IEnumerable Vs IQueryable

Many developers get confused between IEnumerable and IQueryable. When it comes to writing code, both look very similar. However, there are many differences between them which need to be taken care of while writing code. Both have some intended usability scenarios for which they are made.

Listed below are the differences between them based on their properties:

	IEnumerable	IQueryable
Namespace	System.Collections Namespace	System.Linq Namespace
Derives from	No base interface	Derives from IEnumerable
Deferred Execution	Supported	Supported
Lazy Loading	Not Supported	Supported
How does it work	While querying data from database, IEnumerable executes select query on server side, load data in-memory on client side and then filter data. Hence does more work and becomes slow.	While querying data from database, IQueryable executes select query on server side with all filters. Hence does less work and becomes fast.
Suitable for	LINQ to Object and LINQ to XML queries	LINQ to SQL queries
Custom Query	Doesn’t support	Supports using CreateQuery and Execute methods
Extension method parameter	Extension methods supported in IEnumerable takes functional objects.	Extension methods supported in IEnumerable takes expression objects, i.e., expression tree.
When to use	When querying data from in-memory collections like List, Array, etc.	When querying data from out-memory (like remote database, service) collections.
Best Uses	In-memory traversal	Paging

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