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As explained earlier, VB.NET has a new iteration syntax called foreach. The foreach
statement can only be applied to objects of classes that implement the
IEnumerable interface. The IEnumerable interface exposes the enumerator, which
supports a simple iteration over a collection. Enumerators are intended to be
used only to read data in the collection and cannot be used to modify the
underlying collection. The enumerator does not have exclusive access to the
collection. To understand what happens in the background, consider the code
snippet in Listing 5.57.
Listing 5.57: Enumerator Example 1
For
Each
i As
Integer
In
a
Console.WriteLine(i)
Next
This code functions just like the while loop used in Listing 5.58.
Listing 5.58: Enumerator Example 2
a = x.GetEnumerator()
While
a.MoveNext()
Console.WriteLine(a.Current)
End
While
Before entering the statement block, the compiler generates the code to call the
method GetEnumerator of the object passed as the second parameter in the foreach
statement. The GetEnumerator method must return an object, having a property
named Current, of type similar to the first argument of the foreach statement.
Also this object must have a MoveNext method of return type bool. This method
informs the runtime when to terminate the loop.
When an enumerator is instantiated, it takes a snapshot of the current state of
the collection. If changes are made to the collection, such as the addition,
modification, or deletion of elements, the snapshot gets out of sync and the
enumerator throws an InvalidOperationException. Two enumerators instantiated
from the same collection simultaneously can have different snapshots of the
collection.
If the enumerator is positioned before the first element in the collection or
after the last element in the collection, the enumerator is in an invalid state.
In that case, calling Current throws an exception.
The enumerator is positioned before the first element in the collection
initially. The Reset function brings the enumerator back to this position. The
MoveNext method must be called to advance the enumerator to the first element of
the collection before reading the value of Current, after an enumerator is
created or after a Reset. The Current property returns the same object until
either MoveNext or Reset is called.
Once the end of the collection is passed, the enumerator is in an invalid state
and calling MoveNext returns false.
Calling Current throws an exception if the last call to MoveNext returned false.
With this information under your belt, you should insert your enumerating code
inside a try-catchfinally block to prevent unexpected exits.
Conclusion
Hope this article would have helped you in understanding Enumerators in VB.NET.
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Manish Tewatia
Manish is Very much interested in Microsoft & LifeStyle Accessory Designand working with Microsoft technologies. His expert areas are ASP.NET, ADO.NET, C# .NET, WPF, WCF, Windows Phone 7, Android, SQL Server, HTML , XAML, etc… He is doing MCA and he love to drive fast bikes.
inspired from Mr. Mahesh Chand.
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