title: Is there a class like Dictionary<> in C#, but for just keys, no values?
body: <p>I guess another way to phrase this would be "Is there a class like <code>List&lt;&gt;</code> in C#, but optimized for checking whether a particular value is present?" I'm sure for a small set of values <code>List&lt;&gt;.Contains</code> would probably be fine, but what if I have a set of thousands or millions of values and wanted to find out whether a certain value was in it?</p>
  
  <p>I've implemented this kind of thing in the past by creating a <code>Dictionary&lt;object, int&gt;</code> and setting the value to 0 for every key, but this feels really clunky. And now there's Stack Overflow, where my stupid question can be transformed into education for thousands (dozens, even). So here it is!</p>
  
  <p>I'm not even sure what such a class would be called, other than maybe Set, so obviously searches on the topic have been... challenging :) </p>