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System.Collections.Generic.KeyNotFoundException

Accessing an item in a dictionary of which the key does not exist will result in a KeyNotFoundException.

Example

Dictionary<string, Person> persons = new Dictionary<string, Person>();
string id = "1";
string personName = persons[id].Name;
int personAge = persons[id].Age;

This will result in an exception because key "1" does not exist in the dictionary.

Solution A

A solution could be to first verify if the key exists before accessing it:

if (persons.ContainsKey(id))
{
    string personName = persons[id].Name;
    int personAge = persons[id].Age;
}

The above code will work without throwing an exception. However, a minor problem with above is that the dictionary needs to be accessed three times: once to check if the key exists and the second and third time to actually get the values. Solution B is therefore preferred.

Solution B

Person person;
if (persons.TryGetValue(id, out person))
{
    string personName = person.Name;
    int personAge = person.Age;
}

This will only access the dictionary once. When the key is present, the value will be accessible through the out variable and the method will return true.

Note

As this is object-oriented programming, when the dictionary has a class object as value, the reference to that value is retrieved. In other words, if you change a property like person.Name = changedName;, this will change the original item in the dictionary. If the dictionary contains an integer type as value, and you execute if(persons.TryGetValue(id, out age)) and later set age = 30;, this will only change the local variable, not the value that is present in the dictionary.

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