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()

{

       //Create some data

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

//Build LINQ query

var simpleQuery = (

    from bike in bikes

    select bike).ToList();  //Query is NOW executed immediately

foreach (var type in simpleQuery)

{

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

}

Console.ReadLine();

}