Attribute lost with yield

Question

There is some code that I'm trying to convert from IList to IEnumerable:

[Something(123)]
public IEnumerable<Foo> GetAllFoos()
{
  SetupSomething();

  DataReader dr = RunSomething();
  while (dr.Read())
  {
    yield return Factory.Create(dr);
  }
}

The problem is, SetupSomething() comes from the base class and uses:

Attribute.GetCustomAttribute(
    new StackTrace().GetFrame(1).GetMethod(), typeof(Something))

yield ends up creating MoveNext(), MoveNext() calls SetupSomething(), and MoveNext() does not have the [Something(123)] attribute.

I can't change the base class, so it appears I am forced to stay with IList or implement IEnumerable manually (and add the attribute to MoveNext()).

Is there any other way to make yield work in this scenario?

Answer 1

You can't use iterators (yield) if you require the stack frame functionality. As you've discovered, this rewrites your method into a custom class that implements IEnumerable<T>.

However, you can easily just rework this to:

[Something(123)]
public IEnumerable<Foo> GetAllFoos()
{
  SetupSomething();

  List<Foo> results = new List<Foo>();
  DataReader dr = RunSomething();
  while (dr.Read())
  {
    results.Add(Factory.Create(dr));
  }
  return results;
}

You lose the deferred execution of the iterator, but it will work properly.

Answer 2

You can wrap the method in another method that does all required preprocessing:

[Something(123)]
public IEnumerable<Foo> GetAllFoos()
{
    SetupSomething();
    return GetAllFoosInternal();
}

private IEnumerable<Foo> GetAllFoosInternal()
{
    DataReader dr = RunSomething();
    while (dr.Read())
    {
        yield return Factory.Create(dr);
    }
}

Answer 3

Could you split your method up, like this?

[Something(123)]
public void GetAllFoosHelper()
{
  SetupSomething(); 
}

public IEnumerable<Foo> GetAllFoos() 
{ 
  GetAllFoosHelper();

  DataReader dr = RunSomething(); 
  while (dr.Read()) 
  { 
    yield return Factory.Create(dr); 
  } 
} 

Answer 4

From your description, it sounds like the problem is that SetupSomething is only looking at the immediate caller on the stack trace. If it looked a little further up (caller's caller) it would find your GetAllFocus call and the desired attribute.

I don't recall off the top of my head, but if yield is creating a MoveNext() implementation only because your class doesn't already implement it, perhaps you can implement your own MoveNext, put the attribute on it, and yield will find and use your MoveNext()? Just a wild guess.

Answer 5

I'm probably missing something, but I can't make sense of using an attribute here. You might as well have written it like this:

public IEnumerable<Foo> GetAllFoos()
{
  SetupSomething(123);
  // etc..
}

A whole heckofalot faster too. And safer, you're dead in the water when the JIT compiler inlines SetupSomething().