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Part II Designing Types
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Using the BitArray class s indexer is incredibly simple:
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// Allocate a BitArray that can hold 14 bits. BitArray ba = new BitArray(14); // Turn all the even-numbered bits on by calling the set accessor. for (Int32 x = 0; x < 14; x++) { ba[x] = (x % 2 == 0); } // Show the state of all the bits by calling the get accessor. for (Int32 x = 0; x < 14; x++) { Console.WriteLine("Bit " + x + " is " + (ba[x] "On" : "Off")); }
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In the BitArray example, the indexer takes one Int32 parameter, bitPos . All indexers must have at least one parameter, but they can have more . These parameters (as well as the return type) can be of any data type (except void) . An example of an indexer that has more than one parameter can be found in the System.Drawing.Imaging.ColorMatrix class, which ships in the System .Drawing .dll assembly . It s quite common to create an indexer to look up values in an associative array . In fact, the System.Collections.Generic.Dictionary type offers an indexer that takes a key and returns the value associated with the key . Unlike parameterless properties, a type can offer multiple, overloaded indexers as long as their signatures differ . Like a parameterless property s set accessor method, an indexer s set accessor method also contains a hidden parameter, called value in C# . This parameter indicates the new value desired for the indexed element . The CLR doesn t differentiate parameterless properties and parameterful properties; to the CLR, each is simply a pair of methods and a piece of metadata defined within a type . As mentioned earlier, different programming languages require different syntax to create and use parameterful properties . The fact that C# requires this[...] as the syntax for expressing an indexer was purely a choice made by the C# team . What this choice means is that C# allows indexers to be defined only on instances of objects . C# doesn t offer syntax allowing a developer to define a static indexer property, although the CLR does support static parameterful properties . Because the CLR treats parameterful properties just as it does parameterless properties, the compiler will emit either two or three of the following items into the resulting managed assembly:
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A method representing the parameterful property s get accessor method . This is emitted only if you define a get accessor method for the property . A method representing the parameterful property s set accessor method . This is emitted only if you define a set accessor method for the property .
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A property definition in the managed assembly s metadata, which is always emitted . There s no special parameterful property metadata definition table because, to the CLR, parameterful properties are just properties .
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For the BitArray class shown earlier, the compiler compiles the indexer as though the original source code were written as follows:
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public sealed class BitArray { // This is the indexer's get accessor method. public Boolean get_Item(Int32 bitPos) { /* ... */ } // This is the indexer's set accessor method. public void set_Item(Int32 bitPos, Boolean value) }
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{ /* ... */ }
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The compiler automatically generates names for these methods by prepending get_ and set_ to the indexer name . Because the C# syntax for an indexer doesn t allow the developer to specify an indexer name, the C# compiler team had to choose a default name to use for the accessor methods; they chose Item . Therefore, the method names emitted by the compiler are get_Item and set_Item . When examining the .NET Framework Reference documentation, you can tell if a type offers an indexer by looking for a property named Item . For example, the System.Collections.Generic.List type offers a public instance property named Item; this property is List s indexer . When you program in C#, you never see the name of Item, so you don t normally care that the compiler has chosen this name for you . However, if you re designing an indexer for a type that code written in other programming languages will be accessing, you might want to change the default name, Item, given to your indexer s get and set accessor methods . C# allows you to rename these methods by applying the System.Runtime.CompilerServices.IndexerNameAttribute custom attribute to the indexer . The following code demonstrates how to do this:
using System; using System.Runtime.CompilerServices; public sealed class BitArray { [IndexerName("Bit")] public Boolean this[Int32 bitPos] { // At least one accessor method is defined here } }
Now the compiler will emit methods called get_Bit and set_Bit instead of get_Item and set_Item . When compiling, the C# compiler sees the IndexerName attribute, and this tells
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