Question

IEnumerable is a query that is lazily evaluated. But apparently my understanding is a bit flawed. I’d expect the following to work:

        // e.Result is JSON from a server
        JObject data = JObject.Parse(e.Result);
        JsonSerializer serializer = new JsonSerializer();

        // LINQ query to transform the JSON into Story objects
        var stories = data["nodes"].Select(
                   obj => obj["node"]).Select(
                        storyData => storyOfJson(serializer, storyData));

        // set a value on each story returned by the query
        foreach (Story story in stories)
        {
            story.Vid = vid;
        }

        // run through the query again, making sure the value was actually set
        foreach (Story story in stories)
        {
            // FAILS - story.VID is 0
            Debug.Assert(story.Vid == vid);
        }

What am I misunderstanding here? How can I alter the results of what this query returns?

Answer 1

Each time you enumerate the stories variable, the Select call runs again, creating a new set of Story objects.

Therefore, each foreach loop runs on a different set of Story instances.

You need to force the LINQ calls to evaluate exactly once by calling .ToArray().
Looping through the resulting array will not re-evaluate the LINQ calls (since it’s a normal array), so you’ll share the same set of Story instances.