How does LINQ defer execution when in a using stat

Question

Imagine I have the following:

private IEnumerable MyFunc(parameter a)
{
   using(MyDataContext dc = new MyDataContext)
   {
      return dc.tablename.Select(row => row.parameter == a);
   }
}

private void UsingFunc()
{
   var result = MyFunc(new a());

   foreach(var row in result)
   {
      //Do something
   }
}

According to the documentation the linq execution will defer till I actual enumerate the result, which occurs in the line at the foreach. However the using statement should force the object to be collected reliably at the end of the call to MyFunct().

What actually happens, when will the disposer run and/or the result run?

Only thing I can think of is the deferred execution is computed at compile time, so the actual call is moved by the compiler to the first line of the foreach, causing the using to perform correctly, but not run until the foreach line? Is there a guru out there who can help?

EDIT: NOTE: This code does work, I just don't understand how.

I did some reading and I realised in my code that I had called the ToList() extension method which of course enumerates the result. The ticked answer's behaviour is perfectly correct for the actual question answered.

Sorry for any confusion.

Answer 1

I would expect that to simply not work; the Select is deferred, so no data has been consumed at this point. However, since you have disposed the data-context (before leaving MyFunc), it will never be able to get data. A better option is to pass the data-context into the method, so that the consumer can choose the lifetime. Also, I would recommend returning IQueryable<T> so that the consumer can "compose" the result (i.e. add OrderBy / Skip / Take / Where etc, and have it impact the final query):

// this could also be an instance method on the data-context
internal static IQueryable<SomeType> MyFunc(
    this MyDataContext dc, parameter a)
{
   return dc.tablename.Where(row => row.parameter == a);
}

private void UsingFunc()
{
    using(MyDataContext dc = new MyDataContext()) {
       var result = dc.MyFunc(new a());

       foreach(var row in result)
       {
           //Do something
       }
    }
}

---

Update: if you (comments) don't want to defer execution (i.e. you don't want the caller dealing with the data-context), then you need to evaluate the results. You can do this by calling .ToList() or .ToArray() on the result to buffer the values.

private IEnumerable<SomeType> MyFunc(parameter a)
{
   using(MyDataContext dc = new MyDataContext)
   {
      // or ToList() etc
      return dc.tablename.Where(row => row.parameter == a).ToArray();
   }
}

If you want to keep it deferred in this case, then you need to use an "iterator block":

private IEnumerable<SomeType> MyFunc(parameter a)
{
   using(MyDataContext dc = new MyDataContext)
   {
      foreach(SomeType row in dc
          .tablename.Where(row => row.parameter == a))
      {
        yield return row;
      }
   }
}

This is now deferred without passing the data-context around.

Answer 2

I just posted another deferred-execution solution to this problem here, including this sample code:

IQueryable<MyType> MyFunc(string myValue)
{
    return from dc in new MyDataContext().Use()
           from row in dc.MyTable
           where row.MyField == myValue
           select row;
}

void UsingFunc()
{
    var result = MyFunc("MyValue").OrderBy(row => row.SortOrder);
    foreach(var row in result)
    {
        //Do something
    }
}

The Use() extension method essentially acts like a deferred using block.