LINQ : Query Execution — Deferred vs Immediate

A LINQ query can be written to fetch
the results immediately or at a latter point in time. To keep it
simple, LINQ queries are often executed when the query is iterated
over (via a foreach loop) — as opposed to when the query variable is
created. This is considered a “deferred” execution; however,
queries can also be immediately executed when needed. For example,
immediate execution is useful when the query results need to be
cached or re-used.

From a syntax perspective, a query will
execute immediately when the query returns a singleton (Min, Max,
Count, Average, etc) or when it ends with .ToList, .ToArray,
.ToDictionary, etc.

Deferred Example:

public static void DelayedExecString()

{

//Create some data

string[] bikes = new string[] { “Yamaha”, “Honda”, “Ducati”, “Suzuki” };

//Build LINQ query

var simpleQuery =

from bike in bikes

select bike

;

//Query is NOW executed in a deferred manner

foreach (var type in simpleQuery)

{

Console.WriteLine(“Bike type is {0}”, type);

}

Console.ReadLine();

}

Immediate Example:

public static void ImmediateExecString()

{