Question

I want a method to update certain entries of an IEnumerable. I found that doing a foreach over the entries and updating the values failed as in the background I was cloning the collection. This was because my IEnumerable was backed by some LINQ->SQL queries.

By changing the method to take a List I have changed this behavior, Lists are always mutable and hence the method changes the actual objects in the list. Rather than demand a List is passed is there a Mutable interface I can use?

    // Will not behave as consistently for all IEnumerables
    public void UpdateYesterday (IEnumerable<Job> jobs) {
          foreach (var job in jobs.Where(x => x.date == Yesterday)) {
             job.done = true;
          }
    }

    ...

    public class Job {  
         ...           
         public DateTime Date { get; set; }
    }

Answer 1

You’re not changing the list at all here – the collection itself could be immutable and this code would still “work”. You’re changing the data within the items in the collection.

Imagine a row of houses – that’s pretty immutable (creating or destroying a house is tricky) but you can still change the contents of those houses.

So, what you really need to know is whether the elements of the collection are going to be cloned or not… whether you’ll get a collection of “new” jobs each time you perform the query, or whether it’ll use the existing objects.