Collection Interface In .NET

Introduction

.NET framework provides interfaces that implements by collections in language to provide functionality of iterating over objects in collection, adding and removing object from collection to randomly access object from collection.

As different interfaces provide different set of functionality most of the developers has problem when to use which interface to achieve functionality. The following post provides information about interfaces implemented by collection.

Interfaces

The following diagram is for relation between the interfaces.


Note:

1. Class diagram are not having all the methods but contains important method that belongs to each collection interface.
   
   
2. Collection interface is available in both generic and non-generic form, so in diagram obj type is object in nongeneric form and obj type is T(template type) in generic form.

	Functionality Provided	Read	Count	Add & Remove	Index Based Read	Index Based Add & Remove
IEnumerable	Provide Read only Collection. Allow to read each object of collection in forward only mode.	Y	N	N	N	N
ICollection	Allow to modify collection. Allow to get size of collection. Allow to Add and Remove object in/from collection.	Y Inherited)	Y	Y	N	N
IReadOnlyCollection	Allow to read collection. Allow to get size of collection.	Y (Inherited)	Y	N	N	N
IList	Allows to access collection by Index. Allow to Add and Remove object in/from collection by index.	Y (Inherited)	Y (Inherited)	Y (Inherited)	Y	Y
IReadOnlyList	Allow to read collection by Index.	Y (Inherited)	Y (Inherited)	N	Y	N

Note:
In above diagram (Inherited), the columns indicate that the features are inherited from parent and to find out from which parent one must look in the interface collection diagram.

So from above table three main interfaces functionality concluded in following way:

IEnumerable – interface provide minimum functionality which is Enumration.

ICollection – interface provide medium functionality which is getting size, adding, removing and clearing collection i.e. modification of collection. As it inherited from IEnumerable so includes functionality of IEnumerable.

IList – interface provide full functionality which is index base accessing of collection element, index base adding, index base removing from collection. As it inherited from ICollection it includes functionality of Enumerable and ICollection.

The following are some important things to know:

1. IEnumerable interface under the hood make use of IEnumerator for providing reaonly and forward mode read.
   
   
2. IReadOnly*** and IEnumerable are used for providing readonly collection. But difference is that IEnumerable allows collection to read in forward only mode where IReadOnly*** provide feature of Collection /List but only in readonly mode i.e. without modification feature like add & remove.
   
   
3. IReadOnly is part of collection interface from framework 4.5.

Above table list down the features provided by each interface when collection gets converted to interface type or class implement interface to provide feature of collection.

Conclusion

It’s very important for developers to understand these interfaces because the rule says its always good to depend on interface rather than on the concrete type.