Temporary Entities with Entity Framework

Question

I have an EntityObject named Pathway which relates directly to data in the pathway table. My database also stores rules for Pathway customisations. What I would like to do is create a Pathway object in my code that is the resultant of the Pathway + PathwayCustomisations. The resultant should never find it's way back to the database, it is simply a temporary projection used in the code.

public static Pathway ApplyCustomisation(Pathway p, ICollection<PathwayCustomisation> c)
{
    Pathway resultant = new Pathway();
    if (!p.PathwayModule.IsLoaded) p.PathwayModule.Load();
    foreach (PathwayModule m in p.PathwayModule)
    {
        resultant.PathwayModule.Add(m);
    }
    foreach (PathwayCustomisation i in c)
    {
        switch (i.Command)
        {
            case "ADD":
                resultant.PathwayModule.Add(i.PathwayModule);
                break;
            case "DELETE":
                resultant.PathwayModule.Remove(i.PathwayModule);
                break;
        }
    }
    return resultant;
}

This method chokes at the first hurdle because I am adding PathwayModule entities to a second Pathway when they can only belong to one in the model/database:

CoursePlanner.Test.PathwayTest.ApplyCustomisation:
System.InvalidOperationException : Collection was modified; enumeration operation may not execute.

Is there a way to work with Entity projections easily? Am I approaching the problem properly?

Edit:

I still get the exception when with just the first part of the method:

public static Pathway ApplyCustomisation(Pathway p, ICollection<PathwayCustomisation> c)
{
    Pathway resultant = new Pathway();
    if (!p.PathwayModule.IsLoaded) p.PathwayModule.Load();
    foreach (PathwayModule m in p.PathwayModule)
    {
        resultant.PathwayModule.Add(m);
    }
    return resultant;
}

The above enumeration is not modifying the same collection that is being enumerated, it is simply adding the items to a second collection. This code gives the same exception.

.NET3.5, C#, VS Express 2008

Thanks,

Daniel

Answer 1

Your exception is caused by the fact that you are modifing a collection while enumerating it.

foreach (Item item in collection)
{
    collection.Add(something); // Not allowed.
    collection.Remove(something); // Not allowed, too.
}

Working with entities like normal objects is no problem - you must only avoid that the entities are attached to an object context (directly or indirectly through related entities) and they will not be persisted to the database.