What is the difference between deferred and immediate execution?

Deferred execution of LINQ queries means that the query is not executed immediately upon its declaration, but only at the moment of enumerating its results. This is achieved by returning an IEnumerable<T> or IQueryable<T> object, which represents the query description.

Immediate execution, on the other hand, occurs immediately upon query declaration. This is characteristic of LINQ methods that return a scalar value (e.g., Count(), Sum()) or a specific collection type (e.g., ToList(), ToArray()).

// Deferred execution
var query = numbers.Where(n => n > 5); // Query not yet executed

// Immediate execution
int count = numbers.Count(); // Query executed immediately
List<int> list = query.ToList(); // Query executed when calling ToList()

Differences in the table:

Characteristic	Deferred execution	Immediate execution
Moment of execution	When enumerating results	Immediately upon query declaration
Return type	IEnumerable<T>, IQueryable<T>	Scalar value, collection
Source changes	Changes after declaration are considered	Changes after declaration are not considered
Efficiency	Potentially higher with partial selection	Execution regardless of usage