For a DAL POCO with int IDs link a child to a parent

LinkChildToParent.cs

    public static class Extensions
    {
        public static void Link<TParent, TChild>(this TParent parent, TChild child, Expression<Func<TChild, int>> link)
            where TParent : HasId
        {
            if (parent == null || child == null) return;
            if (!(link.Body is MemberExpression member)) return;
            var propertyInfo = (PropertyInfo)member.Member;
            propertyInfo.SetValue(child, parent.Id);
        }
    }
    
    // usage
    parent.Link(parent.Child, a => a.ParentId);
    // this sets the value of parent.Child.ParentId to parent.Id