Difference between IQueryable, ICollection, IList & IDictionary interface

Question

All of these interfaces inherit from IEnumerable, which you should make sure you understand. That interface basically lets you use the class in a foreach statement (in C#).

• ICollection is the most basic of the interfaces you listed. It's an enumerable interface that supports a Count and that's about it.
• IList is everything that ICollection is, but it also supports adding and removing items, retrieving items by index, etc. It's the most commonly-used interface for "lists of objects", which is vague I know.
• IQueryable is an enumerable interface that supports LINQ. You can always create an IQueryable from an IList and use LINQ to Objects, but you also find IQueryable used for deferred execution of SQL statements in LINQ to SQL and LINQ to Entities.
• IDictionary is a different animal in the sense that it is a mapping of unique keys to values. It is also enumerable in that you can enumerate the key/value pairs, but otherwise it serves a different purpose than the others you listed.

The MSDN documentation for each of these is decent, so I would start there in rounding out your understanding.

I noticed several issues in @gunny229's answer above which is why I'm writing this post. I've mentioned those issues in comment area of his post as well. To correct that post I would have had to rewrite almost entire post so I thought of creating my own. I don't intend to cater OP's question in entirety but I'm want to point out the difference between IQueryable and IEnumerable when using LINQ to SQL.

I created following structure in DB (DDL script):

CREATE TABLE [dbo].[Employee]([PersonId] [int] NOT NULL PRIMARY KEY,[Salary] [int] NOT NULL)

Here is the record insertion script (DML script):

INSERT INTO [EfTest].[dbo].[Employee] ([PersonId],[Salary])VALUES(1, 20)
INSERT INTO [EfTest].[dbo].[Employee] ([PersonId],[Salary])VALUES(2, 30)
INSERT INTO [EfTest].[dbo].[Employee] ([PersonId],[Salary])VALUES(3, 40)
INSERT INTO [EfTest].[dbo].[Employee] ([PersonId],[Salary])VALUES(4, 50)
INSERT INTO [EfTest].[dbo].[Employee] ([PersonId],[Salary])VALUES(5, 60)
GO

Now my goal was to simply get top 2 records from Employee table in database. So, I added an ADO.NET Entity Data Model item into my console application pointing to Employee table in my database and started writing LINQ queries.

Code for IQueryable route:

using (var efContext = new EfTestEntities())
{
    IQueryable<int> employees = from e in efContext.Employees  select e.Salary;
    employees = employees.Take(2);

    foreach (var item in employees)
    {
        Console.WriteLine(item);
    }
}

When I started to run this program, I had also started a session of SQL Query profiler on my SQL Server instance and here is the summary of execution:

1. Total number of queries fired: 1
2. Query text: SELECT TOP (2) [c].[Salary] AS [Salary] FROM [dbo].[Employee] AS [c]

It is just that IQueryable is smart enough to apply the Top (2) clause on database server side itself so it brings only 2 out of 5 records over the wire. Any further in-memory filtering is not required at all on client computer side.

Code for IEnumerable route:

using (var efContext = new EfTestEntities())
{
    IEnumerable<int> employees = from e in efContext.Employees  select e.Salary;
    employees = employees.Take(2);

    foreach (var item in employees)
    {
        Console.WriteLine(item);
    }
}

Summary of execution in this case:

1. Total number of queries fired: 1
2. Query text captured in SQL profiler: SELECT [Extent1].[Salary] AS [Salary] FROM [dbo].[Employee] AS [Extent1]

Now the thing is IEnumerable brought all the 5 records present in Salary table and then performed an in-memory filtering on the client computer to get top 2 records. So more data (3 additional records in this case) got transferred over the wire unnecessarily.

IQueryable,IList,IDictionary,ICollection inherits IEnumerable Interface. All the interfaces of type collection will inherits IEnumerable Interface.

Differences Between IEnumerable and IQueryable IEnumerable:-when you are running a query which returns of type IEnumerable. IEnumerable will first Executes the first query and then executes the sub queries written for that query.

Example:-If you want get the top 10 population Records from a Particular country then the query we will use in LinQ is

IEnumerable _Population=from s in India select s.population;//First Query _Population=_Population.take(10);//Second Query

Now if we execute this query First Query will be executed first the IEnumerable will get the Records of all the population from the sql server then it will store the data in the In Process memory and then it executes the top 10 population in the next Query.(Execution is taken place for two times on sql server).

if we execute the same query by using IQueryable .

IQueryable _Population=from s in India select s.population;//First Query _Population=_Population.take(10);//Second Query

in this IQueryable it will execute the two Queries at the same time in the sense it will get the population which is of top 10 in a single Query instead of getting the data and filtering the first 10 again.(One time execution on sql server).

Conclusion for IQueryable and IEnumerable

1. If you are writing the query against the data in the database then use IQueryable.
2. If you are querying against the in process data then use IEnumerable. IEnumerable has a methods GetEnumerator(),Current() and MoveNext().