Add -> Adds an item to the dictionary if item already exists in the dictionary an exception will be thrown.
Indexer or Dictionary[Key] => Add Or Update. If the key doesn't exist in the dictionary, a new item will be added. If the key exists then the value will be updated with the new value.
dictionary.add will add a new item to the dictionary, dictionary[key]=value will set a value to an existing entry in the dictionary against a key. If the key is not present then it (indexer) will add the item in the dictionary.
Dictionary dict = new Dictionary();
dict.Add("Test", "Value1");
dict["OtherKey"] = "Value2"; //Adds a new element in dictionary
Console.Write(dict["OtherKey"]);
dict["OtherKey"] = "New Value"; // Modify the value of existing element to new value
Console.Write(dict["OtherKey"]);
In the above example, in first place dict["OtherKey"] = "Value2"; will add a new value in the dictionary because it doesn't exist, and in second place it will modify the value to New Value.
