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Part I:
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For example, this opens a read-only file:
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FileStream fin = new FileStream("testdat", FileModeOpen, FileAccessRead);
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When you are done with a file, you must close it This can be done by calling Close( ) Its general form is shown here: void Close( ) Closing a file releases the system resources allocated to the file, allowing them to be used by another file As a point of interest, Close( ) works by calling Dispose( ), which actually frees the resources
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NOTE The using statement, described in 20, offers a way to automatically close a file when
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it is no longer needed This approach is beneficial in many file-handling situations because it provides a simple means to ensure that a file is closed when it is no longer needed However, to clearly illustrate the fundamentals of file handling, including the point at which a file can be closed, this chapter explicitly calls Close( ) in all cases
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Reading Bytes from a FileStream
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FileStream defines two methods that read bytes from a file: ReadByte( ) and Read( ) To read a single byte from a file, use ReadByte( ), whose general form is shown here: int ReadByte( ) Each time it is called, it reads a single byte from the file and returns it as an integer value It returns 1 when the end of the file is encountered Possible exceptions include NotSupportedException (the stream is not opened for input) and ObjectDisposedException (the stream is closed) To read a block of bytes, use Read( ), which has this general form: int Read(byte[ ] array, int offset, int count) Read( ) attempts to read up to count bytes into array starting at array[offset] It returns the number of bytes successfully read An IOException is thrown if an I/O error occurs Several other types of exceptions are possible, including NotSupportedException, which is thrown if reading is not supported by the stream The following program uses ReadByte( ) to input and display the contents of a text file, the name of which is specified as a command-line argument Note the program first checks that a filename has been specified before trying to open the file
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/* Display a text file To use this program, specify the name of the file that you want to see For example, to see a file called TESTCS, use the following command line ShowFile TESTCS */ using System; using SystemIO;
14:
Using I/O
class ShowFile { static void Main(string[] args) { int i; FileStream fin; if(argsLength != 1) { ConsoleWriteLine("Usage: ShowFile File"); return; } try { fin = new FileStream(args[0], FileModeOpen); } catch(IOException exc) { ConsoleWriteLine("Cannot Open File"); ConsoleWriteLine(excMessage); return; // File can't be opened, so stop the program } // Read bytes until EOF is encountered try { do { i = finReadByte(); if(i != -1) ConsoleWrite((char) i); } while(i != -1); } catch(IOException exc) { ConsoleWriteLine("Error Reading File"); ConsoleWriteLine(excMessage); } finally { finClose(); } } }
PART I
Notice that the program uses two try blocks The first catches any I/O exceptions that might prevent the file from being opened If an I/O error occurs, the program terminates Otherwise, the second try block monitors the read operation for I/O exceptions Thus, the second try block executes only if fin refers to an open file Also, notice that the file is closed in the finally block associated with the second try block This means that no matter how the do loop ends (either normally or because of an error), the file will be closed Although not an issue in this specific example (because the entire program ends at that point anyway), the advantage to this approach, in general, is that if the code that accesses a file terminates because of some exception, the file is still closed by the finally block This ensures that the file is closed in all cases In some situations, it may be easier to wrap the portions of a program that open the file and access the file within a single try block (rather than separating the two) For example, here is another, shorter way to write the ShowFile program:
// Display a text file Compact version using System;
Part I:
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