Why doesn't Dictionary<TKey, TValue> have an IEnumerable<KeyValuePair<TKey, TValue>> ctor?

Answer 1

Totally unofficial guess:

If a constructor allowed an IEnumerable<KeyValuePair<TKey, TValue>> you would have been able to supply multiple items with the same key and what would be the expected behavior of that?

E.g. you could have done something like this:

var kvps = new[]
{
    new KeyValuePair<int, string>(1, "Foo"),
    new KeyValuePair<int, string>(1, "Bar"),
    new KeyValuePair<int, string>(1, "Baz")        
}
var dic = new Dictionary<int, string>(kvps);

You could argue that this should simply throw an exception to be consistent with the behavior of the Add method, but I would guess that the design team thought it would be a greater source of confusion than actually useful...

Answer 2

  public static Dictionary<T, U> ToDictionary(
      this IEnumerable<KeyValuePair<T, U>> source)
  {
    return source.ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
  }