code 39 vb.net Streams, Files, and I/O in VB.NET

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10. Streams, Files, and I/O
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Table 10-10. The TextReader and TextWriter Classes
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Member TextReader.Null TextWriter.Null TextReader.Close() TextWriter.Close() TextReader.Peek() TextReader.Read() TextReader.ReadBlock() TextReader.ReadLine() TextReader.ReadToEnd() TextReader.Synchronized TextWriter.Synchronized TextWriter.Encoding TextWriter.FormatProvider TextWriter.NewLine TextWriter.Flush() TextWriter.Write()
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Description Provides a TextReader with no data to read. Provides a TextWriter that acts as a data sink. Closes the reader or writer. Returns the next character that will be returned with a Read method call. Reads either a single character or an array of characters. Performs a blocking read operation to obtain a specified number of characters. Reads a line of text, returned as a string. Reads all of the characters from the current position to the end of the reader, returned as a string. Creates a thread-safe wrapper around the reader or writer. Returns the Encoding that is used to write character data. Returns the object used to control formatting. See 7 for more information on formatting. Gets or sets the line separator string. Flushes any buffered data. Writes a range of types, including primitives and strings. Although there is an overloaded form that accepts an object argument, the object isn't serialized. The data written is the result of calling ToString on the argument value. Writes a range of types, including primitives and strings, followed by the line terminator string.
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TextWriter.WriteLine()
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These classes provide the basis for character-oriented I/O, and the .NET Framework contains two sets of implementation classes, detailed in the following sections.
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StreamReader and StreamWriter
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The StreamReader and StreamWriter classes implement the functionality of the TextReader and TextWriter classes against streams, making character-based I/O available for all of the base stream types. As a convenience, overloaded constructors for these classes will accept file paths for input or output, allowing the programmer to use character I/O without having to explicitly create instances of FileStream. The following example demonstrates a simple use of these classes, following the model of the preceding example.
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MemoryStream x_stream = new MemoryStream(); StreamWriter x_writer = new StreamWriter(x_stream); x_writer.WriteLine("C# for Java Developers"); x_writer.Flush();
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10. Streams, Files, and I/O x_stream.Seek(0, SeekOrigin.Begin); StreamReader x_reader = new StreamReader(x_stream); Console.WriteLine(x_reader.ReadLine());
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StringReader and StringWriter
The StringReader and StringWriter classes use strings held in memory as the backing store. The StringReader class takes a string as the constructor argument, and calls to Read return characters sequentially from the string. The StringWriter class uses an instance of System.Text.StringBuilder to store characters processed using the Write method. The contents of the string are available through the ToString method. These classes represent characterbased implementation of the broad functionality provided by the System.IO.MemoryStream class.
Synchronizing Streams
The .NET stream classes do not provide synchronization support for instance members; the programmer must assume the responsibility for ensuring safe access to stream instances. Members that are public and static are guaranteed to be safe.
Streams Summary
Although the .NET stream classes are few in number, they manage to provide the majority of the features available in Java. Some of the specialized streams and readers are missing, but the ability of the same stream instance to both read and write data belies the sparse appearance of the class set. The implementation of the reader and writer classes seems out of keeping with the slick design of the stream classes, and the lack of a common base class leads to a lack of abstraction. The mixed model of defining classes that are both base and pass-through implementations is far from ideal.
Asynchronous I/O
The preceding section was concerned with using streams to perform synchronous I/O operations, in which a call to a read or write method blocks until the operation completes. This section discusses the .NET support for asynchronous I/O, which allows read and write operations to be performed without blocking. Version 1.4 of the Java platform includes support for asynchronous, or nonblocking, I/O, but the .NET model differs so much that we see little benefit in providing a comparison. Asynchronous I/O is byte-oriented, not character-oriented. All classes derived from System.IO.Stream can be used in asynchronous I/O. Importantly, none of the reader and writer classes detailed in the preceding section can be used. To demonstrate asynchronous I/O, we'll concentrate on using the FileStream class. Since all streams can be used for asynchronous I/O, the model laid out in this section can be applied to any of the stream classes. At the heart of all asynchronous operations is the System.AsyncCallback delegate, which will be invoked when an asynchronous operation has completed. The signature for this delegate is a method that accepts an instance of System.IAsyncResult and returns void. The following example demonstrates reading an English transcript of the Magna Carta using asynchronous calls:
10. Streams, Files, and I/O using using using using System; System.IO; System.Text; System.Threading;
namespace MagnaCarta { class MagnaCartaDemo { private byte[] o_byte_arr; private FileStream o_stream; private AsyncCallback o_callback; MagnaCartaDemo() { o_byte_arr = new byte[100]; o_stream = new FileStream("magnacarta.txt", FileMode.Open, FileAccess.Read, FileShare.Read, 100, true); o_callback = new AsyncCallback(readFinished); o_stream.BeginRead(o_byte_arr, 0, o_byte_arr.Length, o_callback, null); Console.WriteLine(">>>> Current thread is " + "not blocking!");
private void readFinished(IAsyncResult p_result) { int x_bytes_read = o_stream.EndRead(p_result); if (x_bytes_read > 0) { Console.Write(Encoding.Default.GetString( o_byte_arr, 0, x_bytes_read)); o_stream.BeginRead(o_byte_arr, 0, o_byte_arr.Length, o_callback, null); } else { o_stream.Close(); } } public static void Main() { new MagnaCartaDemo() ; } } }
In the class constructor, we define a byte array that will be used to read data from the stream; since we are dealing with FileStream, we are limited to byte, rather than character, operations. The next step is to create the stream that will be used to read from the file. Notice that we use the constructor form, which explicitly enables asynchronous operations.
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