16: Building Custom Collections in Visual C#.NET

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16: Building Custom Collections
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#region IEnumerable Members SystemCollectionsIEnumerator SystemCollectionsIEnumerableGetEnumerator() { return GetEnumerator(); } #endregion }
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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
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Listing 1617: Using foreach with BinaryTree<string>
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// 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"), new BinaryTree<string>("Mary Augusta Hickey")); // Grandparents (Mother's side) jfkFamilyTreeSubItemsSecondSubItems = new Pair<BinaryTree<string>>( new BinaryTree<string>("John Francis Fitzgerald"), new BinaryTree<string>("Mary Josephine Hannon")); foreach (string name in jfkFamilyTree) { ConsoleWriteLine(name); }
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Iterators
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OUTPUT 166:
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John Fitzgerald Kennedy Joseph Patrick Kennedy Patrick Joseph Kennedy Mary Augusta Hickey Rose Elizabeth Fitzgerald John Francis Fitzgerald Mary Josephine Hannon
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BEGINNER TOPIC struct versus class An interesting side effect of defining Pair<T> as a struct rather than a class is that SubItemsFirst and SubItemsSecond cannot be assigned directly The following will produce a compile error indicating that SubItems cannot be modified, because it is not a variable :
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jfkFamilyTreeSubItemsFirst = new BinaryTree<string>("Joseph Patrick Kennedy");
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The issue is that SubItems is a property of type Pair<T>, a struct Therefore, when the property returns the value, a copy of _SubItems is made, and assigning First on a copy that is promptly lost at the end of the statement would be misleading Fortunately, the C# compiler prevents this To overcome the issue, don t assign it (see the approach in Listing 1617), use class rather than struct for Pair<T>, don t create a SubItems property and instead use a field, or provide properties in BinaryTree<T> that give direct access to _SubItems members
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Canceling Further Iteration: yield break Sometimes you might want to cancel further iteration You can do this by including an if statement so that no further statements within the code are executed However, you can also jump back to the call site, causing MoveNext() to return false Listing 1618 shows an example of such a method
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16: Building Custom Collections
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Listing 1618: Escaping Iteration via yield break
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public SystemCollectionsGenericIEnumerable<T> GetNotNullEnumerator() { if((First == null) || (Second == null)) { yield break; } yield return Second; yield return First; }
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This method cancels the iteration if either of the elements in the Pair<T> class is null A yield break statement is similar to placing a return statement at the top of a function when it is determined that there is no work to do It is a way to exit from further iterations without surrounding all remaining code with an if block As such, it allows multiple exits, and therefore, you should use it with caution because casual reading of the code may miss the early exit
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ADVANCED TOPIC How Iterators Work When the C# compiler encounters an iterator, it expands the code into the appropriate CIL for the corresponding enumerator design pattern In the generated code, the C# compiler first creates a nested private class to implement the IEnumerator<T> interface, along with its Current property and a MoveNext() method The Current property returns a type corresponding to the return type of the iterator Listing 1614 of Pair<T> contains an iterator that returns a T type The C# compiler examines the code contained within the iterator and creates the necessary code within the MoveNext method and the Current property to mimic its behavior For the Pair<T> iterator, the C# compiler generates roughly equivalent code (see Listing 1619)
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