IEnumerable vs IQueryable

You might have seen IEnumerable, IEnumerable<T> and IQueryable, IQueryable<T> being used in lot of LINQ queries. Today we are going to discuss these items in details.

• IEnumerable - Exposes an enumerator, which supports a simple iteration over a non-generic collection. (what it means in code is iterating a collection using foreach loop)
• Extensively used in .NET non-generic collections.

• IEnumerable<T> - Exposes an enumerator, which supports a simple iteration over a collection of a specified type. "T" represents the type of data to enumerate. 
• Derived from IEnumerable interface.
• Extensively used in .NET generic collections.
• Since interfaces can't be extend using extension methods, the static class Enumerable is used for extending it by adding the Extension Methods. Example: Select, Any, Join, Count etc.

• Enumerable - Provides a set of static methods for querying objects that implements System.Collections.Generic.IEnumerable<T>

• IQueryable - Provide functionality to evaluate queries against a specific data source wherein the type of data is not specified.
• Derived from IEnumerable interface.
• Third party data providers must implement IQueryable or IQueryable<T> interface, in order to support LINQ.

• IQuerable<T> - Provide functionality to evaluate queries against a specific data source wherein the type of data is know. "T" represents the type of data in the data source.  
• Derived from IEnumerable, IEnumerable<T>, and IQueryable interfaces.
• Since interfaces can't be extended using extension methods, the static class Queryable is used for extending it by adding the Extension Methods. Example: Select, Any, Join, Count etc.

• Queryable - Provides a set of static methods for querying objects that implements System.Linq.IQueryable<T>

Similarity between IEnumerable and IQuerable

• Both can move forward only over a collection, they can’t move backward and between the items.

Difference between IEnumerable and IQuerable

• Namespace - IEnumerable is in System.Collections, where as IQuerable is in System.Linq.
• Derived from - No base interface for IEnumerable where as IQueryable is derived from IEnumerable.
• How does it work - While querying data from database, IEnumerable executes select query on the server, load data in-memory on client side and then filters data, where as IQuerable executes query on server side with all filters. Since IEnumerable does more work, it is slow compare to IQuerable.
• Suitable for - IEneumerable is suitable for LINQ to Object and LINQ to XML queries, where as IQuerable is suitable for LINQ to SQL queries.
• When to use - 
• Use IEnumerable while querying data in-memory collections, example:List, Array etc. Use IQuerable data from out-memory (like remote databse or services) collections. 
• Use IQuerable when you want to apply filter before the query is being executed. 
• Use IQuerable, when lazy loading is needed (you get what you need, not the entire list of records).
• Extension method parameter - IEnumerable uses Func, where as IQuerable uses Expression Tree.
• Custom query - IEnumerable doesn't support where as IQueryable supports using CreateQuery and Execute methods.
• Deferred Execution - Supported in both.
• Lazy loading - IEnemerable doesn't support lazy loading hence it is not suitable for paging like scenarios where as IQuerable supports lazy loading hence suitable for paging like scenarios.
• Best uses - IEnumerable is best used in in-memory traversal. IQueryable is best used in paging  like scenarios. 

Example: Lets say we have a need to get the first 10 employees from the database and the database contains 1000 employees.

• If we use IEnumerable to get the first 10 employees, we will be end up loading all the 1000 employees and then selecting first 10.

           IEnumerable<Employee> employee = dbContext.Employees.Take(10).ToList();

•  If we use IQuerable, we get only first 10 emplyees from the query which saves a lot of resource.

           IQuerable<Employee> employee = dbContext.Employees.Take(10).ToList();