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they are never invoked when using C# s as or is operators .
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To really understand operator overload methods and conversion operator methods, I strongly encourage you to examine the System.Decimal type as a role model . Decimal defines several constructors that allow you to convert objects from various types to a Decimal . It also offers several ToXxx methods that let you convert a Decimal object to another type . Finally, the type defines several conversion operators and operator overload methods as well .
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Extension Methods
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The best way to understand C# s extension methods feature is by way of an example . In the StringBuilder Members section in 14, Chars, Strings, and Working with Text, I mention how the StringBuilder class offers fewer methods than the String class for manipulating a string and how strange this is, considering that the StringBuilder class is the preferred way of manipulating a string because it is mutable . So, let s say that you would
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Part II Designing Types
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like to define some of these missing methods yourself to operate on a StringBuilder . For example, you might want to define your own IndexOf method as follows:
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public static class StringBuilderExtensions { public static Int32 IndexOf(StringBuilder sb, Char value) { for (Int32 index = 0; index < sb.Length; index++) if (sb[index] == value) return index; return -1; } }
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Now that you have defined this method, you can use it as the following code demonstrates:
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StringBuilder sb = new StringBuilder("Hello. My name is Jeff."); // The initial string
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// Change period to exclamation and get # characters in 1st sentence (5). Int32 index = StringBuilderExtensions.IndexOf(sb.Replace('.', '!'), '!');
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This code works just fine, but is it not ideal from a programmer s perspective . The first problem is that a programmer who wants to get the index of a character within a StringBuilder must know that the StringBuilderExtensions class even exists . The second problem is that the code does not reflect the order of operations that are being performed on the StringBuilder object, making the code difficult to write, read, and maintain . The programmer wants to call Replace first and then call IndexOf; but when you read the last line of code from left to right, IndexOf appears first on the line and Replace appears second . Of course, you could alleviate this problem and make the code s behavior more understandable by rewriting it like this:
// First, change period to exclamation mark sb.Replace( . , ! ); // Now, get # characters in 1st sentence (5) Int32 index = StringBuilderExtensions.IndexOf(sb, ! );
However, a third problem exists with both versions of this code that affects understanding the code s behavior . The use of StringBuilderExtensions is overpowering and detracts a programmer s mind from the operation that is being performed: IndexOf . If the StringBuilder class had defined its own IndexOf method, then we could rewrite the code above as follows:
// Change period to exclamation and get # characters in 1st sentence (5). Int32 index = sb.Replace('.', '!').IndexOf('!');
Wow, look how great this is in terms of code maintainability! In the StringBuilder object, we re going to replace a period with an exclamation mark and then find the index of the exclamation mark .
8 Methods
Now, I can explain what C# s extension methods feature does . It allows you to define a static method that you can invoke using instance method syntax . Or, in other words, we can now define our own IndexOf method and the three problems mentioned above go away . To turn the IndexOf method into an extension method, we simply add the this keyword before the first argument:
public static class StringBuilderExtensions { public static Int32 IndexOf(this StringBuilder sb, Char value) { for (Int32 index = 0; index < sb.Length; index++) if (sb[index] == value) return index; return -1; } }
Now, when the compiler sees code like this:
Int32 index = sb.IndexOf('X');
the compiler first checks if the StringBuilder class or any of its base classes offers an instance method called IndexOf that takes a single Char parameter . If an existing instance method exists, then the compiler produces IL code to call it . If no matching instance method exists, then the compiler will look at any static classes that define static methods called IndexOf that take as their first parameter a type matching the type of the expression being used to invoke the method . This type must also be marked with the this keyword . In this example, the expression is sb, which is of the StringBuilder type . In this case, the compiler is looking specifically for an IndexOf method that takes two parameters: a StringBuilder (marked with the this keyword) and a Char . The compiler will find our IndexOf method and produce IL code that calls our static method . OK so this now explains how the compiler improves the last two problems related to code understandability that I mentioned earlier . However, I haven t yet addressed the first problem: how does a programmer know that an IndexOf method even exists that can operate on a StringBuilder object The answer to this question is found in Microsoft Visual Studio s Intellisense feature . In the editor, when you type a period, Visual Studio s IntelliSense window opens to show you the list of instance methods that are available . Well, that IntelliSense window also shows you any extension methods that exist for the type of expression you have to the left of the period . Figure 8-1 shows Visual Studio s IntelliSense window; the icon for an extension method has a down arrow next to it, and the tooltip next to the method indicates that the method is really an extension method . This is truly awesome because it is now easy to define your own methods to operate on various types of objects and have other programmers discover your methods naturally when using objects of these types .
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