StreamWriting for Old Walter in C#

Create Code 3/9 in C# StreamWriting for Old Walter

Everything goes smoothly when I enter some random text into TestFile1 txt When I try to open TestFile1txt again, however, the program spits out a message, the gist of which is The file already exists, with the filename attached The path to the file is tortured because the current directory is the directory in which Visual Studio put the executable file Correcting my mistake, I enter an acceptable filename such as TestFile2txt without complaint

Now that you ve seen FileStream and StreamWriter in action, I should point out the more usual way to do stream writing in C# inside a using statement:

using(<someresource>) { // Use the resource }

The using statement is a construct that automates the process of cleaning up after using a stream On encountering the closing curly brace of the using block, C# manages flushing the stream and closing it for you (To flush a stream is to push any last bytes left over in the stream s buffer out to the associated file before it gets closed Think of pushing a handle to drain the last water out of your trout stream) Using using eliminates the common error of forgetting to flush and close a file after writing to it Don t leave open files lying around Without using, you d need to write:

Stream fileStream = null; TextWriter writer = null; try { // Create and use the stream, then } finally { streamFlush(); streamClose(); stream = null; }

Note how I declared the stream and writer above the try block (so they re visible throughout the method) I also declared the fileStream and writer variables using abstract base classes rather than the concrete types FileStream and StreamWriter That s a good practice I set them to null so the compiler won t complain about uninitialized variables

The preferred way to write the key I/O code in the FileWrite example looks more like this:

// Prepare the file stream FileStream fs = FileOpen(fileName, FileModeCreateNew, FileAccessWrite); // Pass the fs variable to the StreamWriter constructor in the using statement using (StreamWriter sw = new StreamWriter(fs)) { // sw exists only within the using block, which is a local scope // Read one string at a time from the console, outputting each to the // FileStream open for writing ConsoleWriteLine( Enter text; enter blank line to stop ); while (true) { // Read next line from Console; quit if line is blank string input = ConsoleReadLine(); if (inputLength == 0) { break; } // Write the line just read to output file via the stream swWriteLine(input); // Loop back up to get another line and write it } } // sw goes away here, and fs is now closed So fs = null; // Make sure you can t try to access fs again

The items in parentheses after the using keyword are its resource acquisition section, where you allocate one or more resources such as streams, readers/writers, fonts, and so on (If you allocate more than one resource, they have to be of the same type) Following that section is the enclosing block, bounded by the outer curly braces The using statement s block is not a loop The block only defines a local scope, like the try block or a method s block (Variables defined within the block, including its head, don t exist outside the block Thus the Stream Writer sw isn t visible outside the using block) I discuss scope in Book I At the top of the preceding example, in the resource-acquisition section, you set up a resource in this case, create a new StreamWriter wrapped around the already-existing FileStream Inside the block is where you carry out all your I/O code for the file At the end of the using block, C# automatically flushes the StreamWriter, closes it, and closes the FileStream, also flushing any bytes it still contains to the file on disk Ending the using block also disposes the StreamWriter object see the warning and the technical discussion coming up