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Collections, Enumerators, and Iterators
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After calling Reset( ), enumeration will again begin at the start of the collection Thus, you must call MoveNext( ) before obtaining the first element In IEnumerator<T>, the methods MoveNext( ) and Reset( ) work in the same way Two other points: First, you cannot use an enumerator to change the collection that is being enumerated Thus, enumerators are read-only relative to the collection Second, any change to the collection under enumeration invalidates the enumerator
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Using an Enumerator
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Before you can access a collection through an enumerator, you must obtain one Each of the collection classes provides a GetEnumerator( ) method that returns an enumerator to the start of the collection Using this enumerator, you can access each element in the collection, one element at a time In general, to use an enumerator to cycle through the contents of a collection, follow these steps: 1 Obtain an enumerator to the start of the collection by calling the collection s GetEnumerator( ) method 2 Set up a loop that makes a call to MoveNext( ) Have the loop iterate as long as MoveNext( ) returns true 3 Within the loop, obtain each element through Current Here is an example that implements these steps It uses an ArrayList, but the general principles apply to any type of collection, including the generic collections
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// Demonstrate an enumerator using System; using SystemCollections; class EnumeratorDemo { static void Main() { ArrayList list = new ArrayList(1); for(int i=0; i < 10; i++) listAdd(i); // Use enumerator to access list IEnumerator etr = listGetEnumerator(); while(etrMoveNext()) ConsoleWrite(etrCurrent + " "); ConsoleWriteLine(); // Re enumerate the list etrReset(); while(etrMoveNext()) ConsoleWrite(etrCurrent + " "); ConsoleWriteLine(); } }
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In general, when you need to cycle through a collection, a foreach loop is more convenient to use than an enumerator However, an enumerator gives you a little extra control by allowing you to reset the enumerator at will
Using IDictionaryEnumerator
When using a non-generic IDictionary, such as Hashtable, you will use an IDictionaryEnumerator instead of an IEnumerator when cycling through the collection The IDictionaryEnumerator inherits IEnumerator and adds three properties The first is DictionaryEntry Entry { get; } Entry obtains the next key/value pair from the enumerator in the form of a DictionaryEntry structure Recall that DictionaryEntry defines two properties, called Key and Value, which can be used to access the key or value contained within the entry The other two properties defined by IDictionaryEnumerator are shown here: object Key { get; } object Value { get; } These allow you to access the key or value directly An IDictionaryEnumerator is used just like a regular enumerator, except that you will obtain the current value through the Entry, Key, or Value properties rather than Current Thus, after obtaining an IDictionaryEnumerator, you must call MoveNext( ) to obtain the first element Continue to call MoveNext( ) to obtain the rest of the elements in the collection MoveNext( ) returns false when there are no more elements Here is an example that enumerates the elements in a Hashtable through an IDictionaryEnumerator:
// Demonstrate IDictionaryEnumerator using System; using SystemCollections; class IDicEnumDemo { static void Main() { // Create a hash table Hashtable ht = new Hashtable(); // Add elements to the table htAdd("Tom", "555 3456"); htAdd("Mary", "555 9876"); htAdd("Todd", "555 3452"); htAdd("Ken", "555 7756"); // Demonstrate enumerator IDictionaryEnumerator etr = htGetEnumerator(); ConsoleWriteLine("Display info using Entry");
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Collections, Enumerators, and Iterators
while(etrMoveNext()) ConsoleWriteLine(etrEntryKey + ": " + etrEntryValue); ConsoleWriteLine(); ConsoleWriteLine("Display info using Key and Value directly"); etrReset(); while(etrMoveNext()) ConsoleWriteLine(etrKey + ": " + etrValue);
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