C# 0 The Complete Reference

PART I

C h a p t e r   1 4 :  

U s i n g   I / O  

375

PART IPART I

    int i;

    FileStream fin;

    if(args.Length != 1) {

      Console.WriteLine("Usage: ShowFile File");

      return;

    }

    try {

      fin = new FileStream(args[0], FileMode.Open);

    } catch(IOException exc) {

      Console.WriteLine("Cannot Open File");

      Console.WriteLine(exc.Message);

      return;

    }

    // Read bytes until EOF is encountered.

    do {

      try {

        i = fin.ReadByte();

      } catch(IOException exc) {

        Console.WriteLine("Error Reading File");

        Console.WriteLine(exc.Message);

        break;

      }

      if(i != -1) Console.Write((char) i);

    } while(i != -1);

    fin.Close();

  }

}

Writing to a File

To write a byte to a file, use the 

WriteByte( )

 method. Its simplest form is shown here:

void WriteByte(byte 

val

)

This method writes the byte specified by 

val

 to the file. If the underlying stream is 

not opened for output, a 

NotSupportedException

 is thrown. If the stream is closed, 

ObjectDisposedException

 is thrown.

You can write an array of bytes to a file by calling 

Write( )

. It is shown here:

void Write(byte[ ] 

buf

, int 

offset

, int 

numBytes

)

Write( )

 writes 

numBytes

 bytes from the array 

buf,

 beginning at 

buf

[

offset

], to the file. The 

number of bytes written is returned. If an error occurs during writing, an 

IOException

 is 

thrown. If the underlying stream is not opened for output, a 

NotSupportedException

 is 

thrown. Several other exceptions are also possible.

As you may know, when file output is performed, often that output is not immediately 

written to the actual physical device. Instead, output is buffered by the operating system 

until a sizable chunk of data can be written all at once. This improves the efficiency of the 

system. For example, disk files are organized by sectors, which might be anywhere from 128 

bytes long, on up. Output is usually buffered until an entire sector can be written all at once. 

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376

 

P a r t   I :  

T h e   C #   L a n g u a g e

However, if you want to cause data to be written to the physical device whether the buffer 

is full or not, you can call 

Flush( )

, shown here:

void Flush( )

An

IOException

 is thrown on failure. If the stream is closed, 

ObjectDisposedException

 is 

thrown.

Once you are done with an output file, you must remember to close it using 

Close( )

.

Doing so ensures that any output remaining in a disk buffer is actually written to the disk. 

Thus, there is no reason to call 

Flush( )

 before closing a file.

Here is a simple example that writes to a file:

// Write to a file.

using System;

using System.IO;

class WriteToFile {

  static void Main(string[] args) {

    FileStream fout;

    // Open output file.

    try {

      fout = new FileStream("test.txt", FileMode.Create);

    } catch(IOException exc) {

      Console.WriteLine("Cannot Open File");

      Console.WriteLine(exc.Message);

      return;

    }

    // Write the alphabet to the file.

    try {

      for(char c = 'A'; c <= 'Z'; c++)

        fout.WriteByte((byte) c);

    } catch(IOException exc) {

      Console.WriteLine("Error Writing File");

      Console.WriteLine(exc.Message);

    }

    fout.Close();

  }

}

The program first opens a file called 

test.txt

 for output. It then writes the uppercase 

alphabet to the file. Finally, it closes the file. Notice how possible I/O errors are handled by 

the

try

/

catch

 blocks. After this program executes, 

test.txt

 will contain the following output:

ABCDEFGHIJKLMNOPQRSTUVWXYZ

Using FileStream to Copy a File

One advantage to the byte-oriented I/O used by 

FileStream

 is that you can use it on any 

type of file—not just those that contain text. For example, the following program copies 

any type of file, including executable files. The names of the source and destination files 

are specified on the command line.

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