Deferred Execution

Introduction

There are subtleties regarding the timing of LINQ query execution. There are essentially two ways of executing a LINQ query:

• Deferred execution: The LINQ query only executes when we try to address an item from a resulting collection.

• Immediate execution: The LINQ query executes immediately after construction.

Let’s explore these in detail.

Deferred execution

There are three steps to creating and running a LINQ query:

1. Creating or attaining a data source
2. Creating a query
3. Executing the query

Deferred execution means that our query isn’t executed the moment it’s created:

// Step 1. Creating or attaining a data source
var countryList = new List<string>()
{
    "Germany", "Thailand", "France", "Spain",
    "Italy", "United States", "Romania", "Russia",
    "Greece", "Argentina", "Canada", "Mexico",
    "Ireland", "Albania", "Slovenia", "Kazakhstan"
};

// Step 2. Creating a query
var countriesThatStartWithC = from country in countryList
                                where country.StartsWith("C")
                                select country;

// But the query hasn't run yet!

Rather, the query runs when we want to access an element inside the resulting collection:

// The query is run here and the result is stored in the variable
Console.WriteLine(countriesThatStartWithC[0]); // Canada

To prove that the query doesn’t run until we access an element in the resulting collection, we can try to alter the data source after we construct the query: