如何为LINQ to SQL编写Upsert？

using (var context = new DataContext())
{
    var newEntity = new Entity { Property1 = "Value1", Property2 = "Value2" };
    context.Entities.InsertOnSubmit(newEntity);
    context.SubmitChanges();
}

using (var context = new DataContext())
{
    var existingEntity = context.Entities.Single(e => e.Id == id);
    existingEntity.Property1 = "NewValue1";
    existingEntity.Property2 = "NewValue2";
    context.SubmitChanges();
}

using (var context = new DataContext())
{
    var entity = context.Entities.SingleOrDefault(e => e.Id == id);

    if (entity == null)
    {
        entity = new Entity { Id = id, Property1 = "Value1", Property2 = "Value2" };
        context.Entities.InsertOnSubmit(entity);
    }
    else
    {
        entity.Property1 = "NewValue1";
        entity.Property2 = "NewValue2";
    }

    context.SubmitChanges();
}