified when the file is created but can change during the lifetime of a file.

To create a file, a user must first decide where it would be located: this is a requirement. A file can be located on the root drive. Alternatively, a file can be positioned inside of an existing folder. Based on security settings, a user may not be able to create a file just anywhere in the (file system of the) computer. Once the user has decided where the file would reside, there are various means of creating files that the users are trained to use. When creating a file, the user must give it a name following the rules of the operating system combined with those of the file system. The most fundamental piece of information a file must have is a name.

Once the user has created a file, whether the file is empty or not, the operating system assigns basic pieces of information to it. Once a file is created, it can be opened, updated, modified, renamed, etc.

Streaming Prerequisites

 

Introduction

To support file processing, the .NET Framework provides the System::IO namespace that contains many different classes to handle almost any type of file operation you may need to perform. Therefore, to perform file processing, you can include the System::IO namespace in your project.

The parent class of file processing is Stream. With Stream, you can store data to a stream or you can retrieve data from a stream. Stream is an abstract class, which means that you cannot use it to declare a variable in your application. As an abstract class, Stream is used as the parent of the classes that actually implement the necessary operations. You will usually use a combination of classes to perform a typical operation. For example, some classes are used to create a stream object while some others are used to write data to the created stream.

Practical LearningPractical Learning: Introducing Streaming

  1. Start Microsoft Visual C++/CLI or Visual Studio and create a Windows Forms Application named ClarksvilleIceCream2
  2. In the Properties window, change the form's Text to Ice Cream Vending Machine
  3. In the Common Controls section of the Toolbox, click ToolTip and click the form
  4. In the Menu and Toolbars section of the Toolbox, click StatusStrip and click the form
  5. Design the form as follows:
     
    Control Name Text ToolTip on toolTip1 Additional Properties
    GroupBox        
    Label   Please enter your initials and click Open:    
    TextTox txtInitials   First enter your initials. Then click Open  
    Button btnFileProcessing Open First enter your initials. Then click Open  
    Label   Order Date:    
    DateTimePicker dtpOrderDate   Click the arrow to select a date Format: Short
    Label   Order Time:    
    DateTimePicker dtpOrderTime   Click each section, then click one of the arrows to change its value Format: Time
    ShowUpDown: True
    Label   Flavor:    
    ComboBox cboFlavors   Click the arrow to display a list, then select a flavor from the list DropDownStyle: DropDownList
    Label   Container:    
    ComboBox cboContainers   Click to display the list of containers and select one DropDownStyle: DropDownList
    Label   Ingredient:    
    ComboBox cboIngredients   Display the list of ingredients and make the customer's choice DropDownStyle: DropDownList
    Label   Scoops:    
    TextBox txtScoops 1 Enter the number of scoops (1, 2, or 3) to fill the container TextAlign: Right
    Label   Order Total:    
    TextBox txtOrderTotal 0.00 This displays the total amount of this order TextAlign: Right
    Label   Amount Tended:    
    TextBox txtAmountTended   Amount you inserted into the machine  
    Label   Change    
    TextBox txtChange   Amount given back to you  
    Button btnClose Close Click to end  
  6. Click the combo box to the right of the Flavor label. Then, in the Properties, click the ellipsis button Ellipsis of Items property and create the list with:
     
    Vanilla
    Cream of Cocoa
    Chocolate Chip
    Cherry Coke
    Butter Pecan
    Chocolate Cookie
    Chunky Butter
    Organic Strawberry
    Chocolate Brownies
    Caramel Au Lait
  7. Click OK
  8. Click the combo box to the right of the Container label. Then, in the Properties, click the ellipsis button Ellipsis of Items property and create the list with:
     
    Cone
    Cup
    Bowl
  9. Click OK
  10. Click the combo box to the right of the Ingredient label. Then, in the Properties, click the ellipsis button Ellipsis of Items property and create the list with:
     
    None
    Peanuts
    Mixed Nuts
    M & M
    Cookies
  11. Click OK
  12. On the form, click the Scoops text box and click the Events button of the Properties window 
  13. In the Events section of the Properties window, look for and double-click Leave to generate its event
  14. Implement it as follows:
     
    System::Void txtScoops_Leave(System::Object^  sender, System::EventArgs^  e)
    {
        double PriceContainer  = 0.00,
               PriceIngredient = 0.00,
               PriceScoops     = 0.00,
    	       OrderTotal      = 0.00;
    	int NumberOfScoops     = 1;
    
        // The price of a container depends on which one the customer selected
        if( cboContainers->Text == L"Cone" )
    		PriceContainer = 0.55;
        else if( cboContainers->Text == L"Cup" )
    		PriceContainer = 0.75;
        else
    		PriceContainer = 1.15;
            
        // Find out if the customer wants any ingredient at all
        if( this->cboIngredients->Text == L"None" )
    		PriceIngredient = 0.00;
    	else
    		PriceIngredient = 0.95;
            
        try {
    	// Get the number of scoops
    	NumberOfScoops = int::Parse(this->txtScoops->Text);
    	
    	if( NumberOfScoops == 1 )
    		PriceScoops = 1.85;
    	else if( NumberOfScoops == 2 )
    		PriceScoops = 2.55;
    	else // if( NumberOfScoops == 3 )
    		PriceScoops = 3.25;
            
        // Make sure the user selected a flavor, 
        // otherwise, there is no reason to process an order
        if( this->cboFlavors->Text != L"" )
    	{
    		OrderTotal = PriceScoops + PriceContainer + PriceIngredient;
    		this->txtOrderTotal->Text = OrderTotal.ToString("F");
    		txtAmountEntered->Text = OrderTotal.ToString("F");
    
    		txtAmountEntered->Focus();
    
    		System::Windows::FormsDialogResult answer = 
    			MessageBox::Show(
    			    L"Do you want to save this order to remember it "
                                L"the next time you come to "
                                L"get your ice scream?",
    			    L"Ice Cream Vending Machine",
    			    MessageBoxButtons::YesNo,
    			    MessageBoxIcon::Question);
    
    		if( answer == System::Windows::Forms::DialogResult::Yes )
    		{
    			MessageBox::Show(
    				L"Please enter your initials and click Save");
    			btnFileProcessing->Text = L"Save";
    			txtInitials->Text = L"";
    			txtInitials->Focus();
    		}
    	}
        }
        catch(FormatException ^)
        {
    	MessageBox::Show(L"The value you entered for the scoops is not valid"
    	              L"\nOnly natural numbers such as 1, 2, or 3 are allowed"
    	                 L"\nPlease try again");
        }
    }
  15. Return to the form
  16. On the form, click the Amount Entered text box and, in the Events section of the Properties window, generate the Leave event
  17. Implement it as follows:
     
    System::Void txtAmountEntered_Leave(System::Object^  sender, 
    		System::EventArgs^  e)
    {
        double TotalOrder,
        AmountEntered = 0.00,
        Change;
    
        // Get the value of the total order. Actually, this value 
        // will be provided by the main form
        TotalOrder = double::Parse(this->txtOrderTotal->Text);
    	
        try 
        {
    	// The amount tended will be entered by the user
    	AmountEntered = double::Parse(this->txtAmountEntered->Text);
        }
        catch(FormatException ^)
        {
    	MessageBox::Show(L"The amount you entered is not "
    			 L"valid - Please try again!");
        }
    
        // Calculate the difference of both values, assuming 
        // that the amount tended is higher
        Change = AmountEntered - TotalOrder;
    
        // Display the result in the Difference text box
        this->txtChange->Text = Change.ToString("F");
    }
  18. Execute the application. Here is an example:
     
  19. Close the form and return to Visual Studio

The Name of a File

Before performing file processing, one of your early decisions will consist of specifying the type of operation you want the user to perform. For example, the user may want to create a brand new file, open an existing file, or perform a routine operation on a file. In all or most cases, whether you are creating a new file or manipulating an existing one, you must specify the name of the file. You can do this by declaring a String variable but, as we will learn later on, most classes used to create a stream can take a string that represents the file.

If you are creating a new file, there are certainly some rules you must observe. The name of a file follows the directives of the operating system. On MS DOS and Windows 3.X (that is, prior to Microsoft Windows 9X), the file had to use the 8.3 format. The actual name had to have a maximum of 8 characters with restrictions on the characters that could be used. The user also had to specify three characters after a period. The three characters, known as the file extension, were used by the operating system to classify the file. That was all necessary for those 8-bit and 16-bit operating systems. Various rules have changed. For example, the names of folders and files on Microsoft Windows >= 95 can have up to 255 characters. The extension of the file is mostly left to the judgment of the programmer but the files are still using extensions. Applications can also be configured to save different types of files; that is, files with different extensions.

Author Note At the time of this writing, the rules for file names for Microsoft Windows were on the MSDN web site at Windows Development\Windows Base Services\Files and I/O\SDK Documentation\Storage\Storage Overview\File Management\Creating, Deleting, and Maintaining Files\Naming a File (because it is a web site and not a book, its pages can change anytime).

Based on this, if you declare a String variable to hold the name of the file, you can simply initialize the variable with the necessary name and its extension. Here is an example:

System::Void btnSave_Click(System::Object^  sender, System::EventArgs^  e)
{
	 String ^ NameOfFile = L"Employees.spr";
}

Practical LearningPractical Learning: Specifying the Name of a File

  1. To add a new form to the project, on the main menu, click Project -> Add Class...
  2. In the Templates section, click Windows Form and set the Name to FileProcessing
  3. Click Add and design the form as follows:
     
    Control Name Text Other Properties
    Label lblMessage Message: Modifiers: Public
    TextBox txtInitials   Modifiers: Public
    Button btnFileProcessing Process DialogResult: OK
    Modifiers: Public
  4. Return to the first form
  5. Right-click the form and click View Code
  6. In the top section of the file, just under #pragma once, type #include "FileProcessing.h"
  7. Return to the form
  8. Double-click the Close button and implement its Click event as follows:
     
    System::Void btnClose_Click(System::Object^  sender, System::EventArgs^  e)
    {
        System::Windows::Forms::DialogResult answer =
    	 MessageBox::Show(L"Do you want to save this order to remember it "
                             L"the next time you come to "
                             L"get your ice scream?",
    			 L"Ice Cream Vending Machine",
    			 MessageBoxButtons::YesNo,
    			 MessageBoxIcon::Question);
    
        if( answer == System::Windows::Forms::DialogResult::Yes )
        {
    	MessageBox::Show(L"In the following form, please enter "
    			 L"your initials and click Save");
    	FileProcessing ^ frmFileProcessing = gcnew FileProcessing;
    	frmFileProcessing->lblMessage->Text = "Enter your initials and click Save";
    	frmFileProcessing->btnFileProcessing->Text = L"Save";
    	frmFileProcessing->txtInitials->Text = L"";
    	frmFileProcessing->txtInitials->Focus();
    
    	if( frmFileProcessing->ShowDialog() ==
    		 System::Windows::Forms::DialogResult::OK )
    	{
    	    if( frmFileProcessing->txtInitials->Text == L"" )
    		MessageBox::Show(L"Invalid File Name: Empty Initials");
    	    else
    	    {
    		String ^ NameOfFile = frmFileProcessing->txtInitials->Text;
    	    }
    	 }
        }
        else
    	 MessageBox::Show("Good Bye: It was a delight serving you");
    
        Close();
    }

The Path to a File

If you declare a string as above, the file will be created in the folder as the application. Otherwise, you can create your new file anywhere in the hard drive or on another medium. To do that, you must provide a complete path where the file will reside. A path is a string that specifies the drive (such as A:, C:, or D:, etc). The sections of a complete path are separated by a backslash. For example, a path can be made of a folder followed by the name of the file. An example would be

C:\Palermo.tde 

A path can also consist of a drive followed by the name of the folder in which the file will be created. Here is an example:

C:\Program Files\Palermo.tde

A path can also indicate that the file will be created in a folder that itself is inside of another folder. In this case, remember that the names of folders must be separated by backslashes.

In Lesson 2, we saw that the backslash character is used to create or manage escape sequences and it can be included in a string value to make up an escape sequence. Because of this, every time you include a backslash in a string, the compiler thinks that you are trying to provide an escape sequence. In this case, if the combination of the backslash and the character that follows the backslash is not recognized as an escape sequence, you would get an error. To solve this problem, you have two alternatives. To indicate that the backslash must be considered as a character in its own right, you can double it. Here are examples:

System::Void btnSave_Click(System::Object^  sender, System::EventArgs^  e)
{			
    String ^ NameOfFile = L"C:\\Documents and "
			  L"Settings\\Business Records\\Employees.spr";
}

In the same way, you can declare a String variable to represent the name of an existing file that you plan to use in your program. You can also represent its path.

When providing a path to the file, if the drive you specify doesn't exist or cannot be read, the compiler would consider that the file doesn't exist. If you provide folders that don't exist in the drive, the compiler would consider that the file doesn't exist. This also means that the compiler will not create the folder(s) (the .NET Framework provides all means to create a folder but you must ask the compiler to create it; simply specifying a folder that doesn't exist will not automatically create it, even if you are creating a new file). Therefore, it is your responsibility to make sure that either the file or the path to the file is valid. As we will see in the next section, the compiler can check the existence of a file or path.

The .NET Support for Files

 

Introduction

The primary support of a file as an object is provided by a .NET Framework class called File. This static class is equipped with various types of (static) methods to create, save, open, copy, move, delete, or check the existence of a file.

File Existence

One of the valuable operations that the File class can perform is to check the existence of the file you want to use. For example, if you are creating a new file, you may want to make sure it doesn't exist already because if you try to create a file that exists already, the compiler may first delete the old file before creating the new one. This could lead to unpredictable result, especially because such a file is not sent to the Recycle Bin. On the other hand, if you are trying to open a file, you should first make sure the file exists, otherwise the compiler will not be able to open a file it cannot find.

To check the existence of a file, the File class provides the Exists method. Its syntax is:

public:
    static bool Exists(String ^path);

If you provide only the name of the file, the compiler would check it in the folder of the application. If you provide the path to the file, the compiler would check its drive, its folder(s) and the file itself. In both cases, if the file exists, the method returns true. If the compiler cannot find the file, the method returns false. It's important to know that if you provided a complete path to the file, any slight mistake would produce a false result.

File Creation

Besides checking the existence of the file, the File class can be used to create a new file. To support this operation, the File class is equipped with the Create() method that is overloaded with two versions as follows:

public:
    static FileStream ^ Create(String ^ path);
public:
    static FileStream ^ Create(String ^ path, int buffersize);

In both cases, the File.Create() method returns a Stream value, in this case a FileStream value. As the File.Create() method indicates, it takes the name or path of the file as argument. If you know or want to specify the size, in bytes, of the file, you can use the second version.

To provide the same operation of creating a file, you can use the Open() method of the File class. It is overloaded in three versions as follows:

public:
    static FileStream ^ Open(String ^ path, FileMode mode);
public:
    static FileStream ^ Open(String ^ path, 
			     FileMode mode, 
			     FileAccess access);
public:
   static FileStream Open(String ^ path,
                          FileMode mode,
       			  FileAccess access,
   			  FileShare share);

Access to a File

In order to perform an operation on a file, you must specify to the operating system how to proceed. One of the options you have is to indicate the type of access that will be granted on the file. This access is specified using the FileAccess enumerator. The members of the FileAccess enumerator are:

  • FileAccess::Write: New data can be written to the file
  • FileAccess.Read: Existing data can be read from the file
  • FileAccess.ReadWrite: Existing data can be read from the file and new data be written to the file

File Sharing

In standalone workstations, one person is usually able to access and open a file then perform the necessary operations on it. In networked computers, you may create a file that different people can access at the same time or you may make one file access another file to retrieve information. For example, suppose you create an application for a fast food restaurant that has two or more connected workstations and all workstations save their customers orders to a common file. In this case, you must make sure that any of the computers can access the file to save an order. An employee from one of these workstations must also be able to open the file to retrieve a customer order for any necessary reason. You can also create a situation where one file holds an inventory of the items of a store and another file holds the customers orders. Obviously one file would depend on another. Based on this, when an operation must be performed on a file, you may have to specify how a file can be shared. This is done through the FileShare enumerator.

The values of the FileShare enumerator are:

  • FileShare.Inheritable: Allows other file handles to inherit from this file
  • FileShare.None: The file cannot be shared
  • FileShare.Read: The file can be opened and read from
  • FileShare::Write: The file can be opened and written to
  • FileShare.ReadWrite: The file can be opened to write to it or read from it

The Mode of a File

Besides the access to the file, another option you will most likely specify to the operating system is referred to as the mode of a file. It is specified through the FileMode enumerator. The members of the FileMode Enumerator are:

  • FileMode.Append: If the file already exists, the new data will be added to its end. If the file doesn't exist, it will be created and the new data will be added to it
  • FileMode.Create: If the file already exists, it will be deleted and a new file with the same name will be created. If the file doesn't exist, then it will be created
  • FileMode.CreateNew: If the new already exists, the compiler will throw an error. If the file doesn't exist, it will be created
  • FileMode.Open: If the file exists, it will be opened. If the file doesn't exist, an error would be thrown
  • FileMode.OpenOrCreate: If the file already exists, it will be opened. If the file doesn't exist, it will be created
  • FileMode.Truncate: If the file already exists, its contents will be deleted completely but the file will be kept, allowing you to write new data to it. If the file doesn't exist, an error would be thrown

Fundamentals of File Streaming

 

Introduction

File streaming consists of performing one of the routine operations on a file, such as creating it or opening it. This basic operation can be performed using a class called FileStream. You can use a FileStream object to get a stream ready for processing. As one of the most complete classes of file processing of the .NET Framework, FileStream is equipped with all necessary properties and methods. To use it, you must first declare a variable of it. The class is equipped with nine constructors.

One of the constructors (the second) of the FileStream class has the following syntax:

public:
    FileStream(String ^ path, FileMode mode);

This constructor takes as its first argument the name or the file or its path. The second argument specifies the type of operation to perform on the file. Here is an example:

System::Void btnSave_Click(System::Object^  sender, System::EventArgs^  e)
{
    String ^ NameOfFile = L"Persons.spr";

    FileStream ^ fstPersons = gcnew FileStream(NameOfFile,
					       FileMode::Create);
}

Stream Writing

A streaming operation is typically used to create a stream. Once the stream is ready, you can write data to it. The writing operation is perform through various classes. One of these classes is BinaryWriter.

The BinaryWriter class can be used to write values of primitive data types (char, int, float, double, etc). To use a BinaryWriter value, you can first declare its variable. To do this, you would use one of the class' three constructors. One of its constructors (the second) has the following syntax:

public:
    BinaryWriter(Stream ^ output);

This constructor takes as argument a Stream value, which could be a FileStream variable. Here is an example:

System::Void btnSave_Click(System::Object^  sender, System::EventArgs^  e)
{
    String ^ NameOfFile = L"Persons.spr";

    FileStream ^ fstPersons = gcnew FileStream(NameOfFile,
					       FileMode::Create);
    BinaryWriter ^ wrtPersons = gcnew BinaryWriter(fstPersons);
}

Most classes that are used to add values to a stream are equipped with a method called Write. This is also the case for the BinaryWriter class. This method takes as argument the value that must be written to the stream. The method is overloaded so that there is a version for each primitive data type. Here is an example that adds strings to a newly created file:

System::Void btnSave_Click(System::Object^  sender, System::EventArgs^  e)
{
    String ^ NameOfFile = L"Persons.spr";

    FileStream ^ fstPersons = gcnew FileStream(NameOfFile,
					       FileMode::Create);
    BinaryWriter ^ wrtPersons = gcnew BinaryWriter(fstPersons);

    wrtPersons->Write(txtPerson1->Text);
    wrtPersons->Write(txtPerson2->Text);
    wrtPersons->Write(txtPerson3->Text);
    wrtPersons->Write(txtPerson4->Text);

    txtPerson1->Text = L"";
    txtPerson2->Text = L"";
    txtPerson3->Text = L"";
    txtPerson4->Text = L"";
}

Stream Closing

When you use a stream, it requests resources from the operating system and uses them while the stream is available. When you are not using the stream anymore, you should free the resources and make them available again to the operating system so that other services can use them. This is done by closing the stream.

To close a stream, you can can call the Close() method of the class(es) you were using. Here are examples:

System::Void btnSave_Click(System::Object^  sender, System::EventArgs^  e)
{
    String ^ NameOfFile = L"Persons.spr";

    FileStream ^ fstPersons = gcnew FileStream(NameOfFile,
					       FileMode::Create);
    BinaryWriter ^ wrtPersons = gcnew BinaryWriter(fstPersons);

    wrtPersons->Write(txtPerson1->Text);
    wrtPersons->Write(txtPerson2->Text);
    wrtPersons->Write(txtPerson3->Text);
    wrtPersons->Write(txtPerson4->Text);

    wrtPersons->Close();
    fstPersons->Close();

    txtPerson1->Text = L"";
    txtPerson2->Text = L"";
    txtPerson3->Text = L"";
    txtPerson4->Text = L"";
}

Practical LearningPractical Learning: Writing to a Stream

  1. Change the code of the btnClose_Click event as follows:
     
    System::Void btnClose_Click(System::Object^  sender, System::EventArgs^  e)
    {
        System::Windows::Forms::DialogResult answer =
    	 MessageBox::Show(L"Do you want to save this order to remember it "
                             L"the next time you come to "
                             L"get your ice scream?",
    			 L"Ice Cream Vending Machine",
    			 MessageBoxButtons::YesNo,
    			 MessageBoxIcon::Question);
    
        if( answer == System::Windows::Forms::DialogResult::Yes )
        {
    	 MessageBox::Show(L"In the following form, please enter "
    			  L"your initials and click Save");
    	 FileProcessing ^ frmFileProcessing = gcnew FileProcessing;
    	 frmFileProcessing->lblMessage->Text = 
    		"Enter your initials and click Save";
    	 frmFileProcessing->btnFileProcessing->Text = L"Save";
    	 frmFileProcessing->txtInitials->Text = L"";
    	 frmFileProcessing->txtInitials->Focus();
    
    	 if( frmFileProcessing->ShowDialog() == 
    		System::Windows::Forms::DialogResult::OK )
    	 {
    	     if( frmFileProcessing->txtInitials->Text == L"" )
    		 MessageBox::Show(L"Invalid File Name: Empty Initials");
    	     else
    	     {
    		 String ^ NameOfFile = 
    			frmFileProcessing->txtInitials->Text + L".icr";
    
    		 FileStream ^ stmIceCream =
    			gcnew FileStream(NameOfFile, FileMode::Create);
    		 BinaryWriter ^ bnwIceCream =
    	                    gcnew BinaryWriter(stmIceCream);
    
    		 bnwIceCream->Write(dtpOrderDate->Value.ToShortDateString());
    		 bnwIceCream->Write(dtpOrderTime->Value.ToShortTimeString());
    		 bnwIceCream->Write(cboFlavors->Text);
    		 bnwIceCream->Write(cboContainers->Text);
    		 bnwIceCream->Write(cboIngredients->Text);
    		 bnwIceCream->Write(txtScoops->Text);
    		 bnwIceCream->Write(txtOrderTotal->Text);
    
    		 bnwIceCream->Close();
    		 stmIceCream->Close();
    
    		 MessageBox::Show(L"The order has been saved");
    	     }
    	 }
        }
        else
    	 MessageBox::Show("Good Bye: It was a delight serving you");
    
        Close();
    }
  2. Scroll completely in the file and, under the other using namespace lines, type
    using namespace System::IO;
  3. Execute the application and test it. Here is an example:
     
  4. Close the DOS window

Stream Reading

As opposed to writing to a stream, you may want to read existing data from it. Before doing this, you can first specify your intent to the streaming class using the FileMode enumerator. This can be done using the FileStream class as follows:

System::Void btnOpen_Click(System::Object^  sender, System::EventArgs^  e)
{
    String ^ NameOfFile = L"Persons.spr";

    FileStream ^ fstPersons = gcnew FileStream(NameOfFile, FileMode::Open);
}

Once the stream is ready, you can get prepared to read data from it. To support this, you can use the BinaryReader class. This class provides two constructors. One of the constructors (the first) has the following syntax:

public:
    BinaryReader(Stream ^ input);

This constructor takes as argument a Stream value, which could be a FileStream object. After declaring a FileStream variable using this constructor, you can read data from it. To do this, you can call an appropriate method. This class provides an appropriate method for each primitive data type.

After using the stream, you should close it to reclaim the resources it was using. This is done by calling the Close() method.

Here is an example of using the mentioned methods:

System::Void btnOpen_Click(System::Object^  sender, System::EventArgs^  e)
{
    String ^ NameOfFile = L"Persons.spr";

    FileStream ^ fstPersons = gcnew FileStream(NameOfFile, FileMode::Open);
    BinaryReader ^ rdrPersons = gcnew BinaryReader(fstPersons);
    String ^ strLine = nullptr;

    strLine = rdrPersons->ReadString();
    txtPerson1->Text = strLine;
    strLine = rdrPersons->ReadString();
    txtPerson2->Text = strLine;
    strLine = rdrPersons->ReadString();
    txtPerson3->Text = strLine;
    strLine = rdrPersons->ReadString();
    txtPerson4->Text = strLine;

    rdrPersons->Close();
    fstPersons->Close();
}

Practical LearningPractical Learning: Reading From a Stream

  1. Display the main form
  2. Double-click outside of any control, such as to the left side of the group box to generate the Load event of the form
     
  3. Implement the event as follows:
     
    System::Void Form1_Load(System::Object^  sender, System::EventArgs^  e)
    {
        FileProcessing ^ frmFileProcessing = gcnew FileProcessing;
    
        frmFileProcessing->lblMessage->Text =
    	L"Enter your initials and click Open";
        frmFileProcessing->btnFileProcessing->Text = L"Open";
        frmFileProcessing->txtInitials->Text = L"";
        frmFileProcessing->txtInitials->Focus();
    
        if( frmFileProcessing->ShowDialog() == 
    		System::Windows::Forms::DialogResult::OK )
        {
    	 String ^ NameOfFile =
    		frmFileProcessing->txtInitials->Text + L".icr";
    
    	 String ^ OrderDate;
    	 String ^ OrderTime;
    	 String ^ SelectedFlavor;
    	 String ^ SelectedContainer;
    	 String ^ SelectedIngredient;
    	 int Scoops;
    	 double OrderTotal;
    
    	 FileStream ^ stmIceCream =
    			 gcnew FileStream(NameOfFile, FileMode::Open);
    	 BinaryReader ^ bnrIceCream =
    			 gcnew BinaryReader(stmIceCream);
    
    	 // Find out if this order was previously saved in the machine
    	 if( File::Exists(NameOfFile) )
    	 {
    	     // If so, open it
    	     OrderDate = bnrIceCream->ReadString();
    	     OrderTime = bnrIceCream->ReadString();
    	     SelectedFlavor = bnrIceCream->ReadString();
    	     SelectedContainer = bnrIceCream->ReadString();
    	     SelectedIngredient = bnrIceCream->ReadString();
    	     Scoops = bnrIceCream->ReadInt32();
    	     OrderTotal = bnrIceCream->ReadDouble();
                    
    	     // And display it to the user
     	     dtpOrderDate->Value = DateTime::Parse(OrderDate);
    	     dtpOrderTime->Value = DateTime::Parse(OrderTime);
    	     cboFlavors->Text = SelectedFlavor;
    	     cboContainers->Text = SelectedContainer;
    	     cboIngredients->Text = SelectedIngredient;
    	     txtScoops->Text = Scoops.ToString();
    	     txtOrderTotal->Text = OrderTotal.ToString(L"F");
                    
    	     bnrIceCream->Close();
    	     stmIceCream->Close();
    	 }
    	 else
    	     MessageBox::Show(L"It looks like you have not previously "
    		              L"ordered an ice cream here");
        }
    }
  4. Execute the application and test it. Here is an example:
     
  5. Close the form
 
 

Previous Copyright 2007, yevol.com Next