score:16

Accepted answer

you can also use the linq enumerable.todictionary, specifying the key and value as null:

var mylist = new list<string> { "first", "second" };
dictionary<string, string> dict = mylist.todictionary(
    item => item,         // key 
    item => (string)null  // value
);

note that you need to cast "null" to your element type, otherwise c# can't infer the type of the second lambda expression.

score:1

just using a loop is simple enough:

foreach (var item in mylist)
   mydictionary.add(item, null);

this assumes, of course, that you have already created the dictionary.


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