How and When LINQ Queries are Translated and Evaluated?

Question

I have a LINQ to SQL Query as below:

var carIds = from car in _db.Cars
           where car.Color == 'Blue'
           select car.Id;

The above will be translated into the below sql finally:

select Id from Cars 
where Color = 'Blue'

I've read that the phases are:

1. LINQ to Lambda Expression
2. Lambda Expression to Expression Trees
3. Expression Trees to SQL Statements
4. SQL Statements gets executed

So, my questions are when and how does this get translated and executed?

I know that phase 4 happens at runtime when my "carIds" variable get accessed within a foreach loop.

foreach(var carId in carIds) // here?
{
  Console.Writeline(carId) // or here?
}

What about the other phases? when do they occur? at compile time or runtime? at which line (on the definition or after the definition, when accessed or before it gets accessed)?

Answer 1

What you are talking about is deferred execution - essentially, the linq to SQL query will be executed at the time you attempt to enumerate the results (e.g. iterate round it, .ToArray() it etc).

In your example, the statement is executed on the following line:

foreach(var carId in carIds)

See this MSDN article on LINQ and Deferred Execution