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The problem with an implementation such as Listing 147 is that if two such loops interleaved each other one foreach inside another, both using the same collection the collection must maintain a state indicator of the current element so that when MoveNext() is called, the next element can be determined The problem is that one interleaving loop can affect the other (The same is true of loops executed by multiple threads) To overcome this problem, the collection classes do not support IEnumerator<T> and IEnumerator interfaces directly As shown in Figure 141, there is a second interface, called IEnumerable<T>, whose only method is GetEnumerator() The purpose of this method is to return an object that supports IEnumerator<T> Instead of the collection class maintaining the state, a different class, usually a nested class so that it has access to the internals of the collection, will support the IEnumerator<T> interface and will keep the state of the iteration loop Using this pattern, the C# equivalent of a foreach loop will look like the code shown in Listing 148
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Listing 148: A Separate Enumerator Maintaining State during an Iteration
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SystemCollectionsGenericStack<int> stack = new SystemCollectionsGenericStack<int>(); int number; SystemCollectionsGenericStack<int>Enumerator enumerator; // // If IEnumerable<T> is implemented explicitly, // then a cast is required // ((IEnumerable<int>)stack)GetEnumerator(); enumerator = stackGetEnumerator(); while (enumeratorMoveNext()) { number = enumeratorCurrent; ConsoleWriteLine(number); }
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Cleaning Up Following Iteration
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Since the classes that implement the IEnumerator<T> interface maintain the state, sometimes you need to clean up the state after it exits the loop (because either all iterations have completed or an exception is thrown) To achieve this, the IEnumerator<T> interface derives from IDisposable Enumerators that implement IEnumerator do not necessarily implement IDisposable, but if they do, Dispose() will be called as well This enables the calling of Dispose() after the foreach loop exits The C# equivalent of the final CIL code, therefore, looks like Listing 149
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Listing 149: Compiled Result of foreach on Collections
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SystemCollectionsGenericStack<int> stack = new SystemCollectionsGenericStack<int>(); SystemCollectionsGenericStack<int>Enumerator enumerator; IDisposable disposable; enumerator = stackGetEnumerator(); try { int number; while (enumeratorMoveNext()) { number = enumeratorCurrent; ConsoleWriteLine(number);
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} } finally { // Explicit cast used for IEnumerator<T> disposable = (IDisposable) enumerator; disposableDispose(); // // // // // // // } IEnumerator will use the as operator unless IDisposable support is known at compile time disposable = (enumerator as IDisposable); if (disposable != null) { disposableDispose(); }
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Notice that because the IDisposable interface is supported by IEnumerator<T>, the using statement can simplify the code in Listing 149 to that shown in Listing 1410
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Listing 1410: Error Handling and Resource Cleanup with using
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SystemCollectionsGenericStack<int> stack = new SystemCollectionsGenericStack<int>(); int number; using( SystemCollectionsGenericStack<int>Enumerator<int> enumerator = stackGetEnumerator()) { while (enumeratorMoveNext()) { number = enumeratorCurrent; ConsoleWriteLine(number); } }
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However, recall that the CIL also does not directly support the using keyword, so in reality, the code in Listing 149 is a more accurate C# representation of the foreach CIL code
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Do Not Modify Collections during foreach Iteration 3 showed that the compiler prevents assignment of the foreach variable (number) As is demonstrated in Listing 1410, an assignment to
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Standard Query Operators number would not be a change to the collection element itself, so the C#
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compiler prevents such an assignment altogether In addition, neither the element count within a collection nor the items themselves can generally be modified during the execution of a foreach loop If, for example, you called stackPush(42) inside the foreach loop, it would be ambiguous whether the iterator should ignore or incorporate the change to stack in other words, whether iterator should iterate over the newly added item or ignore it and assume the same state as when it was instantiated Because of this ambiguity, an exception of type SystemInvalidOperationException is generally thrown upon reaccessing the enumerator if the collection is modified within a foreach loop, reporting that the collection was modified after the enumerator was instantiated
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