barcode generator project in vb.net Figure 7-21. The structure of an extension method in Visual C#.NET

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Figure 7-21. The structure of an extension method
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CHAPTER 7 CLASSES AND INHERITANCE
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The following code shows a full program, including class MyData and extension method Average declared in class ExtendMyData. Notice that method Average is invoked exactly as if it were an instance member of MyData! Figure 7-21 illustrates the code. Classes MyData and ExtendMyData together act like the desired class, with three methods. namespace ExtensionMethods { sealed class MyData { private double D1, D2, D3; public MyData(double d1, double d2, double d3) { D1 = d1; D2 = d2; D3 = d3; } public double Sum() { return D1 + D2 + D3; } } static class ExtendMyData Keyword and type { public static double Average(this MyData md) { Declared static return md.Sum() / 3; } } class Program { static void Main() { MyData md = new MyData(3, 4, 5); Console.WriteLine("Sum: {0}", md.Sum()); Console.WriteLine("Average: {0}", md.Average()); } } Invoke as an instance member of the class } This code produces the following output: Sum: 12 Average: 4
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CHAPTER 7 CLASSES AND INHERITANCE
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External Methods
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An external method is a method that does not have an implementation in the declaration. Often the implementation is in a language other than C#. External methods are marked with the extern modifier and do not have an implementation in the class declaration. The implementation is replaced by a semicolon. Keyword public static extern int GetCurrentDirectory(int size, StringBuilder buf); No implementation Connecting the declaration with the implementation is implementation-dependent, but is often done using the DllImport attribute. Attributes are covered in detail in 24. For example, the following code uses an external method, GetCurrentDirectory, whose implementation is the Win32 system call for getting a string that contains the current directory. using System; using System.Text; using System.Runtime.InteropServices; namespace ExternalMethod { class MyClass { [DllImport("kernel32", SetLastError=true)] public static extern int GetCurrentDirectory(int a, StringBuilder b); } class Program { static void Main( ) { const int MaxDirLength = 250; StringBuilder sb = new StringBuilder(); sb.Length = MaxDirLength; MyClass.GetCurrentDirectory(MaxDirLength, sb); Console.WriteLine(sb); } } }
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CHAPTER 7 CLASSES AND INHERITANCE
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This code produces the following output:
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C:\BookPrograms\ExternalMethod\ExternalMethod\bin\Debug
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Expressions and Operators
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Expressions Literals Order of Evaluation Simple Arithmetic Operators The Remainder Operator Relational and Equality Comparison Operators Increment and Decrement Operators Conditional Logical Operators Logical Operators Shift Operators Assignment Operators The Conditional Operator Unary Arithmetic Operators User-Defined Type Conversions Operator Overloading The typeof Operator
CHAPTER 8 EXPRESSIONS AND OPERATORS
Expressions
This chapter defines expressions and describes the operators provided by C#. It also explains how you can define the C# operators to work with your user-defined classes. An expression is a string of operators and operands. Some of the constructs that can act as operands are Literals Constants Variables Method calls Element accessors, such as array accessors and indexers Other expressions The C# operators take one, two, or three operands. An operator Takes its operands as input Performs an action Returns a value, based on the action Expressions can be combined, using operators, to create other expressions, as shown in this expression, with three operators and four operands:
Evaluating an expression is the process of applying each operator to its operands, in the proper sequence, to produce a value. The value is returned to the position at which the expression was evaluated. There, it might in turn be an operand in an enclosing expression. Besides the value returned, some expressions also have side effects, such as setting a value in memory.
CHAPTER 8 EXPRESSIONS AND OPERATORS
Literals
Literals are numbers or strings typed into the source code that represent a specific, set value of a specific type. For example, the following code shows literals of six types. Notice, for example, the difference between the double literal and the float literal. static void Main() Literals { Console.WriteLine("{0}", 1024); Console.WriteLine("{0}", 3.1416); Console.WriteLine("{0}", 3.1416F); Console.WriteLine("{0}", true); Console.WriteLine("{0}", 'x'); Console.WriteLine("{0}", "Hi there"); } The output of this code is the following: 1024 3.1416 3.1416 True x Hi there Because literals are written into the source code, their values must be known at compile time. Several of the predefined types have their own forms of literal: Type bool has two literals: true and false. For reference type variables, literal null means that the variable is not set to a reference in memory.
// // // // // //
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