Best lookup data structure to store only the keys (Dictionary without value)

What is the best data structure in .Net with high performance lookup, like a binary tree implementation, but to store only the keys (string keys) ?

We need only to check if a certain key is present in the collection. Like:

Dictonary<string, object> myKeys;
myKeys.Add("key1", null);
myKeys.Add("key2", null);
// Dozens or hundreds keys

Assert.IsTrue(myKeys.Contains("key1"));