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The X++ language syntax provides two composite types: arrays and containers. These composite types are useful for aggregating values of primitive types. However, you can't store class objects in arrays or containers.

Collection classes are used to store objects. They let you create arrays, lists, sets, maps, and structs that can hold any data type, even objects. For maximum performance, the classes are implemented in C++ (they are system classes). Collection classes were previously known as foundation classes. The collection classes are Array, List, Map, Set, and Struct.

• Array – This class resembles the array type in the X++ language, but it can hold values of any single type, even objects and records. Objects are accessed in a specific order.
• List – This class contains elements that are accessed sequentially. Unlike an array, the List class provides an addStart method. Like the Set class, the List class provides the getEnumerator and getIterator methods. You can use an iterator to insert and delete items from a List object.
• Map – This class associates a key value with another value.
• Set – This class holds values of any single type. Values aren't stored in the sequence in which they are added. Instead, the Set object stores the value in a manner that optimizes performance for the in method. A Set object ignores any attempt to add a value that the Set object is already storing. Unlike the Array class, the Set class provides the in and remove methods.
• Struct – This class can contain values of more than one type. It's used to group information about a specific entity.

The constructor for every collection class except Struct takes a type parameter that is an element of the Types system enum. The collection instance can store items of that type only. The Types::AnyType enum element is a special case that can't be used to construct a collection object, such as a Set object. The null value can't be stored as an element in a Set object. Additionally, null can't be a key in a Map object. You can iterate through a collection object by using an iterator or enumerator. Here are typical examples that show how you can obtain an iterator.

new MapIterator(myMap)
myMap.getEnumerator()

For Set objects, if any elements are added or removed after an iterator is created, the iterator instance can no longer be used to read from or step through the collection.

For Map objects, as for Set objects, if any elements are removed, the iterator is no longer valid. However, a MapIterator object remains valid even after a call to the Map.insert method, regardless of whether the key is new, or whether the key already exists and only the value is being updated in the Map element. Code that calls Map.insert and depends on the iterator object remaining valid might fail if it's run as .NET Framework CIL.

You can use the collection classes to form more complex classes. For example, you can easily implement a stack by using a list where elements are always added to the beginning of the list. The newest element then occupies the top of the stack.

You can also extend the collection classes. For example, you can extend the List class to create a list of customer records where the operations are type-safe. In this case, the derived collection class will accept only customer records.

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