Difference of Dictionary.Add vs Dictionary[key]=value [duplicate]

Question

What is the difference between the Dictionary.Add method and the indexer Dictionary[key] = value?

Answer 1

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.

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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<string, string> dict = new Dictionary<string, string>();
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.

Answer 2

Dictionary.Add throws an exception if the key is already present. [] when used for setting an item doesn't (it does if you try to access it for read).

x.Add(key, value); // will throw if key already exists or key is null
x[key] = value; // will throw only if key is null
var y = x[key]; // will throw if key doesn't exists or key is null

Answer 3

The documentation for Add makes this very clear, I feel:

You can also use the Item property to add new elements by setting the value of a key that does not exist in the Dictionary(Of TKey, TValue); for example, myCollection[myKey] = myValue (in Visual Basic, myCollection(myKey) = myValue). However, if the specified key already exists in the Dictionary(Of TKey, TValue), setting the Item property overwrites the old value. In contrast, the Add method throws an exception if a value with the specified key already exists.

(Note that the Item property corresponds to the indexer.)

Answer 4

The behavior is identical when the key does not exist in the dictionary: the item will be added in both cases.

The behavior differs when the key already exists. dictionary[key] = value will update the value mapped to the key, while dictionary.Add(key, value) will instead throw an ArgumentException.