Iqueryable vs Ienumerable a comparison.

There is always a doubt in developers mind regarding usage of IQueryable and IEnumerable.

IQueryable extends IEnumerable. IQueryable should be used while working with remote databases. 

Well, here are some facts which can help resolving your doubt and can strengthen your decision-making as which one to use.

IEnumerable has just GetEnumerator() method. Whereas IQueryable extends IEnumerable.

Some important points about Ienumberable<T>

• It's read-only.
• Iterates only in forward direction.
• Doesn't supports adding or removing objects.
• It's a base interface for any generic collection

When to use IEnumberable:

• Working with read-only collection
• Need to read objects in the forward direction.
• Want to iterate collection using for-each.

Some important points about IQueryable<T>

• It implements IEnumerable so the results can be iterated using for-each.
• Best suited for querying against the remote data source.

When to use IQueryable

• your data source is queryable.
• Data needs to be applied 
• The deferred way is needed to load the data.

	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 the database, IEnumerable executes a select query on the server side, load data in-memory on the client side and then filter data. Hence does more work and becomes slow.	While querying data from the database, IQueryable executes a select query on the 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

Table from code project