LINQ Deferred Execution

Syntax

var query = collection.Where(predicate);  // not executed yet

Examples

List<int> numbers = new List<int> { 1, 2, 3 };

var query = numbers.Where(n =>
{
    Console.WriteLine($"Checking {n}");
    return n > 1;
});

Console.WriteLine("Query defined, but not run yet");
var result = query.ToList();  // THIS is when "Checking..." actually prints

List<int> numbers = new List<int> { 1, 2, 3 };
var evens = numbers.Where(n => n % 2 == 0);

numbers.Add(4);
numbers.Add(6);

Console.WriteLine(string.Join(", ", evens));  // 2, 4, 6 - includes the new items!

List<int> numbers = new List<int> { 1, 2, 3 };
List<int> evensSnapshot = numbers.Where(n => n % 2 == 0).ToList();  // runs NOW

numbers.Add(4);

Console.WriteLine(string.Join(", ", evensSnapshot));  // 2 - unaffected by the later Add

Best practices

• Call .ToList() or .ToArray() when you need a stable snapshot of results that will not change if the underlying source is modified later
• Be cautious about enumerating the same deferred query multiple times when the source (or the query itself) is expensive to compute - each enumeration re-runs the whole pipeline
• Remember methods like Count(), Sum(), First(), and ToList() all trigger immediate execution of everything before them in the chain
• Watch for 'multiple enumeration' bugs where a deferred query is enumerated more than once unintentionally (e.g. once in an if check, again in a loop) - materialize it first with .ToList() to avoid redundant work

At a glance

Purpose

Applications on the .NET platform

File extension

.cs

Runs in

.NET runtime

Usually used with

.NET SDK and libraries

Specifications & further reading

• C# language reference

Related C# documentation