Programming C#, 2nd Edition

Programming C#, 2nd Edition

5.0 1
by Jesse Liberty
     
 

This volume is aimed at experienced programmers and Web developers. It focuses on the features and programming patterns that are new to C# and fundamental to the programming of Web services and applications.See more details below

Overview

This volume is aimed at experienced programmers and Web developers. It focuses on the features and programming patterns that are new to C# and fundamental to the programming of Web services and applications.

Editorial Reviews

Explores the details of the C# language, how to write .NET applications, and the .NET common language runtime library. Topics include the collection classes provided by the framework class library, desktop applications with Windows forms, and web applications with web forms. The second edition is updated to VS.NET 1.0. Annotation c. Book News, Inc., Portland, OR (booknews.com)

Product Details

ISBN-13:
9780596003098
Publisher:
O'Reilly Media, Incorporated
Publication date:
02/01/2002
Edition description:
Second Edition
Pages:
648
Product dimensions:
7.04(w) x 9.21(h) x 1.15(d)

Read an Excerpt

Chapter 18: Attributes and Reflection

Throughout this book, I have emphasized that a .NET application contains code, data, and metadata. Metadata is information about the data--that is, information about the types, code, assembly, and so forth--that is stored along with your program. This chapter will explore how some of that metadata is created and used.

Attributes are a mechanism for adding metadata, such as compiler instructions and other data about your data, methods, and classes, to the program itself. Attributes are inserted into the metadata and are visible through ILDasm and other metadata-reading tools.

Reflection is the process by which a program can read its own metadata. A program is said to reflect on itself, extracting metadata from its assembly and using that metadata either to inform the user or to modify its own behavior.

Attributes

An attribute is an object that represents data you want to associate with an element in your program. The element to which you attach an attribute is referred to as the target of that attribute. For example, the attribute:

[NoIDispatch]

is associated with a class or an interface to indicate that the target class should derive from IUnknown rather than IDispatch, when exporting to COM. COM interface programming is discussed in detail in Chapter 22.

In Chapter 17, you saw this attribute:

[assembly: AssemblyKeyFile("c:\myStrongName.key")]

This inserts metadata into the assembly to designate the program's StrongName.

Intrinsic Attributes

Attributes come in two flavors: intrinsic and custom. Intrinsic attributes are supplied as part of the Common Language Runtime (CLR), and they are integrated into .NET. Custom attributes are attributes you create for your own purposes.

Most programmers will use only intrinsic attributes, though custom attributes can be a powerful tool when combined with reflection, described later in this chapter.

Attribute Targets

If you search through the CLR, you'll find a great many attributes. Some attributes are applied to an assembly, others to a class or interface, and some, such as [WebMethod], to class members. These are called the attribute targets. Possible attribute targets are detailed in Table 18-1.

Table 18-1: Possible attribute targets

Member Name

Usage

All

Applied to any of the following elements: assembly, class, class member, delegate, enum, event, field, interface, method, module, parameter, property, return value, or struct

Assembly

Applied to the assembly itself

Class

Applied to instances of the class

ClassMembers

Applied to classes, structs, enums, constructors, methods, properties, fields, events, delegates, and interfaces

Constructor

Applied to a given constructor

Delegate

Applied to the delegated method

Enum

Applied to an enumeration

Event

Applied to an event

Field

Applied to a field

Interface

Applied to an interface

Method

Applied to a method

Module

Applied to a single module

Parameter

Applied to a parameter of a method

Property

Applied to a property (both get and set, if implemented)

ReturnValue

Applied to a return value

Struct

Applied to a struct

Applying Attributes

You apply attributes to their targets by placing them in square brackets immediately before the target item. You can combine attributes, either by stacking one on top of another:

[assembly: AssemblyDelaySign(false)]
[assembly: AssemblyKeyFile(".\\keyFile.snk")]

or by separating the attributes with commas:

[assembly: AssemblyDelaySign(false),
   assembly: AssemblyKeyFile(".\\keyFile.snk")]

TIP:   You must place assembly attributes after all using statements and before any code.

Many intrinsic attributes are used for interoperating with COM, as discussed in detail in Chapter 22. You've already seen use of one attribute ([WebMethod]) in Chapter 16. You'll see other attributes, such as the [Serializable] attribute, used in the discussion of serialization in Chapter 19.

The System.Runtime namespace offers a number of intrinsic attributes, including attributes for assemblies (such as the keyname attribute), for configuration (such as debug to indicate the debug build), and for version attributes.

You can organize the intrinsic attributes by how they are used. The principal intrinsic attributes are those used for COM, those used to modify the Interface Definition Language (IDL) file from within a source-code file, attributes used by the ATL Server classes, and attributes used by the Visual C++ compiler.

Perhaps the attribute you are most likely to use in your everyday C# programming (if you are not interacting with COM) is [Serializable]. As you'll see in Chapter 19, all you need to do to ensure that your class can be serialized to disk or to the Internet is add the [Serializable] attribute to the class:

[serializable]
class MySerializableClass

The attribute tag is put in square brackets immediately before its target--in this case, the class declaration.

The key fact about intrinsic attributes is that you know when you need them; the task will dictate their use.

Custom Attributes

You are free to create your own custom attributes and use them at runtime as you see fit. Suppose, for example, that your development organization wants to keep track of bug fixes. You already keep a database of all your bugs, but you'd like to tie your bug reports to specific fixes in the code.

You might add comments to your code along the lines of:

// Bug 323 fixed by Jesse Liberty 1/1/2005.

This would make it easy to see in your source code, but there is no enforced connection to Bug 323 in the database. A custom attribute might be just what you need. You would replace your comment with something like this:

[BugFixAttribute(323,"Jesse Liberty","1/1/2005") 
Comment="Off by one error"]

You could then write a program to read through the metadata to find these bug-fix notations and update the database. The attribute would serve the purposes of a comment, but would also allow you to retrieve the information programmatically through tools you'd create.

Declaring an Attribute

Attributes, like most things in C#, are embodied in classes. To create a custom attribute, you derive your new custom attribute class from System.Attribute:

public class BugFixAttribute : System.Attribute

You need to tell the compiler with which kinds of elements this attribute can be used (the attribute target). You specify this with (what else?) an attribute:

[AttributeUsage(AttributeTargets.Class |
    AttributeTargets.Constructor |
    AttributeTargets.Field |
    AttributeTargets.Method |
    AttributeTargets.Property,
    AllowMultiple = true)]

AttributeUsage is an attribute applied to attributes: a meta-attribute. It provides, if you will, meta-metadata--that is, data about the metadata. For the AttributeUsage attribute constructor, you pass two arguments. The first argument is a set of flags that indicate the target--in this case, the class and its constructor, fields, methods, and properties. The second argument is a flag that indicates whether a given element might receive more than one such attribute. In this example, AllowMultiple is set to true, indicating that class members can have more than one BugFixAttribute assigned.

Naming an Attribute

The new custom attribute in this example is named BugFixAttribute. The convention is to append the word Attribute to your attribute name. The compiler supports this by allowing you to call the attribute with the shorter version of the name. Thus, you can write:

[BugFix(123, "Jesse Liberty", "01/01/05", Comment="Off by one")]

The compiler will first look for an attribute named BugFix and, if it does not find that, will then look for BugFixAttribute.

Constructing an Attribute

Every attribute must have at least one constructor. Attributes take two types of parameters, positional and named. In the BugFix example, the programmer's name and the date are positional parameters, and comment is a named parameter. Positional parameters are passed in through the constructor and must be passed in the order declared in the constructor:

public BugFixAttribute(int bugID, string programmer, 
string date)
{
    this.bugID = bugID;
    this.programmer = programmer;
    this.date = date;
}

Named parameters are implemented as properties:

public string Comment
{
     get
     {
         return comment;
     }
     set
     {
         comment = value;
     }
}

It is common to create read-only properties for the positional parameters:

public int BugID
{
     get
     {
         return bugID;
     }
}

Using an Attribute

Once you have defined an attribute, you can put it to work by placing it immediately before its target. To test the BugFixAttribute of the preceding example, the following program creates a simple class named MyMath and gives it two functions. You'll assign BugFixAttributes to the class to record its code-maintenance history...

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