Dictionaries and null keys

C# does not let you initialize a dictionary entry by simply assigning a value to a non-existant key in the dictionary. So instead of

  mydictionary[key] = value;    

...where "key" does not yet exist in the dictionary, it is necessary to do something like this...

  if (mydictionary.ContainsKey(key))

  {

    mydictionary[key] += value;

    //or

    mydictionary[key] = value;

  }

  else

  {

    mydictionary.Add(key, value);

  }