what the difference between add and [] in the dictionary operation

Question

Dictionary dict;

what's the diff between

dict.add(key, value) and dict[key] = value

Answer 1

dict[key] = value will add the value if the key doesn't exist, otherwise it will overwrite the value with that (existing) key.

Example:

var dict = new Dictionary<int, string>();
dict.Add(42, "foo");
Console.WriteLine(dict[42]);
dict[42] = "bar";  // overwrite
Console.WriteLine(dict[42]);
dict[1] = "hello";  // new
Console.WriteLine(dict[1]);
dict.Add(42, "testing123"); // exception, already exists!

Answer 2

As Ahmad noted, dictionary[key] = value; will add the value if the key doesn't exist, or overwrite if it does.

On the other hand, dictionary.Add(key, value); will throw an exception if key exists.