Replace entity in context with a different instance of the same entity

Question

I have an entity which is not connected to my dbcontext. I want to change that. However there is already another instance of the same entity attached to dbcontext. If I just add my new entity, I get an error, that an entity with the same primary key is already attached.
I tried multiple different variants of removing the old entity from dbcontext without any success. How can I replace the old instance with the new one?
Note: I don't want to copy the values, I want to attach this very instance of my new entity to dbcontext.

var entity = new MyEntity { Id = 1 };
var logicalDuplicate = dbcontext.Set<MyEntity >().Local
    .FirstOrDefault(e => e.Equals(entity));
if (logicalDuplicate != null)
{
    // remove logicalDuplicate from dbcontext
}
dbcontext.MyEntity.Attach(entity);

For clarification: I have overridden Equals to check for Id instead of reference.

Answer 1

Try this:

if (logicalDuplicate != null)
{
    dbcontext.Entry(logicalDuplicate).State = EntityState.Detached;
    dbcontext.MyEntity.Attach(entity);
    dbcontext.Entry(entity).State = EntityState.Modified;
}
else
{
    dbcontext.MyEntity.Add(entity);
}

How to get related entries

I investigated that and want to share with my results. I used reflection as short way to get entity properties names. But it's possible to get it without reflection as mentioned @Florian Haider. You can use answer and this.

// Found loaded related entries that can be detached later.
private HashSet<DbEntityEntry> relatedEntries;

private DbContext context;

private List<string> GetPropertiesNames(object classObject)
{
    // TODO Use cache for that.
    // From question https://stackoverflow.com/questions/5851274/how-to-get-all-names-of-properties-in-an-entity
    var properties = classObject.GetType().GetProperties(BindingFlags.DeclaredOnly |
                                                              BindingFlags.Public |
                                                              BindingFlags.Instance);
    return properties.Select(t => t.Name).ToList();
}

private void GetRelatedEntriesStart(DbEntityEntry startEntry)
{
    relatedEntries = new HashSet<DbEntityEntry>();

    // To not process start entry twice.
    relatedEntries.Add(startEntry);
    GetRelatedEntries(startEntry);
}

private void GetRelatedEntries(DbEntityEntry entry)
{
    IEnumerable<string> propertyNames = GetPropertiesNames(entry.Entity);
    foreach (string propertyName in propertyNames)
    {
        DbMemberEntry dbMemberEntry = entry.Member(propertyName);
        DbReferenceEntry dbReferenceEntry = dbMemberEntry as DbReferenceEntry;
        if (dbReferenceEntry != null)
        {
            if (!dbReferenceEntry.IsLoaded)
            {
                continue;
            }

            DbEntityEntry refEntry = context.Entry(dbReferenceEntry.CurrentValue);
            CheckReferenceEntry(refEntry);
        }
        else
        {
            DbCollectionEntry dbCollectionEntry = dbMemberEntry as DbCollectionEntry;
            if (dbCollectionEntry != null && dbCollectionEntry.IsLoaded)
            {
                foreach (object entity in (ICollection)dbCollectionEntry.CurrentValue)
                {
                    DbEntityEntry refEntry = context.Entry(entity);
                    CheckReferenceEntry(refEntry);
                }
            }
        }
    }
}

private void CheckReferenceEntry(DbEntityEntry refEntry)
{
    // Add refEntry.State check here for your need.
    if (!relatedEntries.Contains(refEntry))
    {
        relatedEntries.Add(refEntry);
        GetRelatedEntries(refEntry);
    }
}