RE: [nhusers] Modify the collections of an entity "collection was modified" exception

[Pete Appleton]

Change
        var toRemove = firstCollection.Where(item => secondCollection.All(x => 
x.Key != item.Key));
to
        var toRemove = firstCollection.Where(item => secondCollection.All(x => 
x.Key != item.Key)).ToArray();

As it stands, your code is lazily iterating toRemove; as soon as something's 
removed from it, you'll get exception you described.  This is not related to 
NHibernate, it's a standard 'feature' of .NET :).

/Pete

-----Original Message-----
From: nhusers@googlegroups.com on behalf of Matteo Migliore
Sent: Thu 7/4/2013 11:58 AM
To: nhusers@googlegroups.com
Subject: [nhusers] Modify the collections of an entity "collection was 
modified" exception
 
Hi,

I obtain the error "Collection was modified; enumeration operation may not 
execute." when I try to save (merge)
a loaded entity.

I use AutoMapper to write the values from the DTO (command) to the loaded 
entity and this method to synchronize
the collection between the entity and the command:

public static ICollection<TFirst> Synchronize<TFirst, TSecond>(
        this ICollection<TFirst> first,
        IEnumerable<TSecond> second,
        Func<TSecond, TFirst> convert = null,
        Func<TFirst, int> firstHash = null,
        Func<TSecond, int> secondHash = null)
{
        if (firstHash == null)
        {
                firstHash = x => x.GetHashCode();
        }

        if (secondHash == null)
        {
                secondHash = x => x.GetHashCode();
        }

        if (convert == null)
        {
                convert = x => Mapper.Map<TFirst>(x);
        }

        var firstCollection = first.ToDictionary(x => firstHash(x), x => x);
        var secondCollection = second.ToDictionary(x => secondHash(x), x => x);

        var toAdd = secondCollection.Where(item => firstCollection.All(x => 
x.Key != item.Key)).Select(x => convert(x.Value));

        foreach (var item in toAdd)
        {
                first.Add(item);
        }

        var toRemove = firstCollection.Where(item => secondCollection.All(x => 
x.Key != item.Key));

        foreach (var item in toRemove)
        {
                first.Remove(item.Value);
        }

        return first;
}

How can I fix the problem?

Thanks,
Matteo

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