Input and Output in .NET

Input and Output in .NET
To make a crude generalization, the input/output functions in the .NET Framework can be divided into two broad categories, irrespective of the data storage (disk, memory, etc.) that is being written to or read from.

Data can be treated as a stream of bytes or characters. For example, we could read 500 bytes from a file and write them to a memory buffer. Data can also be treated as a set of objects. Reading and writing the objects is referred to as deserializing and serializing the objects. We can serialize (write) the list of Customer objects to disk. We can then deserialize (read) the list of Customer objects back into memory.

The System::IO namespace has several classes for reading and writing to various types of storage while treating the data as bytes or characters. Serialization functionality can be found in various places in the .NET framework. The System::Runtime::Serialization namespace handles serialization of the Common Type System. The System::Xml::Serialization namespace handles XML serialization.

Stream Classes
Stream is an abstract class that is the basis for reading from and writing bytes to some storage such as a file. It supports both synchronous and asynchronous reading and writing. Asynchronous methods are discussed later in this chapter. The Stream class has the typical methods that you would expect: Read, Write, Seek, Flush, and Close.

The FileStream class is derived from Stream to represent the reading and writing of files as a series of bytes. The FileStream constructor builds the actual stream instance. The overridden Stream methods implement the reading and writing to the file.

Other classes derived from Stream include MemoryStream, BufferedStream, and NetworkStream (in System::Net::Sockets).

The FileStream example (in the FileIO directory with the IO examples) illustrates how to use the Stream classes. If the file does not exist, a new file is created and the numbers from 0 to 9 are written to the file. If the file already exists, the code starts reading 5 bytes from the end of the file and then writes them out. (You should run the example twice. The first time creates and writes the file, and the second time reads and displays the file.)

      unsigned char data __gc[] =
         new unsigned char __gc [10];
      FileStream *fs = new FileStream(
         "FileStreamTest.txt", FileMode::OpenOrCreate);
      if (fs->Length == 0)
      {
         Console::WriteLine("Writing Data...");
         for (short i = 0; i < 10; i++)
            data[i] = (unsigned char)i;
         fs->Write(data, 0, 10);
      }
      else
      {
         fs->Seek(-5, SeekOrigin::End);
         int count = fs->Read(data, 0, 10);
         for (int i = 0; i < count; i++)
         {
            Console::WriteLine(data[i]);
         }
      }
      fs->Close();

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Python and XML

Chapter 1
Python and XML

Python and XML are two very different animals, each with a rich history. Python is a full-scale programming language that has grown from scripting world roots in a very organic way, through the vision and guidance of Python's inventor, Guido van Rossum. Guido continues to take into account the needs of Python developers as Python matures. XML, on the other hand, though strongly impacted by the ideas of a small cadre of visionaries, has grown from standards-committee roots. It has seen both quiet adoption and wrenching battles over its future. Why bother putting the two technologies together?

Before the Python/XML combination, there seemed no easy or effective way to work with XML in a distributed environment. Developers were forced to rely on a variety of tools used in awkward combination with one other. We used shell scripting and Perl to process text and interact with the operating system, and then used Java XML API's for processing XML and network programming. The shell provided an excellent means of file manipulation and interaction with the Unix system, and Perl was a good choice for simple text manipulation, providing access to the Unix APIs. Unfortunately, neither sported a sophisticated object model. Java, on the other hand, featured an object-oriented environment, a robust platform API for network programming, threads, and graphical user interface (GUI) application development. But with Java, we found an immediate lack of text manipulation power; scripting languages typically provided strong text processing. Python presented a perfect solution, as it combines the strengths of all of these various options.

The Power of Python and XML

Now that we've introduced you to the world of XML, we'll look at what Python brings to the table. We'll review the Python features that apply to XML, and then we'll give some specific examples of Python with XML. As a very high-level language, Python includes many powerful data structures as part of the core language and libraries. The more recent versions of Python, from 2.0 onward, include excellent support for Unicode and an impressive range of encodings, as well as an excellent (and fast!) XML parser that provides character data from XML as Unicode strings. Python's standard library also contains implementations of the industry-standard DOM and SAX interfaces for working with XML data, and additional support for alternate parsers and interfaces is available.

Of course, this much could be said of other modern high-level languages as well. Java certainly includes an impressive library of highly usable data structures, and Perl offers equivalent data structures also. What makes Python preferable to those languages and their libraries? There are several features, of which we briefly discuss the most important:

• Python source code is easy to read and maintain.
• The interactive interpreter makes it simple to try out code fragments.
• Python is incredibly portable, but does not restrict access to platform-specific capabilities.
• The object-oriented features are powerful without being obscure.

The Power of Python and XML

Now that we've introduced you to the world of XML, we'll look at what Python brings to the table. We'll review the Python features that apply to XML, and then we'll give some specific examples of Python with XML. As a very high-level language, Python includes many powerful data structures as part of the core language and libraries. The more recent versions of Python, from 2.0 onward, include excellent support for Unicode and an impressive range of encodings, as well as an excellent (and fast!) XML parser that provides character data from XML as Unicode strings. Python's standard library also contains implementations of the industry-standard DOM and SAX interfaces for working with XML data, and additional support for alternate parsers and interfaces is available.

Of course, this much could be said of other modern high-level languages as well. Java certainly includes an impressive library of highly usable data structures, and Perl offers equivalent data structures also. What makes Python preferable to those languages and their libraries? There are several features, of which we briefly discuss the most important:

• Python source code is easy to read and maintain.
• The interactive interpreter makes it simple to try out code fragments.
• Python is incredibly portable, but does not restrict access to platform-specific capabilities.
• The object-oriented features are powerful without being obscure.

There are many languages capable of doing what can be done with Python, but it is rare to find all of the "peripheral" qualities of Python in any single language. These qualities do not so much make Python more capable, but they make it much easier to apply, reducing programming hours. This allows more time to be spent finding better ways to solve real problems or just allows the programmer to move on to the next problem. Here we discuss these features in more detail.

Easy to read and maintain

As a programming language, Python exhibits a remarkable clarity of expression. Though some programmers accustomed to other languages view Python's use of significant whitespace with surprise, everyone seems to think it makes Python source code significantly more readable than languages that require more special characters to be introduced to mark structure in the source. Python's structures are not simpler than those of other languages, but the different syntax makes source code "feel" much cleaner in Python.

The use of whitespace also helps avoid having minor stylistic differences, such as the placement of structural braces, so there's a greater degree of visual consistency across code by different programmers. While this may seem like a minor thing to many programmers, the effect is that maintaining code written by another programmer becomes much easier simply because its easier to concentrate on the actual structure and algorithms of the code. For the individual programmer, this is a nice side benefit, but for a business, this results in lower expenses for code maintenance.

Exploratory programming in an interactive interpreter

Many modern high-level programming languages offer interpreters, but few have proved as successful at doing so as Python. Others, such as Java, do not generally offer interpreters at all. If we consider Perl, a language that is arguably very capable when used from a command line, we see that it is not equipped with a rich interpreter. If we start the Perl interpreter without naming a script, it simply waits for us to type a complete script at the console, and then interprets the script when we're done. It does allow us to enter a few commands on the command line directly, but there's no ability to run one statement at a time and inspect the results as we go in order to determine if each bit of code is doing exactly what we expect. With Python, the interactive interpreter provides a rich environment for executing individual statements and testing the results.

Portability without restrictions

The Python interpreter is one of the most portable language interpreters available. It is known to run on platforms ranging from PDAs and other embedded systems to some of the most powerful multiprocessor platforms ever built. It can run on more operating systems than perhaps any other interpreter. Moreover, carefully written application code can share much of this portability. Python provides a great array of abstractions that do just enough to hide platform differences while allowing the programmer to use the services of specific platforms when necessary.

When an application requires access to facilities or libraries that Python does not provide, Python also makes it easy to add extensions that take advantage of these additional facilities. Additional modules can be created (usually in C or C++, but other languages can be used as well) that allow Python code to call on external facilities efficiently.

Powerful but accessible object-orientation

At one time, it was common to hear about how object-oriented programming (OOP) would solve most of the technical problems programmers had to deal with in their code. Of course, programmers knew better, pushed back, and turned the concepts into useful tools that could be applied when appropriate (though how and when it should be applied may always be the subject of debate). Unfortunately, many languages that have strong support for OOP are either very tedious to work with (such as C++ or, to a lesser extent, Java), or they have not been as widely accepted for general use (such as Eiffel).

Python is different. The language supports object orientation without much of the syntactic overhead found in many widely used object-oriented languages, making it very easy to define new object types. Unlike many other languages, Python is highly polymorphic; interfaces are defined in much less stringent ways than in languages such as C++ and Java. This makes it easy to create useful objects without having to write code that exists only to conform to an interface, but that will not actually be used in a particular application. When combined with the excellent advantage taken by Python's standard library of a variety of common interfaces, the value of creating reusable objects is easily recognized, all while the ease of implementing useful interfaces is maintained.

Python Tools for XML

Three major packages provide Python tools for working with XML. These are, from the most commonly used to the largest:

1.     The Python standard library

2.     PyXML, produced by the Python XML Special Interest Group

3.     4Suite, provided by Fourthought, Inc.

The Python standard library provides a minimal but useful set of interfaces to work with XML, including an interface to the popular Expat XML parser, an implementation of the lightweight Simple API for XML (SAX), and a basic implementation of the core Document Object Model (DOM). The DOM implementation supports Level 1 and much of Level 2 of the DOM specification from the W3C, but does not implement most of the optional features. The material in the standard library was drawn from material originally in the PyXML package, and additional material was contributed by leading Python XML developers.

PyXML is a more feature-laden package; it extends the standard library with additional XML parsers, has a much more substantial DOM implementation (including more optional features), has adapters to allow more parsers to support the SAX interface, XPath expression parsing and evaluation, XSLT transformations, and a variety of other helper modules. The package is maintained as a community effort by many of the most active Python/XML programmers.

4Suite is not a superset of the other packages, but is intended to be used in addition to PyXML. It offers additional DOM implementations tailored for different applications, support for the XLink and XPointer specifications, and tools for working with Resource Description Framework (RDF) data.

What Can We Do with It?

Now that we've looked at how we can use XML with Python, we need to look at how we can apply our knowledge of XML and Python to real applications. In the Internet age, this means widely distributed systems operating across the Internet.

There's a lot to working with the Internet beyond XML and the CGI programming done in many of the examples in the book. In case you're not already familiar with this topic, we include an introduction to the facilities in the Python standard library that help create clients and servers for the Internet in Chapter 8. We review how to retrieve data from remote servers, and how to submit form-based requests programmatically and read the result. We then learn to build custom web servers that respond to HTTP requests, allowing us to build servers that do exactly what we need them to.

With these skills under our hat, we proceed to look at the emerging world of "web services." Chapter 9 describes what we mean by web services and introduces the specifications coming out in that area. We look at two packages that allow us to use SOAP to call on web services and demonstrate how to create one in Python.

In Chapter 10, we pull together much of what we've learned with an extended example that demonstrates how it all works together. Using XML as a communications medium, we are able to build an application that uses a variety of technologies and operates in diverse environments.

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XML Extensible Markup Language

XML (Extensible Markup Language) is a flexible way to create common information formats and share both the format and the data on the World Wide Web, intranets, and elsewhere. For example, computer makers might agree on a standard or common way to describe the information about a computer product (processor speed, memory size, and so forth) and then describe the product information format with XML. Such a standard way of describing data would enable a user to send an intelligent agent (a program) to each computer maker's Web site, gather data, and then make a valid comparison. XML can be used by any individual or group of individuals or companies that wants to share information in a consistent way.

XML, a formal recommendation from the World Wide Web Consortium (W3C), is similar to the language of today's Web pages, the Hypertext Markup Language (HTML). Both XML and HTML contain markup symbols to describe the contents of a page or file. HTML, however, describes the content of a Web page (mainly text and graphic images) only in terms of how it is to be displayed and interacted with. For example, the letter "p" placed within markup tags starts a new paragraph. XML describes the content in terms of what data is being described. For example, the word "phonenum" placed within markup tags could indicate that the data that followed was a phone number. This means that an XML file can be processed purely as data by a program or it can be stored with similar data on another computer or, like an HTML file, that it can be displayed. For example, depending on how the application in the receiving computer wanted to handle the phone number, it could be stored, displayed, or dialed.

XML is "extensible" because, unlike HTML, the markup symbols are unlimited and self-defining. XML is actually a simpler and easier-to-use subset of the Standard Generalized Markup Language (SGML), the standard for how to create a document structure. It is expected that HTML and XML will be used together in many Web applications. XML markup, for example, may appear within an HTML page.

Early applications of XML include Microsoft's Channel Definition Format (CDF), which describes a channel, a portion of a Web site that has been downloaded to your hard disk and is then is updated periodically as information changes. A specific CDF file contains data that specifies an initial Web page and how frequently it is updated. Another early application is ChartWare, which uses XML as a way to describe medical charts so that they can be shared by doctors.Applications related to banking, e-commerce ordering, personal preference profiles, purchase orders, litigation documents, part lists, and many others are anticipated.

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XAML

What is XAML?

Expression Studio 2.0

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Extensible Application Markup Language, or XAML (pronounced "zemell"), is an XML-based markup language developed by Microsoft. XAML is the language behind the visual presentation of an application that you develop in Microsoft Expression Blend, just as HTML is the language behind the visual presentation of a Web page. Creating an application in Expression Blend means writing XAML code, either by hand or visually by working in the Design view of Expression Blend.

XAML as the equalizer between design applications

You can export art assets from Microsoft Expression Design 2 as XAML, and then import the XAML into your Expression Blend project. Some other design applications have tools that can convert art assets to XAML. You can search on the Internet for conversion tools that are posted on trusted sites.

For more information about importing XAML, see the topics Import XAML that is exported from Expression Design and Import assets from Expression Design 2 in this User Guide.

You can also import Silverlight 1.0 projects that are created by using a Silverlight template to encode a video project in Microsoft Expression Encoder 2. Additionally, you can modify the Silverlight templates that come with Expression Encoder 2 in Expression Blend 2. For more information, see Import a Silverlight 1.0 site from Expression Encoder 2 and Modify a Silverlight template for Expression Encoder 2 in Expression Blend.

The relationship between XAML and code-behind files

The XAML for any given document in Expression Blend is stored in a .xaml file. If there is underlying code for your XAML document, that code is stored in a file of the same name, with the additional extension of .cs or .vb. For example, if your XAML document is named Window1.xaml, the code-behind file will be called Window1.xaml.cs if the programming language of the code is C#.

Note:

In Silverlight 1.0 projects, the language used in the code-behind files is JavaScript. For more information, see Quick start with Silverlight 1.0.

For information about how to create XAML documents with or without code-behind files, see the topic  Create a new document or project in this User Guide.

When you build your project, the WPF parser reads the. Xaml files for that project and reports any resulting errors. Likewise, when you open an existing project in Expression Blend, the XAML parser reads the. Xaml files that are included in your project folder and attempts to parse the elements and display the documents on the art board in Design view. In both cases, if the parser encounters errors, the art board is disabled, and Expression Blend displays an error message with a link to open XAML view so that you can resolve the errors. The parsing errors are also reported on the Errors tab in the Results panel. For more information, see the topics Editing XAML, Handling errors, and Debug Expression Blend applications in this User Guide. For information about the syntax of individual controls, see Control Library or search for XAML elements in the Class Library reference, both on MSDN.

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