Instead of placing all code into one monolithic binary file, C# and the underlying CLI platform allow you to spread code across multiple assemblies This enables you to reuse assemblies across multiple executables

The HelloWorldexe program is one of the most trivial programs you can write Real-world programs are more complex, and as complexity increases, it helps to organize the complexity by breaking programs into multiple parts To do this, developers move portions of a program into separate compiled units called class libraries or, simply, libraries Programs then reference and rely on class libraries to provide parts of their functionality The power of this concept is that two programs can rely on the same class library, thereby sharing the functionality of that class library across the two programs and reducing the total amount of code needed In other words, it is possible to write features once, place them into a class library, and allow multiple programs to include those features by referencing the same class library Later on, when developers fix a bug or add functionality to the class library, all the programs will have access to the increased functionality, just because they continue to reference the now improved class library

To reuse the code within a different assembly, it is necessary to reference the assembly when running the C# compiler Generally, the referenced assembly is a class library, and creating a class library requires a different assembly target from the default console executable targets you created thus far

Console executable: This is the default type of assembly, and all compilation thus far has been to a console executable (Leaving off the /target option or specifying /target:exe creates a console executable) Class library: Classes that are shared across multiple executables are generally defined in a class library (/target:library) Windows executable: Windows executables are designed to run in the Microsoft Windows family of operating systems and outside of the command console (/target:winexe) Module: In order to facilitate multiple languages within the same assembly, code can be compiled to a module and multiple modules can be combined to form an assembly ( /target:module) Assemblies to be shared across multiple applications are generally compiled as class libraries Consider, for example, a library dedicated to functionality around longitude and latitude coordinates To compile the Coordinate, Longitude, and Latitude classes into their own library, you use the command line shown in Output 94

>csc /target:library /out:Coordinatesdll Coordinatecs IAnglecs Latitudecs Longitudecs Arccs Microsoft (R) Visual C# 2005 Compiler version 8005072742 for Microsoft (R) Windows (R) 2005 Framework version 2050727 Copyright (C) Microsoft Corporation 2001-2005 All rights reserved

Assuming you use NET and the C# compiler is in the path, this builds an assembly library called Coordinatesdll

Just as classes serve as an encapsulation boundary for behavior and data, assemblies provide a similar boundar among groups of types Developers can break a system into assemblies and then share those assemblies with multiple applications or integrate them with assemblies provided by third parties

By default, a class without any access modifier is defined as internal The result is that the class is inaccessible from outside the assembly Even though another assembly references the assembly containing the class, all internal classes within the referenced assemblies will be inaccessible

Just as private and protected provide levels of encapsulation to members within a class, C# supports the use o access modifiers at the class level for control over the encapsulation of the classes within an assembly The access modifiers available are public and internal , and in order to expose a class outside of the assembly, the class must be marked as public Therefore, before compiling the Coordinatesdll assembly, it is necessary to modify the type declarations as public (see Listing 912)