Why use ICollection and not IEnumerable or List T on many-many one-many relationships

Question

I see this a lot in tutorials  with navigation properties as ICollection lt T gt    Is this a mandatory requirement for Entity Framework  Can I use IEnumerable   What s the main purpose of using ICollection instead of IEnumerable or even List lt T gt

User · Accepted Answer

Usually what you choose will depend on which methods you need access to. In general - IEnumerable<> (MSDN: http://msdn.microsoft.com/en-us/library/system.collections.ienumerable.aspx) for a list of objects that only needs to be iterated through, ICollection<> (MSDN: http://msdn.microsoft.com/en-us/library/92t2ye13.aspx) for a list of objects that needs to be iterated through and modified, List<> for a list of objects that needs to be iterated through, modified, sorted, etc (See here for a full list: http://msdn.microsoft.com/en-us/library/6sh2ey19.aspx).

From a more specific standpoint, lazy loading comes in to play with choosing the type. By default, navigation properties in Entity Framework come with change tracking and are proxies. In order for the dynamic proxy to be created as a navigation property, the virtual type must implement ICollection.

A navigation property that represents the "many" end of a relationship must return a type that implements ICollection, where T is the type of the object at the other end of the relationship. -Requirements for Creating POCO Proxies^MSDN

More information on Defining and Managing Relationships^MSDN

User · Answer

There are some basics difference between ICollection and IEnumerable   IEnumerable - contains only GetEnumerator method to get Enumerator and allows looping ICollection contains additional methods  Add  Remove  Contains  Count  CopyTo ICollection is inherited from IEnumerable With ICollection you can modify the collection by using the methods like add remove  You don t have the liberty to do the same with IEnumerable    Simple Program   using System  using System Collections  using System Collections Generic   namespace StackDemo       class Program                static void Main string   args                        List lt Person gt  persons   new List lt Person gt                 persons Add new Person  John  30                persons Add new Person  Jack   27                 ICollection lt Person gt  personCollection   persons              IEnumerable lt Person gt  personEnumeration   persons                  IEnumeration                IEnumration Contains only GetEnumerator method to get Enumerator and make a looping             foreach  Person p in personEnumeration                                                                  Console WriteLine  Name  0   Age  1    p Name  p Age                                 ICollection                ICollection Add Remove Contains Count CopyTo                ICollection is inherited from IEnumerable             personCollection Add new Person  Tim   10                 foreach  Person p in personCollection                                Console WriteLine  Name  0   Age  1    p Name  p Age                                     Console ReadLine                          class Person               public string Name   get  set            public int Age   get  set            public Person string name  int age                        this Name   name              this Age   age

User · Answer

Navigation properties are typically defined as virtual so that they can take advantage of certain Entity Framework functionality such as lazy loading    If a navigation property can hold multiple entities  as in many-to-many or one-to-many relationships   its type must be a list in which entries can be added  deleted  and updated  such as ICollection   https   www asp net mvc overview getting-started getting-started-with-ef-using-mvc creating-an-entity-framework-data-model-for-an-asp-net-mvc-application

User · Answer

Lets try thinking outside of the box with by logic and understand clearly these three interfaces in your question   When the class of some instance implements the System Collection IEnumerable interface then  in simple words  we can say that this instance is both enumerable and iterable  which means that this instance allows somehow in a single loop to go get pass traverse iterate over through all the items and elements that this instance contains   This means that this is also possible to enumerate all the items and elements that this instance contains   Every class that implements the System Collection IEnumerable interface also implements the GetEnumerator method that takes no arguments and returns an System Collections IEnumerator instance   Instances of System Collections IEnumerator interface behaves very similar to C   iterators   When the class of some instance implements the System Collection ICollection interface then  in simple words  we can say that this instance is some collection of things   The generic version of this interface  i e  System Collection Generic ICollection  is more informative because this generic interface explicitly states what is the type of the things in the collection   This is all reasonable  rational  logical and makes sense that System Collections ICollection interface inherits from System Collections IEnumerable interface  because theoretically every collection is also both enumerable and iterable and this is theoretically possible to go over all the items and elements in every collection   System Collections ICollection interface represents a finite dynamic collection that are changeable  which means that exist items can be removed from the collection and new items can be added to the same collection   This explains why System Collections ICollection interface has the  Add  and  Remove  methods   Because that instances of System Collections ICollection interface are finite collections then the word  finite  implies that every collection of this interface always has a finite number of items and elements in it   The property Count of System Collections ICollection interface supposes to return this number   System Collections IEnumerable interface does not have these methods and properties that System Collections ICollection interface has  because it does not make any sense that System Collections IEnumerable will have these methods and properties that System Collections ICollection interface has   The logic also says that every instance that is both enumerable and iterable is not necessarily  a collection and not necessarily changeable   When I say changeable  I mean that don t immediately think that you can add or remove something from something that is both enumerable and iterable   If I just created some finite sequence of prime numbers  for example  this finite sequence of prime numbers is indeed an instance of System Collections IEnumerable interface  because now I can go over all the prime numbers in this finite sequence in a single loop and do whatever I want to do with each of them  like printing each of them to the console window or screen  but this finite sequence of prime numbers is not an instance of System Collections ICollection interface  because this is not making sense to add composite numbers to this finite sequence of prime numbers   Also you want in the next iteration to get the next closest larger prime number to the current prime number in the current iteration  if so you also don t want to remove exist prime numbers from this finite sequence of prime numbers   Also you probably want to use  code and write  yield return  in the GetEnumerator method of the System Collections IEnumerable interface to produce the prime numbers and not allocating anything on the memory heap and then task the Garbage Collector  GC  to both deallocate and free this memory from the heap  because this is obviously both waste of operating system memory and decreases performance   Dynamic memory allocation and deallocation on the heap should be done when invoking the methods and properties of System Collections ICollection interface  but not when invoking the methods and properties of System Collections IEnumerable interface  although System Collections IEnumerable interface has only 1 method and 0 properties    According to what others said in this Stack Overflow webpage  System Collections IList interface simply represents an orderable collection and this explains why the methods of System Collections IList interface work with indexes in contrast to these of System Collections ICollection interface   In short System Collections ICollection interface does not imply that an instance of it is orderable  but System Collections IList interface does imply that   Theoretically ordered set is special case of unordered set   This also makes sense and explains why System Collections IList interface inherits System Collections ICollection interface

User · Answer

Responding to your question about List lt T gt    List lt T gt  is a class  specifying an interface allows more flexibility of implementation   A better question is  why not IList lt T gt     To answer that question  consider what IList lt T gt  adds to ICollection lt T gt   integer indexing  which means the items have some arbitrary order  and can be retrieved by reference to that order   This is probably not meaningful in most cases  since items probably need to be ordered differently in different contexts

User · Answer

What I have done in the past is declare my inner class collections using IList lt Class gt   ICollection lt Class gt or IEnumerable lt Class gt   if static list  depending on whether or not I will have to do any number of the following in a method in my repository  enumerate  sort order or modify  When I just need to enumerate  and maybe sort  over objects then I create a temp List lt Class gt to work with the collection within an IEnumerable method  I think this practice would only be effective if the collection is relatively small  but it may be good practice in general  idk  Please correct me if there is evidence as to why this would not good practice

User · Answer

I remember it this way    IEnumerable has one method GetEnumerator   which allows one to read through the values in a collection but not write to it   Most of the complexity of using the enumerator is taken care of for us by the for each statement in C    IEnumerable has one property   Current  which returns the current element  ICollection implements IEnumerable and adds few additional properties the most use of which is Count   The generic version of ICollection implements the Add   and Remove   methods  IList implements both IEnumerable and ICollection  and add the integer indexing access to items  which is not usually required  as ordering is done in database

User · Answer

The basic idea of using ICollection is a provide an interface to readonly-access to some finite amount of data  In fact you have a ICollection Count property  IEnumerable is more suitable for some chain of the data where you read till some logical point  some condition esplicitly specified by consumer or till the end of the enumeration

User · Answer

ICollection lt T gt  is used because the IEnumerable lt T gt  interface provides no way of adding items  removing items  or otherwise modifying the collection

[c#] Why use ICollection and not IEnumerable or List<T> on many-many/one-many relationships?

Examples related to c#

Examples related to entity-framework

Examples related to code-first

Examples related to icollection