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CHAPTER 7 CLASSES AND INHERITANCE
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Class Access Modifiers
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A class can be seen and accessed by other classes in the system. This section covers the accessibility of classes. Although I will use classes in the explanations and examples, since that is what you are familiar with at this point in the text, the accessibility rules also apply to the other types I will cover later. The term visible is sometimes used for the term accessible. They can be used interchangeably. There are two levels of class accessibility public and internal. A class marked public can be accessed by code from any assembly in the system. To make a class visible to other assemblies, use the public access modifier, as shown here. Keyword public class MyBaseClass { ... A class marked internal can be seen only by classes within its own assembly. This is the default accessibility level, so unless you explicitly specify the modifier public in the class declaration, code outside the assembly cannot access the class. You can explicitly declare a class as internal by using the internal access modifier. Keyword internal class MyBaseClass { ... Figure 7-13 illustrates the accessibility of internal and public classes from outside the assembly. Class MyClass is not visible to the classes in the assembly on the left, because it is marked internal. Class OtherClass, however, is visible to the classes on the left, because it is marked public.
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Figure 7-13. Classes from other assemblies can access public classes but cannot access internal classes.
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CHAPTER 7 CLASSES AND INHERITANCE
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Inheritance Between Assemblies
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So far, I have been declaring derived classes in the same assembly where the base class is declared. But C# also allows you to derive a class from a base class defined in a different assembly. To do this, the following must be true: The base class must be declared public, so that it can be accessed from outside its assembly. You must include a reference in your Visual Studio project, to the assembly containing the base class. To make it easier to refer to the classes and types in the other assembly, without using their fully qualified names, place a using directive at the top of the source file, with the namespace containing the classes or types you want to access.
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Note Adding a reference to the other assembly and adding a using directive are two separate things. Adding the reference to the other assembly tells the compiler where the required types are defined. Adding the using directive allows you to reference other classes without having to use their fully qualified names. 10 covers this in detail.
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For example, the following two code segments, from different assemblies, show how easy it is to inherit a class from another assembly. The first code listing creates an assembly that contains the declaration of a class called MyBaseClass, which has the following characteristics: It is declared in a source file called BaseClass.cs, and inside a namespace declared as BaseClassNS. It is declared public, so that it can be accessed from other assemblies. It contains a single member, a method called PrintMe, that just writes out a simple message identifying the class. // Source file name BaseClass.cs using System; Namespace containing declaration of base class namespace BaseClassNS { Declare the class public, so it can be seen outside the assembly. public class MyBaseClass { public void PrintMe() { Console.WriteLine("I am MyBaseClass"); } } }
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CHAPTER 7 CLASSES AND INHERITANCE
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The second assembly contains the declaration of a class called DerivedClass that inherits from MyBaseClass, declared in the first assembly. The source file is named Program.cs. Figure 7-14 illustrates the two assemblies. DerivedClass has an empty body but inherits method PrintMe from MyBaseClass. Main creates an object of type DerivedClass and calls its inherited method PrintMe. // Source file name Program.cs using System; using BaseClassNS; Namespace containing declaration of base class namespace UsesBaseClass { Base class in other assembly class DerivedClass: MyBaseClass { // Empty body } class Program { static void Main( ) { DerivedClass mdc = new DerivedClass(); mdc.PrintMe(); } } } This code produces the following output:
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