mysvcServiceImages = db.ServiceImages
.Where(e => e.MySvcID == mysvcMySvcID).ToList();

List<ServiceImage> list = new List<ServiceImage>();

foreach (ServiceImage sim in mysvcServiceImages)
{
    ServiceImage siEx = serviceImages
        .Where(e => e.MySvcID == sim.MySvcID &&
        e.ImageName == sim.ImageName).FirstOrDefault();
    if (siEx == null)
    {
        list.Add(sim);
    }
}

foreach (ServiceImage sim in list)
{
    mysvcServiceImages.Remove(db.ServiceImages.First(z => z.MySvcID == sim.MySvcID && 
        z.ImageName == sim.ImageName));
}

foreach (ServiceImage sim in serviceImages)
{
    ServiceImage siEx = mysvcServiceImages
        .Where(e => e.MySvcID == sim.MySvcID &&
        e.ImageName == sim.ImageName).FirstOrDefault();
    if (siEx == null)
    {
        db.Entry(sim).State = EntityState.Added;
    }
}
db.SaveChanges();

if (detachedChild == null)
                            {
                                parent.ServiceImages.Remove(child);
                                db.ServiceImages.Remove(child);
                            }

db.DeleteObject(child);

var parent = db.Services.Find(mysvc.MySvcID); 
if (parent != null && parent.ServiceImages!=null && mysvc.ServiceImages != null)
{
    foreach (var child in mysvc.ServiceImages)
    {
        var attachedChild =
            parent.ServiceImages.FirstOrDefault(z => z.MySvcID == child.MySvcID &&
            z.ImageName == child.ImageName);
        if (attachedChild == null)
        {
            ServiceImage simg = child.Clone();
            parent.ServiceImages.Add(simg);
        }
    }
}
db.SaveChanges();

x.Message = "Нарушено ограничение количества элементов. 
Роль \"ServiceImage_mysvc_Target\" связи \"MySvc.Models.ServiceImage_mysvc\" имеет количество элементов 1 или 0..1."