A LINQ query can be a recipe rather than a snapshot

First, build a mental model

Where and Select on an IEnumerable commonly build a recipe. Enumeration performs the work. ToList materializes a result at that moment; later source mutations do not add elements to that list.

1. Where filters elements; Select transforms each retained element; GroupBy organizes groups for later work.
2. Deferred execution means declaring a query and executing it can happen at different times.
3. Materializing with ToList captures current result elements, though referenced mutable elements can still be shared.
4. IQueryable providers may translate expression trees to another system such as SQL. An operation supported in memory is not automatically translatable or efficient on a database.

var scores = new List<int> { 10, 30 };
var query = scores.Where(score => score >= 20);
scores.Add(40);
var snapshot = query.ToList();
scores.Add(50);
Console.WriteLine(string.Join(",", snapshot));
Console.WriteLine(string.Join(",", query));

1. Declare

   query describes the filter; it has not created an independent result list.

2. Materialize

   ToList runs after 40 is added, producing 30 and 40.

3. Enumerate again

   The final enumeration sees the source's later 50 as well.