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CHAPTER 11 NAMESPACES
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} The code in Listing 11-16 looks good. But it if you open the example project for this chapter, you ll see that the code won t compile. Figure 11-4 shows you the Visual Studio screen where one of the errors has been flagged.
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Figure 11-4. Visual Studio highlighting the hiding problem At first glance, the Visual Studio error doesn t make any sense, and it can take a while to figure out what is happening the first time that you come across this problem. Somewhere else in your namespace is a class called System. In my example project, there is a second code file that contains the code in Listing 11-17. Listing 11-17. The Problematic Code namespace AuditSystem { class System { // class body } } The problem here is that C# looks in the local namespace for your type before looking elsewhere. The AuditSystem namespace contains a class called System, so my calls to the System.Console class are assumed to relate to the AuditSystem.System class, and the error arises because there is no member in the AuditSystem.System class called Console. You can sort this out by using the alias techniques or by using fully qualified names as covered in the previous sections, but there is another way, which is to use the global keyword. If you prefix your statement with global::, you tell the C# compiler not to look for a
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CHAPTER 11 NAMESPACES
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local type that matches the name but to start with the global namespace and go from there. Listing 11-18 shows how this keyword can be used to fix the problems in the previous example. Listing 11-18. Unhiding a Class by Using the global Keyword namespace AuditSystem { class Audit { public static void Main(string[] args) { global::System.Console.WriteLine("Hello World"); global::System.Console.WriteLine("Press enter to finish"); global::System.Console.ReadLine(); } } }
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Namespaces are a flexible way of grouping types together to help organize your program. You don t have to use namespaces in your code, but you do have to know how they work so that you can take full advantage of the .NET Framework class library, which contains thousands of useful types. The flexibility of namespaces can lead to some problems, especially when it comes to ambiguity of type names. We saw how to use different techniques to work around name conflicts and name ambiguity, and although most programmers don t encounter these problems regularly, it is good to know that they can be resolved when the need arises.
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C H A P T E R 12
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Interfaces, Structs, and Enums
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The previous chapters have been focused on classes, but C# supports other categories of types, three of which are the subject of this chapter: interfaces, structs, and enums. Must C# programmers won t use these types as frequently as classes, but each of these three has its place in the programming tool set, and a working knowledge of them is useful, not least because you will find examples of each when you come to use the .NET Framework. Table 12-1 provides the summary for this chapter. Table 12-1. Quick Problem/Solution Reference for 12
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Problem
Impose consistent features on types that are otherwise unrelated. Extend the set of members that an interface defines. Define an interface across multiple code files. Avoid member ambiguity when implementing interfaces that specify the same members. Create a custom value type.
Solution
Define and implement an interface.
Listings
12-1 through 6, 12-10, 12-11, 1 2-14, 12-15 12-7, 12-8
Derive one interface from another.
Define a partial interface.
12-9
Explicitly implement an interface.
12-12, 12-13
Define a struct.
12-16 through 12-20 12-21 12-22 through 12-25 12-26, 12-27
Duplicate the values of a struct. Work with a defined range of values. Combine enum values.
Copy a struct value. Define and use an enum.
Use the Flags attribute and assign explicit numeric values that increment in powers of two.
CHAPTER 12 INTERFACES, STRUCTS, AND ENUMS
Using Interfaces
Interfaces contain a set of specifications for methods and other members. A class is said to implement an interface when it contains implementations for all the members defined in an interface. In the following sections, I ll show you how to define, implement, and use interfaces and explain how they differ from other C# features such as abstract classes.
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