qr code generator c# example The C# Language in C#

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The C# Language
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but are not technically a feature of the C# language) However, there is one part of C# that relates directly to the assembly: the internal access modifier, which is examined next
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The internal Access Modifier
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In addition to the access modifiers public, private, and protected, which you have been using throughout this book, C# also defines internal The internal modifier declares that a member is known throughout all files in an assembly, but unknown outside that assembly Thus, in simplified terms, a member marked as internal is known throughout a program, but not elsewhere The internal access modifier is particularly useful when creating software components The internal modifier can be applied to classes and members of classes and to structures and members of structures The internal modifier can also be applied to interface and enumeration declarations You can use protected in conjunction with internal to produce the protected internal access modifier pair The protected internal access level can be given only to class members A member declared with protected internal access is accessible within its own assembly or to derived types Here is an example that uses internal:
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// Use internal using System; class InternalTest { internal int x; } class InternalDemo { static void Main() { InternalTest ob = new InternalTest(); obx = 10; // can access -- in same file ConsoleWriteLine("Here is obx: " + obx); } }
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Inside InternalTest, the field x is declared internal This means that it is accessible within the program, as its use in InternalDemo shows, but unavailable outside the program
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CHAPTER
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Runtime Type ID, Reflection, and Attributes
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his chapter discusses three interrelated and powerful features: runtime type identification, reflection, and attributes Runtime type ID is the mechanism that lets you identify a type during the execution of a program Reflection is the feature that enables you to obtain information about a type Using this information, you can construct and use objects at runtime This feature is very powerful because it lets a program add functionality dynamically, during execution An attribute describes a characteristic of some element of a C# program For example, you can specify attributes for classes, methods, and fields, among others Attributes can be interrogated at runtime, and the attribute information obtained Attributes use both runtime type identification and reflection
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Runtime Type Identification
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Runtime type identification (RTTI) allows the type of an object to be determined during program execution RTTI is useful for many reasons For example, you can discover precisely what type of object is being referred to by a base-class reference Another use of RTTI is to test in advance whether a cast will succeed, preventing an invalid cast exception Runtime type identification is also a key component of reflection C# includes three keywords that support runtime type identification: is, as, and typeof Each is examined in turn
Testing a Type with is
You can determine if an object is of a certain type by using the is operator Its general form is shown here: expr is type Here, expr is an expression that describes an object whose type is being tested against type If the type of expr is the same as, or compatible with, type, then the outcome of this operation is true Otherwise, it is false Thus, if the outcome is true, expr is some form of type As it applies to is, one type is compatible with another if both are the same type, or if a reference, boxing, or unboxing conversion exists
Part I:
The C# Language
Here is an example that uses is:
// Demonstrate is using System; class A {} class B : A {} class UseIs { static void Main() { A a = new A(); B b = new B(); if(a is A) ConsoleWriteLine("a is an A"); if(b is A) ConsoleWriteLine("b is an A because it is derived from A"); if(a is B) ConsoleWriteLine("This won t display -- a not derived from B"); if(b is B) ConsoleWriteLine("B is a B"); if(a is object) ConsoleWriteLine("a is an object"); } }
The output is shown here:
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