print barcode labels in vb.net Part II Designing Types in Visual C#.NET

Generation PDF-417 2d barcode in Visual C#.NET Part II Designing Types

Part II Designing Types
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FIguRE 8-1 Visual Studio s IntelliSense window, showing extension methods
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Rules and Guidelines
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There are some additional rules and guidelines that you should know about extension methods:
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C# supports extension methods only; it does not offer extension properties, extension events, extension operators, and so on . Extension methods (methods with this before their first argument) must be declared in non-generic, static classes . However, there is no restriction on the name of the class; you can call it whatever you want . Of course, an extension method must have at least one parameter, and only the first parameter can be marked with the this keyword . The C# compiler looks only for extension methods defined in static classes that are themselves defined at the file scope . In other words, if you define the static class nested within another class, the C# compiler will emit the following message: "error
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CS1109: Extension method must be defined in a top-level static class; StringBuilderExtensions is a nested class."
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Since the static classes can have any name you want, it takes the C# compiler time to find extension methods as it must look at all the file-scope static classes and scan their static methods for a match . To improve performance and also to avoid considering an extension method that you may not want, the C# compiler requires that you import extension methods . For example, if someone has defined a StringBuilderExtensions class in a Wintellect namespace, then a programmer who wants to have access to this class s extension methods must put a using Wintellect; directive at the top of his or her source code file .
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8 Methods
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It is possible that multiple static classes could define the same extension method . If the compiler detects that two or more extension methods exist, then the compiler issues the following message: "error CS0121: The call is ambiguous
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between the following methods or properties: 'StringBuilderExtensions. IndexOf(string, char)' and 'AnotherStringBuilderExtensions. IndexOf(string, char)'." To fix this error, you must modify your source code .
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Specifically, you cannot use the instance method syntax to call this static method anymore; instead you must now use the static method syntax where you explicitly indicate the name of the static class to explicitly tell the compiler which method you want to invoke .
You should use this feature sparingly, as not all programmers are familiar with it . For example, when you extend a type with an extension method, you are actually extending derived types with this method as well . Therefore, you should not define an extension method whose first parameter is System.Object, as this method will be callable for all expression types and this will really pollute Visual Studio s IntelliSense window . There is a potential versioning problem that exists with extension methods . If, in the future, Microsoft adds an IndexOf instance method to their StringBuilder class with the same prototype as my code is attempting to call, then when I recompile my code, the compiler will bind to Microsoft s IndexOf instance method instead of my static IndexOf method . Because of this, my program will experience different behavior . This versioning problem is another reason why this feature should be used sparingly .
Extending Various Types with Extension Methods
In this chapter, I demonstrated how to define an extension method for a class, StringBuilder . I d like to point out that since an extension method is really the invocation of a static method, the CLR does not emit code ensuring that the value of the expression used to invoke the method is not null:
// sb is null StringBuilder sb = null; // Calling extension method: NullReferenceException will NOT be thrown when calling IndexOf // NullReferenceException will be thrown inside IndexOf s for loop sb.IndexOf('X'); // Calling instance method: NullReferenceException WILL be thrown when calling Replace sb.Replace('.', '!');
I d also like to point out that you can define extension methods for interface types as the following code shows:
public static void ShowItems<T>(this IEnumerable<T> collection) { foreach (var item in collection) Console.WriteLine(item); }
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