C# in Depth

194 CHAPTER 7 Concluding C# 2: the final featuresnamespace or type (for example, using Out = System.Console;) and the other justintroduced a namespace into the list of contexts the compiler would search whenlooking for a type (for example, using System.IO;). By and large, this was adequate,but there are a few situations that the language simply couldn’t cope with, and otherswhere automatically generated code would have to go out of its way to make absolutelysure that the right namespaces and types were being used whatever happened.C# 2 fixes these problems, bringing an additional robustness to the language. It’snot that the code being generated is any more robust in terms of execution, but youcan write code that is guaranteed to mean what you want it to regardless of whichother types, assemblies, and namespaces are introduced. These extreme measures arerarely needed outside automatically generated code, but it’s nice to know that they’rethere when you need them. In C# 2 there are three types of aliases: the namespacealiases of C# 1, the global namespace alias, and extern aliases. We’ll start off with the onetype of alias that was already present in C# 1, but we’ll introduce a new way of usingaliases to ensure that the compiler knows to treat it as an alias rather than checking tosee whether it’s the name of another namespace or type.7.4.1 Qualifying namespace aliasesEven in C# 1, it was a good idea to avoid namespace aliases wherever possible. Every sooften you might find that one type name clashed with another—as with our Buttonexample earlier—and so you either had to specify the full name including thenamespace every time you used them, or have an alias that distinguished the two, insome ways acting like a shortened form of the namespace. Listing 7.5 shows an examplewhere the two types of Button are used, qualified by an alias.Listing 7.5Using aliases to distinguish between different Button typesusing System;using WinForms = System.Windows.Forms;using WebForms = System.Web.UI.WebControls;class Test{static void Main(){Console.WriteLine (typeof (WinForms.Button));Console.WriteLine (typeof (WebForms.Button));}}Listing 7.5 compiles without any errors or warnings, although it’s still not as pleasantas it would be if we only needed to deal with one kind of Button to start with. There’sa problem, however—what if someone were to introduce a type or namespace calledWinForms or WebForms? The compiler wouldn’t know what WinForms.Button meant,and would use the type or namespace in preference to the alias. We want to be able totell the compiler that we need it to treat WinForms as an alias, even though it’s availableelsewhere. C# 2 introduces the ::syntax to do this, as shown in listing 7.6.Licensed to Rhona Hadida