How does the Dictionary collection notify about the existence of an already added key?

In C#, the Dictionary<TKey, TValue> collection throws an ArgumentException when attempting to add an element with a key that already exists.

For example:

var dict = new Dictionary<int, string>();
dict.Add(1, "one");
dict.Add(1, "another one"); // An exception will be thrown here

To check for the existence of a key before adding, you can use the ContainsKey method:

if (!dict.ContainsKey(1)) {
    dict.Add(1, "one");
} else {
    Console.WriteLine("Key already exists");
}

Thus, Dictionary does not notify directly but signals an error through an exception when attempting to add a duplicate key.