How do I implement IEnumerable T

Question

I know how to implement the non generic IEnumerable  like this   using System  using System Collections   namespace ConsoleApplication33       class Program               static void Main string   args                        MyObjects myObjects   new MyObjects                myObjects 0    new MyObject     Foo    Hello   Bar   1                myObjects 1    new MyObject     Foo    World   Bar   2                 foreach  MyObject x in myObjects                                Console WriteLine x Foo                   Console WriteLine x Bar                              Console ReadLine                         class MyObject               public string Foo   get  set            public int Bar   get  set               class MyObjects   IEnumerable               ArrayList mylist   new ArrayList             public MyObject this int index                        get   return  MyObject mylist index                 set   mylist Insert index  value                        IEnumerator IEnumerable GetEnumerator                         return mylist GetEnumerator                        However I also notice that IEnumerable has a generic version  IEnumerable lt T gt   but I can t figure out how to implement it   If I add using System Collections Generic  to my using directives  and then change   class MyObjects   IEnumerable   to   class MyObjects   IEnumerable lt MyObject gt    And then right click on IEnumerable lt MyObject gt  and select Implement Interface   gt  Implement Interface  Visual Studio helpfully adds the following block of code   IEnumerator lt MyObject gt  IEnumerable lt MyObject gt  GetEnumerator         throw new NotImplementedException        Returning the non generic IEnumerable object from the GetEnumerator    method doesn t work this time  so what do I put here  The CLI now ignores the non generic implementation and heads straight for the generic version when it tries to enumerate through my array during the foreach loop

User · Answer

Why do you do it manually  yield return automates the entire process of handling iterators   I also wrote about it on my blog  including a look at the compiler generated code    If you really  want to do it yourself  you have to return a generic enumerator too  You won t be able to use an ArrayList any more since that s non-generic  Change it to a List lt MyObject gt  instead  That of course assumes that you only have objects of type MyObject  or derived types  in your collection

User · Answer

make mylist into a List lt MyObject gt   is one option

User · Answer

Note that the IEnumerable lt T gt  allready implemented by the System Collections so another approach is to derive your MyObjects class from System Collections as a base class  documentation       System Collections  Provides the base class for a generic collection    We can later make our own implemenation to override the virtual System Collections methods to provide custom behavior  only for ClearItems  InsertItem  RemoveItem  and SetItem along with Equals  GetHashCode  and ToString from Object   Unlike the List lt T gt  which is not designed to be easily extensible   Example   public class FooCollection   System Collections lt Foo gt                  protected override void InsertItem int index  Foo newItem                base InsertItem index  newItem                Console Write  An item was successfully inserted to MyCollection              public static void Main         FooCollection fooCollection   new FooCollection        fooCollection Add new Foo       OUTPUT  An item was successfully inserted to FooCollection      Please note that driving from collection recommended only in case when custom collection behavior is needed  which is rarely happens  see usage

User · Answer

If you choose to use a generic collection  such as List lt MyObject gt  instead of ArrayList  you ll find that the List lt MyObject gt  will provide both generic and non-generic enumerators that you can use   using System Collections   class MyObjects   IEnumerable lt MyObject gt        List lt MyObject gt  mylist   new List lt MyObject gt          public MyObject this int index                    get   return mylist index               set   mylist Insert index  value                   public IEnumerator lt MyObject gt  GetEnumerator                 return mylist GetEnumerator               IEnumerator IEnumerable GetEnumerator                 return this GetEnumerator

User · Answer

If you work with generics  use List instead of ArrayList  The List has exactly the GetEnumerator method you need   List lt MyObject gt  myList   new List lt MyObject gt

User · Answer

You probably do not want an explicit implementation of IEnumerable lt T gt   which is what you ve shown    The usual pattern is to use IEnumerable lt T gt  s GetEnumerator in the explicit implementation of IEnumerable   class FooCollection   IEnumerable lt Foo gt   IEnumerable       SomeCollection lt Foo gt  foos          Explicit for IEnumerable because weakly typed collections are Bad     System Collections IEnumerator IEnumerable GetEnumerator                    uses the strongly typed IEnumerable lt T gt  implementation         return this GetEnumerator                  Normal implementation for IEnumerable lt T gt      IEnumerator lt Foo gt  GetEnumerator                 foreach  Foo foo in this foos                        yield return foo                nb  if SomeCollection is not strongly-typed use a cast                 yield return  Foo foo                 Or better yet  switch to an internal collection which is                strongly-typed  Such as List lt T gt  or T    your choice                        or  as pointed out  return this foos GetEnumerator

[c#] How do I implement IEnumerable<T>

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