Shorter syntax for casting from a List X to a List Y

Question

I know its possible to cast a list of items from one type to another  given that your object has a public static explicit operator method to do the casting  one at a time as follows   List lt Y gt  ListOfY   new List lt Y gt      foreach X x in ListOfX      ListOfY Add  Y x     But is it not possible to cast the entire list at one time  For example   ListOfY    List lt Y gt  ListOfX

User · Answer

The direct cast var ListOfY    List lt Y gt  ListOfX is not possible because it would require co contravariance of the List lt T gt  type  and that just can t be guaranteed in every case  Please read on to see the solutions to this casting problem   While it seems normal to be able to write code like this   List lt Animal gt  animals    List lt Animal gt   mammalList    because we can guarantee that every mammal will be an animal  this is obviously a mistake   List lt Mammal gt  mammals    List lt Mammal gt   animalList    since not every animal is a mammal   However  using C  3 and above  you can use   IEnumerable lt Animal gt  animals   mammalList Cast lt Animal gt       that eases the casting a little  This is syntactically equivalent to your one-by-one adding code  as it uses an explicit cast to cast each Mammal in the list to an Animal  and will fail if the cast is not successfull   If  you like more control over the casting   conversion process  you could use the ConvertAll method of the List lt T gt  class  which can use a supplied expression to convert the items  It has the added benifit that it returns a List  instead of IEnumerable  so no  ToList   is necessary   List lt object gt  o   new List lt object gt     o Add  one    o Add  two    o Add 3    IEnumerable lt string gt  s1   o Cast lt string gt       fails on the 3rd item List lt string gt  s2   o ConvertAll x   gt  x ToString       succeeds

User · Answer

You can use List lt Y gt  ConvertAll lt T gt   Converter from Y to T

User · Answer

In case when X derives from Y you can also use ToList lt T gt  method instead of Cast lt T gt  listOfX ToList lt Y gt

User · Answer

dynamic data   List lt x gt  val    List lt y gt  val2     IEnumerable data  Cast lt y gt    ToList

User · Answer

To add to Sweko s point   The reason why the cast   var listOfX   new List lt X gt     ListOf lt Y gt  ys    List lt Y gt  listOfX     Compile error  Cannot implicitly cast X to Y   is not possible is because the List lt T gt  is invariant in the Type T and thus it doesn t matter whether X derives from Y  - this is because List lt T gt  is defined as   public class List lt T gt    IList lt T gt   ICollection lt T gt   IEnumerable lt T gt         Other interfaces    Note that in this declaration  type T here has no additional variance modifiers   However  if mutable collections are not required in your design  an upcast on many of the immutable collections  is possible  e g  provided that Giraffe derives from Animal   IEnumerable lt Animal gt  animals   giraffes    This is because IEnumerable lt T gt  supports covariance in T - this makes sense given that IEnumerable implies that the collection cannot be changed  since it has no support for methods to Add or Remove elements from the collection  Note the out keyword in the declaration of IEnumerable lt T gt    public interface IEnumerable lt out T gt    IEnumerable    Here s further explanation for the reason why mutable collections like List cannot support covariance  whereas immutable iterators and collections can    Casting with  Cast lt T gt     As others have mentioned   Cast lt T gt    can be applied to a collection to project a new collection of elements casted to T  however doing so will throw an InvalidCastException if the cast on one or more elements is not possible  which would be the same behaviour as doing the explicit cast in the OP s foreach loop    Filtering and Casting with OfType lt T gt     If the input list contains elements of different  incompatable types  the potential InvalidCastException can be avoided by using  OfType lt T gt    instead of  Cast lt T gt       OfType lt  gt    checks to see whether an element can be converted to the target type  before attempting the conversion  and filters out incompatable types    foreach  Also note that if the OP had written this instead   note the explicit Y y in the foreach   List lt Y gt  ListOfY   new List lt Y gt      foreach Y y in ListOfX        ListOfY Add y       that the casting will also be attempted  However  if no cast is possible  an InvalidCastException will result   Examples  For example  given the simple  C 6  class hierarchy   public abstract class Animal       public string Name   get         protected Animal string name    Name   name       public class Elephant    Animal       public Elephant string name    base name       public class Zebra   Animal       public Zebra string name     base name          When working with a collection of mixed types   var mixedAnimals   new Animal         new Zebra  Zed        new Elephant  Ellie       foreach Animal animal in mixedAnimals            Fails for Zed -  InvalidCastException - cannot cast from Zebra to Elephant       castedAnimals Add  Elephant animal      var castedAnimals   mixedAnimals Cast lt Elephant gt           Also fails for Zed with  InvalidCastException      ToList      Whereas   var castedAnimals   mixedAnimals OfType lt Elephant gt         ToList       Ellie   filters out only the Elephants - i e  Zebras are eliminated   Re  Implicit cast operators  Without dynamic  user defined conversion operators are only used at compile-time   so even if a conversion operator between say Zebra and Elephant was made available  the above run time behaviour of the approaches to conversion wouldn t change   If we add a conversion operator to convert a Zebra to an Elephant   public class Zebra   Animal       public Zebra string name    base name          public static implicit operator Elephant Zebra z                return new Elephant z Name             Instead  given the above conversion operator  the compiler will be able to change the type of the below array from Animal   to Elephant    given that the Zebras can be now converted to a homogeneous collection of Elephants   var compilerInferredAnimals   new          new Zebra  Zed        new Elephant  Ellie        Using Implicit Conversion Operators at run time   As mentioned by Eric  the conversion operator can however be accessed at run time by resorting to dynamic   var mixedAnimals   new Animal      i e  Polymorphic collection       new Zebra  Zed        new Elephant  Ellie       foreach  dynamic animal in mixedAnimals        castedAnimals Add animal        Returns Zed  Ellie

User · Answer

If X can really be cast to Y you should be able to use  List lt Y gt  listOfY   listOfX Cast lt Y gt    ToList      Some things to be aware of  H T to commenters     You must include using System Linq  to get this extension method This casts each item in the list - not the list itself  A new List lt Y gt  will  be created by the call to ToList    This method does not support custom conversion operators    see http   stackoverflow com questions 14523530 why-does-the-linq-cast-helper-not-work-with-the-implicit-cast-operator   This method does not work for an object that has a explicit operator method  framework 4 0

User · Answer

This is not quite the answer to this question  but it may be useful for some  as  SWeko said  thanks to covariance and contravariance  List lt X gt  can not be cast in List lt Y gt   but List lt X gt  can be cast into IEnumerable lt Y gt   and even with implicit cast   Example   List lt Y gt  ListOfY   new List lt Y gt     List lt X gt  ListOfX    List lt X gt  ListOfY     Compile error   but   List lt Y gt  ListOfY   new List lt Y gt     IEnumerable lt X gt  EnumerableOfX   ListOfY      No issue   The big advantage is that it does not create a new list in memory

[c#] Shorter syntax for casting from a List<X> to a List<Y>?

Examples related to c#

Examples related to list

Examples related to casting

Examples related to ienumerable