Iterators in C#.NET

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primitives: CSharpPrimitiveTypes enumerator: Enumerator
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GetEnumerator() Instantiate
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MoveNext() yield return "object" WriteLine()
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MoveNext() yield return "byte" WriteLine()
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Figure 168: Sequence Diagram with yield return
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foreach displays on the screen what CSharpPrimitiveTypes yielded and
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starts the loop again This process continues until there are no more yield return statements within the iterator At that point, the foreach loop at the call site terminates
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More Iterator Examples Before you modify BinaryTree<T>, you must modify Pair<T> to support the IEnumerable<T> interface using an iterator Listing 1614 is an example that yields each element in Pair<T>
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16: Building Custom Collections
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Listing 1614: Using yield to Implement BinaryTree<T>
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public struct Pair<T>: IPair<T>,
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IEnumerable<T>
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{ public Pair(T first, T second) { _first = first; _second = second; } public T First { get{ return _first; } private set{ _first = value; } } private T _first; public T Second { get{ return _second; } private set{ _second = value; } } private T _second; #region IEnumerable<T> public IEnumerator<T> GetEnumerator() { yield return First; yield return Second; } #endregion IEnumerable<T> #region IEnumerable Members SystemCollectionsIEnumerator SystemCollectionsIEnumerableGetEnumerator() { return GetEnumerator(); } #endregion }
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In Listing 1614, the iteration over the Pair<T> data type loops twice: first through yield return First, and then through yield return Second Each time the yield return statement within GetEnumerator() is encountered, the state is saved and execution appears to jump out of the GetEnumerator() method context and into the context of the call site When the second iteration starts, GetEnumerator() begins to execute again with the yield return Second statement SystemCollectionsGenericIEnumerable<T> inherits from SystemCollectionsIEnumerable Therefore, when implementing IEnumerable<T>, it is also necessary to implement IEnumerable In Listing 1614,
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you do so explicitly, and the implementation simply involves a call to IEnumerable<T> s GetEnumerator() implementation This call from IEnumerableGetEnumerator() to IEnumerable<T>GetEnumerator() will always work because of the type compatibility (via inheritance) between IEnumerable<T> and IEnumerable This call from x to y will work in all cases because of type compatibility between IEnumerable and IEnumerator<T> Since the signatures for both GetEnumerator()s are identical (the return type does not distinguish a signature), one or both implementations must be explicit Given the additional type safety offered by IEnumerable<T> s version, you implement IEnumerable s implementation explicitly Listing 1615 uses the Pair<T>GetEnumerator() method and displays "Inigo" and "Montoya" on two consecutive lines
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Listing 1615: Using Pair<T>GetEnumerator() via foreach
Pair<string> fullname = new Pair<string>("Inigo", "Montoya"); foreach (string name in fullname) { ConsoleWriteLine(name); }
Notice that the call to GetEnumerator() is implicit within the foreach loop
Placing a yield return within a Loop It is not necessary to hardcode each yield return statement, as you did in both CSharpPrimitiveTypes and Pair<T> Using the yield return statement, you can return values from inside a loop construct Listing 1616 uses a foreach loop Each time the foreach within GetEnumerator() executes, it returns the next value
Listing 1616: Placing yield return Statements within a Loop
public class BinaryTree<T>: IEnumerable<T> { // #region IEnumerable<T> public IEnumerator<T> GetEnumerator() { // Return the item at this node yield return Value;
16: Building Custom Collections
// Iterate through each of the elements in the pair foreach (BinaryTree<T> tree in SubItems) { if (tree != null) { // Since each element in the pair is a tree, // traverse the tree and yield each // element foreach (T item in tree) { yield return item; } } } } #endregion IEnumerable<T> #region IEnumerable Members SystemCollectionsIEnumerator SystemCollectionsIEnumerableGetEnumerator() { return GetEnumerator(); } #endregion }
In Listing 1616, the first iteration returns the root element within the binary tree During the second iteration you traverse the pair of subelements If the subelement pair contains a non-null value, then you traverse into that child node and yield its elements Note that foreach(T item in tree) is a recursive call to a child node As observed with CSharpPrimitiveTypes and Pair<T>, you can now iterate over BinaryTree<T> using a foreach loop Listing 1617 demonstrates this, and Output 166 shows the results
Listing 1617: Using foreach with BinaryTree<string>
// JFK jfkFamilyTree = new BinaryTree<string>( "John Fitzgerald Kennedy"); jfkFamilyTreeSubItems = new Pair<BinaryTree<string>>( new BinaryTree<string>("Joseph Patrick Kennedy"), new BinaryTree<string>("Rose Elizabeth Fitzgerald")); // Grandparents (Father's side) jfkFamilyTreeSubItemsFirstSubItems = new Pair<BinaryTree<string>>( new BinaryTree<string>("Patrick Joseph Kennedy"),