public void Keys_ReturnsCollectionOfKeys()
{
var sourceKeys = new[] { "A", "B", "C" };
_target.Add(sourceKeys[0], "B");
_target.Add(sourceKeys[1], "C");
_target.Add(sourceKeys[2], "D");
var keys = _target.Keys();
Assert.All(keys, key => sourceKeys.Contains(key));
Assert.All(sourceKeys, key => keys.Contains(key));
}