What is the difference between IQueryable and IEnumerable?

Question

What is the difference between IQueryable and IEnumerable?

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See also What\'s the difference between IQueryable and I

Answer 1

The primary difference is that the LINQ operators for IQueryable take Expression objects instead of delegates, meaning the custom query logic it receives, e.g., a predicate or value selector, is in the form of an expression tree instead of a delegate to a method.

• IEnumerable is great for working with sequences that are iterated in-memory, but
• IQueryable allows for out-of memory things like a remote data source, such as a database or web service.

Query execution:

• Where the execution of a query is going to be performed "in process", typically all that's required is the code (as code) to execute each part of the query.

• Where the execution will be performed out-of-process, the logic of the query has to be represented in data such that the LINQ provider can convert it into the appropriate form for the out-of-memory execution - whether that's an LDAP query, SQL or whatever.

More in:

• LINQ : IEnumerable and IQueryable
• C# 3.0 and LINQ.
• "Returning IEnumerable vs IQueryable"
• Reactive Programming for .NET and C# Developers - An Introduction To IEnumerable, IQueryable, IObservable, and IQbservable
• 2018: "THE MOST FUNNY INTERFACE OF THE YEAR … IQUERYABLE" from Bart De Smet.