platform-compat/docs/DE0006.md

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# DE0006: Non-generic collections shouldn't be used

## Motivation

When .NET was created, generic data types didn't exist, which is why the
collection types in the [`System.Collections`][collections] namespace are untyped. However, since then,
generic data types were introduced and thus a new set of collections
were made available in the [`System.Collections.Generic`][generic] and
[`System.Collections.ObjectModel`][objectmodel] namespaces.

## Recommendation

For new code, you shouldn't use non-generic collections:

* **Error prone**: since non-generic collections are untyped, it requires frequent
  casting between `object` and the actual type you're expecting. Since the compiler
  can't check that your types are consistent, it's easier to put the wrong type in
  the wrong collection.

* **Less performant**: generic collections have the advantage that value types
  don't have to be boxed as object. For instance, a `List<int>` stores its data
  in an `int[]`. That's far better than storing the data in `object[]` as that
  requires boxing.

The following table shows how the non-generic collection types can be
replaced by their generic counterparts from the [`System.Collections.Generic`][generic] or
[`System.Collections.ObjectModel`][objectmodel] namespaces:

| Type                                                                 | Replacement                                                                                   |
|----------------------------------------------------------------------|-----------------------------------------------------------------------------------------------|
| [`ArrayList`][arraylist]                                             | [`List<T>`][list]                                                                             |
| [`CaseInsensitiveComparer`][caseinsensitivecomparer]                 | [`StringComparer.OrdinalIgnoreCase`][ordinalignorecase]                                       |
| [`CaseInsensitiveHashCodeProvider`][caseinsensitivehashcodeprovider] | [`StringComparer.OrdinalIgnoreCase`][ordinalignorecase]                                       |
| [`CollectionBase`][collectionbase]                                   | [`Collection<T>`][collection-1]                                                               |
| [`Comparer`][comparer]                                               | [`Comparer<T>`][comparer-1]                                                                   |
| [`DictionaryBase`][dictionarybase]                                   | [`Dictionary<TKey, TValue>`][dictionary] or [`KeyedCollection<TKey, TItem>`][keyedcollection] |
| [`DictionaryEntry`][dictionaryentry]                                 | [`KeyValuePair<TKey, TValue>`][keyvaluepair]                                                  |
| [`Hashtable`][hashtable]                                             | [`Dictionary<TKey, TValue>`][dictionary]                                                      |
| [`Queue`][queue]                                                     | [`Queue<T>`][queue-1]                                                                         |
| [`ReadOnlyCollectionBase`][readonlycollectionbase]                   | [`ReadOnlyCollection<T>`][readonlycollection]                                                 |
| [`SortedList`][sortedlist]                                           | [`SortedList<TKey, TValue>`][sortedlist-2]                                                    |
| [`Stack`][stack]                                                     | [`Stack<T>`][stack-1]                                                                         |

[arraylist]: https://docs.microsoft.com/dotnet/api/system.collections.arraylist
[caseinsensitivecomparer]: https://docs.microsoft.com/dotnet/api/system.collections.caseinsensitivecomparer
[caseinsensitivehashcodeprovider]: https://docs.microsoft.com/dotnet/api/system.collections.caseinsensitivehashcodeprovider
[collection-1]: https://docs.microsoft.com/dotnet/api/system.collections.objectmodel.collection-1
[collectionbase]: https://docs.microsoft.com/dotnet/api/system.collections.collectionbase
[collections]: https://docs.microsoft.com/dotnet/api/system.collections
[comparer]: https://docs.microsoft.com/dotnet/api/system.collections.comparer
[comparer-1]: https://docs.microsoft.com/dotnet/api/system.collections.generic.comparer-1
[dictionary]: https://docs.microsoft.com/dotnet/api/system.collections.generic.dictionary-2
[dictionarybase]: https://docs.microsoft.com/dotnet/api/system.collections.dictionarybase
[dictionaryentry]: https://docs.microsoft.com/dotnet/api/system.collections.dictionaryentry
[generic]: https://docs.microsoft.com/dotnet/api/system.collections.generic
[hashtable]: https://docs.microsoft.com/dotnet/api/system.collections.hashtable
[keyedcollection]: https://docs.microsoft.com/dotnet/api/system.collections.objectmodel.keyedcollection-2
[keyvaluepair]: https://docs.microsoft.com/dotnet/api/system.collections.generic.keyvaluepair-2
[list]: https://docs.microsoft.com/dotnet/api/system.collections.generic.list-1
[objectmodel]: https://docs.microsoft.com/dotnet/api/system.collections.objectmodel
[ordinalignorecase]: https://docs.microsoft.com/dotnet/api/system.stringcomparer.ordinalignorecase
[queue]: https://docs.microsoft.com/dotnet/api/system.collections.queue
[queue-1]: https://docs.microsoft.com/dotnet/api/system.collections.generic.queue-1
[readonlycollection]: https://docs.microsoft.com/dotnet/api/system.collections.objectmodel.readonlycollection-1
[readonlycollectionbase]: https://docs.microsoft.com/dotnet/api/system.collections.readonlycollectionbase
[stack]: https://docs.microsoft.com/dotnet/api/system.collections.stack
[stack-1]: https://docs.microsoft.com/dotnet/api/system.collections.generic.stack-1
[sortedlist]: https://docs.microsoft.com/dotnet/api/system.collections.sortedlist
[sortedlist-2]: https://docs.microsoft.com/dotnet/api/system.collections.generic.sortedlist-2