A lots of time Confusion between IQueryable and IEnumerable interface because, they are look like same and when we start writing a code often, we will choose a wrong approach between them. From behind this reason was I am unable to know perfectly about these two. Here, we will try to explain a differences.
IQueryable exists in System.Linq Namespace. It only forward only over a collection. This is a better query data from out-memory (like remote database, service) collections. When we access a data from a database IQueryable execute select query on server side with all filters. It support Lazy Loading that was better for paging.
IQueryable<T> is the interface that allows LINQ-to-SQL to work. If we want to refine another query it will executes, if necessary.
That code will execute SQL to only select Students which grade is <5. And
we have also filter all other students which grade above from 5.
Enumerable exists in System.Collections Namespace. It only forward only over a collection. IEnumerable is best to query data from in-memory collections. While we access data from database, IEnumerable interface execute select query on server side and load data in-memory on client side and then filtering data. IEnumerable is suitable for LINQ to Object queries. IEnumerable supports deferred execution. IEnumerable does not support custom query and also be a lazy loading.
IEnumerable<T> is the interface that allows LINQ-to-Object to work. It means that all objects matching the original query will have to be loaded into memory from the database.
This is quite an important difference, and working on IQueryable<T> can in many cases save you from returning too many rows from the database. Another prime example is doing paging: If you use Take andSkip on IQueryable, you will only get the number of rows requested; doing that on an IEnumerable<T> will cause all of your rows to be loaded in memory.
There are many differences as below:
1. IEnumerable is exists under the System.Collections namespace.
2. IEnumerable is suitable for querying data from in-memory collections.
3. While querying data from the database, IEnumerable executes "select query" on the server-side, loads data in-memory on the client-side and then filters the data.
4. IEnumerable is also be better work for LINQ to Object and LINQ to XML queries.