title: Why is Lookup immutable in C#?
body: <p>Unlike <code>Dictionary</code>, you cannot construct a <code>Lookup</code> by adding elements one by one. Do you happen to know the reason?</p>
  
  <p><code>Lookup</code> is just like <code>multimap</code> in C++; why can't we modify it in C#? If we really can't, how can we construct a <code>multimap</code> data structure in C#?</p>