The Problem with Type Unsafe Enumeration in Visual Studio .NET

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Prior to NET 20, the enumeration pattern was supported by the IEnumerable and IEnumerator interfaces The problem with the IEnumerator interface is that its Current property only returns an Object reference, and you have to cast to the object's actual type to use it correctly Besides the lack of type safety this represents, it also can induce performance problems when your collection contains value types, and the casting itself doesn't come for free Value types have to be boxed to be passed as an object reference, which means that an object has to be allocated in garbage-collected memory to contain the value, the value needs to be copied into that object from the stack, and a reference to that memory is what you get back as the object reference Typically what you do next when accessing an item from a collection through the IEnumeratorCurrent property is to cast it back to its appropriate type If that type is a value type, the object reference has to be unboxed to cast it back to the appropriate type (such as int or DateTime) and then copied into the stack-allocated variable Then the object that was allocated becomes available for garbage collection, which increases the workload of the garbage collector All of this has performance costs associated with it that you should avoid As a result, always favor using the generic collection types and interfaces to avoid this problem when operating directly on a collection of a particular type The foreach operator avoids the problem by calling the Current property and MoveNext method directly on the enumerator object instead of doing it through theIEnumerator interface This allows that object's enumerator to return the actual type of the item instead of a vanilla object reference In addition, the foreach operator can operate directly against the actual type of the collection's items, thus avoiding boxing and unboxing for value types if they implement their enumerators correctly
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You might be wondering why two separate interfaces are needed Why not just define one interface that the collection itself implements that directly supports iterating through the collection The answer is that you might want to support more than one form of iteration on a collection, so separating the iteration logic into a separate object lets you plug in additional forms of iteration if you need to You can then expose those additional iteration approaches through properties on the collection class that return an IEnumerator interface reference to an object that iterates through your collection in a different way This pattern also allows you to provide multiple enumerators to the same collection, so your code can maintain more than one current position in the collection If your code will iterate over the collection using multiple threads, then you'll want to look into implementing the ICollection interface to provide thread-safe access to the collection Typically, the object that implements the iteration methods of IEnumerator are created as a nested class within the collection class, but you are free to implement things however you want It should be fairly rare that you would need to do a raw implementation of IEnumerable and IEnumerator, because the built-in collection classes in NET already do that for you IEnumerable and its related IEnumerator interface are really all that is needed to support a read-only presentation of bound data If you can provide a collection reference to a control, it can iterate over the items in the collection and use specific type informationhelper classes like the PropertyDescriptor classor reflection to access the properties on the data items and display them This is how the BindingSource component lets you bind to anything that implements IEnumerable
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