Question

If I were to use IEnumerable instead of var in the code example below, will the SQL be generated only during the execution of the foreach statement? Or will it execute as an when the Linq statements are evaluated?

var query = db.Customers.Where (c => c.Age > 18);
query = query.Where (c => c.State == "CO");
var result = query.Select (c => c.Name);

foreach (string name in result)   // Only now is the query executed!
   Console.WriteLine (name);

Another example:

IEnumerable<Order> query = db.Orders.Where(o => o.Amount > 1000);
int orderCount = query.Count();

Would it be better to use var (or IQueryable) as it would be executed a select count(*)… when .Count() is executed or would it be exactly same with the IEnumerable code shown above?

Answer 1

In general, the SQL is generated when GetEnumerator is called on the IQueryable.

In your example, there is a subtle difference that you may want to consider. Let’s take your original example:

var query = db.Customers.Where (c => c.Age > 18); 
query = query.Where (c => c.State == "CO"); 
var result = query.Select (c => c.Name); 

In this case if you change your first query to IEnumerable query = …, then the second line would use the IEnumerable version of the Where extension (LINQ to Objects) rather than the IQueryable one (LINQ to SQL/EF). As a result, when we start iterating, the first where clause is passed to the database, but the second where clause is performed on the client side (because it has already been converted to an IEnumerable).

Another subtle item you want to be aware of is the following type of code:

var query = db.OrderBy(c => c.State);
query = query.Customers.Where(c => c.Age > 18); // Fails: Widening

In this case, since your original query returns IOrderedQueryable rather than IQueryable. If you try to then assign query to the result of the .Where operation, you’re trying to widen the scope of the return type and the compiler will refuse to perform that widening. As a result, you have to explicitly specify the baseline type rather than using var:

IQueryable<Customer> query = db.OrderBy(c => c.State);  // Is narrowing rather than widening.
query = query.Customers.Where(c => c.Age > 18);