How is an IQueryable expression converted into an SQL query for database execution?

The IQueryable expression in C# represents a deferred data query that is not executed immediately, but only when necessary (for example, when iterating over the results). When you use LINQ queries to a database through an ORM, such as Entity Framework, IQueryable builds an expression tree that describes the query.

When methods that trigger query execution are called (for example, ToList(), First(), Count()), the ORM analyzes this expression tree and converts it into the corresponding SQL query. This SQL query is then sent to the database for execution, and the results are returned and materialized into objects.

Example:

var query = context.Users.Where(u => u.Age > 18); // IQueryable
var list = query.ToList(); // Here, conversion to SQL and execution occur

Thus, the key mechanism is the construction and analysis of the Expression Tree, which allows the ORM to dynamically generate SQL based on the LINQ query.