Question

Assume we have a method like this:

public IEnumerable<T> FirstMethod() {     var entities = from t in context.Products                    where {some conditions}                    select t;      foreach( var entity in entities )     {         entity.SomeProperty = {SomeValue};         yield return entity;        } } 

where context is a DataContext that is generated by Linq to SQL designer.

Does ‘FirstMethod’ load the data into memory from database (because of the foreach loop) or will it still defer-load it until another foreach loop that doesn’t have ‘yield return’ is found in another method like the following?

public void SecondMethod() {     foreach( var item in FirstMethod() )     {         {Do Something}     } } 

Answer 1

The latter (deferred); FirstMethod is an iterator block (because of yield return); this means that you have a chain of iterators. Nothing is read until the final caller starts iterating the data; then each record is read in turn during the final caller’s foreach (between which the connection/command is open).

The using that surrounds foreach (under the bonnet) ensures that the connection is closed if the foreach is abandoned half-way-through.

If you want to load the data earlier, use .ToList() or .ToArray() to buffer the data locally – but note that this breaks ‘composition’ – i.e. the caller can no longer add extra Where etc clauses (which they can if it returns a raw IQueryable<T>).

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Re your question:

public IEnumerable<T> FirstMethod() {     var entities = from t in context.Products                    where {some conditions}                    select t;      foreach( var entity in entities.AsEnumerable() )     {         entity.SomeProperty = {SomeValue};         yield return entity;        } } 

The AsEnumerable is the key here; it ends the composable IQueryable<T> chain, and uses LINQ-to-Objects for the rest.