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CHAPTER 20 ENUMERATORS AND ITERATORS
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The IEnumerator<T> Interface
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The IEnumerator<T> interface uses generics to return an actual derived type, rather than a reference to an object. The IEnumerator<T> interface derives from two other interfaces: the nongeneric IEnumerator interface and the IDisposable interface. It must therefore implement their members. You ve already seen the nongeneric IEnumerator interface and its three members. The IDisposable interface has a single, void, parameterless method called Dispose, which can be used to free unmanaged resources being held by the class. (The Dispose method was described in 6.) The IEnumerator<T> interface itself has a single property, Current, which returns an instance of type T or derived from T rather than a reference of type object. Since both IEnumerator<T> and IEnumerator have a member named Current, you should explicitly implement the IEnumerator version and implement the generic version in the class itself, as shown in Figure 20-6. Figure 20-6 illustrates the implementation of the interface.
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Figure 20-6. Implementing the IEnumerator<T> interface
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CHAPTER 20 ENUMERATORS AND ITERATORS
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The declaration of the class implementing the interface should look something like the pattern in the following code, where T is the type returned by the enumerator: using System.Collections; using System.Collections.Generic; class MyGenEnumerator: IEnumerator< T > { public T Current { get { } } Explicit implementation object IEnumerator.Current { get { ... } } public bool MoveNext() { ... } public void Reset() public void Dispose() ... } { ... } { ... }
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// IEnumerator<T>--Current
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// IEnumerator--Current // IEnumerator--MoveNext // IEnumerator--Reset // IDisposable--Dispose
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CHAPTER 20 ENUMERATORS AND ITERATORS
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For example, the following code implements the ColorEnumerator example using the generic enumerator interface: using System.Collections; using System.Collections.Generic; Substitute type string for T class ColorEnumerator : IEnumerator<string> { string[] Colors; int Position = -1; Returns the type argument type public string Current // Current--generic { get { return Colors[Position]; } } Explicit implementation object IEnumerator.Current // Current--nongeneric { get { return Colors[Position]; } } public bool MoveNext() { if (Position < Colors.Length - 1) { Position++; return true; } else return false; } public void Reset() { Position = -1; } public void Dispose() { } public ColorEnumerator(string[] colors) { Colors = new string[colors.Length]; for (int i = 0; i < colors.Length; i++) Colors[i] = colors[i]; } } // Constructor // MoveNext
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// Reset
CHAPTER 20 ENUMERATORS AND ITERATORS
The IEnumerable<T> Interface
The generic IEnumerable<T> interface is very similar to the nongeneric version, IEnumerable. The generic version derives from IEnumerable, so it must also implement the IEnumerable interface. Like IEnumerable, the generic version also contains a single member, a method called GetEnumerator. This version of GetEnumerator, however, returns a class object implementing the generic IEnumerator<T> interface. Since the class must implement two GetEnumerator methods, you should explicitly implement the nongeneric version and implement the generic version at the class level, as shown in Figure 20-7. Figure 20-7 illustrates the implementation of the interface.
Figure 20-7. Implementing the IEnumerable<T> interface
CHAPTER 20 ENUMERATORS AND ITERATORS
The following code shows a pattern for implementing the generic interface. T is the type returned by the enumerator. using System.Collections; using System.Collections.Generic; class MyGenEnumerable: IEnumerable<T> { public IEnumerator<T> GetEnumerator() { ... } Explicit implementation IEnumerator IEnumerable.GetEnumerator() { ... } ... }
// IEnumerable<T> version
// IEnumerable version
The following code shows the use of the generic enumerable interface: using System.Collections; using System.Collections.Generic; Substitute actual type for T class MyColors : IEnumerable<string> { string[] Colors = { "Red", "Yellow", "Blue" }; Substitute actual type for T public IEnumerator<string> GetEnumerator() { return new ColorEnumerator(Colors); } Explicit implementation IEnumerator IEnumerable.GetEnumerator() { return new ColorEnumerator(Colors); } }
// IEnumerable<T> version
// IEnumerable version
CHAPTER 20 ENUMERATORS AND ITERATORS
Iterators
Enumerable classes and enumerators are used extensively in the .NET collection classes, so it s important that you know how they work. But now that you know how to create your own enumerable classes and enumerators, you might be pleased to learn that, starting with C# 2.0, the language got a much simpler way of creating enumerators and enumerables. In fact, the compiler will create them for you. The construct that produces them is called an iterator. You can use the enumerators and enumerables generated by iterators wherever you would use manually coded enumerators or enumerables. Before I explain the details, let s take a look at two examples. The following method declaration implements an iterator that produces and returns an enumerator. The iterator returns a generic enumerator that returns three items of type string. The yield return statements declare that this is the next item in the enumeration.
Return a generic enumerator. public IEnumerator<string> BlackAndWhite() { yield return "black"; yield return "gray"; yield return "white"; }
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