LINQ provides two different behaviors of Query Execution –
- Deferred Execution
- Immediate Execution
|Deferred/Lazy Operators||Immediate/Greedy Operators|
|Query is not executed at the point of its declaration.||Query is executed at the point of its declaration.|
|Projection Operator – Select, SelectMany
Restriction Operator – WherePaging Operator – Take, Skip
|Aggregate Functions – Count, Average, Min, Max, Sum
Element Operators – First, Last, SingleToList, ToArray, ToDictionary
By default, LINQ uses deferred execution.
When we write a LINQ query, it doesn’t execute by itself. It executes, when we access the query results.In other words, execution of the query is deferred until the query variable is iterated over in a foreach loop.
Benefits of Deferred Execution –
- It avoids unnecessary query execution and hence improves performance.
- Query construction and Query execution are decoupled, so we can create the LINQ query in several steps.
- A deferred execution query is reevaluated when you re-enumerate – hence we always get the latest data.
In the above example, we first create a list of 3 Employees. We then construct the LINQ Query. Once done, we create a new instance of the Employee object.
Now had the query been executed when the LINQ Query was constructed, the result would have been only ‘Samir’. However that is not the right result in this case.
The Output of the program is Samir, Mark
This is because the execution of the query was deferred until the query variable was iterated using a foreach loop.
Immediate execution is the reverse of deferred execution. It forces the LINQ query to execute and gets the result immediately.
To force immediate execution of a query, that return a singleton value, we can use the aggregate operators or element operators.
To force immediate execution of a query that does not produce a singleton value, we can call the ToList method, the ToDictionary method, or the ToArray method on a query or query variable.
The Output of the above program is only ‘Samir’
This is because the execution of the query was done immediately, when it was constructed – since we used the ‘ToList’ operator.
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