invalid key in Dictionary

Question

Why do dictionaries not just return null when an invalid key is used to index into the collection?

Answer 1

Because generic dictionaries could contain instances of a value type, and null is not valid for a value type. For example:

var dict = new Dictionary<string, DateTime>();
DateTime date = dict["foo"]; // What should happen here?  date cannot be null!

You should instead use the TryGetValue method of dictionary:

var dict = new Dictionary<string, DateTime>();
DateTime date;

if (dict.TryGetValue("foo", out date)) {
    // Key was present; date is set to the value in the dictionary.
} else {
    // Key was not present; date is set to its default value.
}

Also, a dictionary that stores reference types will still store null values. And your code might consider "value is null" to be different from "key does not exist."

Answer 2

Microsoft decided that =)
Do an inline check to avoid that.

object myvalue = dict.ContainsKey(mykey) ? dict[mykey] : null;

Answer 3

Try using

Dictionary.TryGetValue Method

or maybe

Dictionary.ContainsKey Method

Answer 4

Practical reason: Because a Dictionary could store a null value. You wouldn't be able to differentiate this case with an exception, in your scenario.