How can I make this Dictionary TryGetValue code more readable?

Question

I'd like to test if an id was not yet known or, if it is known, if the associated value has changed. I'm currently using code similar to this, but it is hard to understand for those not familiar with the pattern. Can you think of a way to make it more readable while keeping it short in LOC?

string id;
string actual;
string stored;

if (!someDictionary.TryGetValue (id, out stored) || stored != actual) {
    // id not known yet or associated value changed.
}

Answer 1

You can write an extension method with a good name:

public static class Utility
{
    public static bool ValueChangedOrUnknown(this Dictionary<string, string> dictionary, string id, string actual)
    {
        string stored = null;
        return (!dictionary.TryGetValue(id, out actual) || stored != actual);
    }
}

so later you can use

string id;
string actual;

if (someDictionary.ValueChangedOrUnknown(id, actual) {
    // id not known yet or associated value changed.
}

Answer 2

So I would most probably break it up and give it meaningful names. This is more to read, but you don't need much to say in comments:

bool isKnown = someDictionary.TryGetValue (id, out stored);
// can only change when it is known
bool valueChanged = isKnown && stored != actual;

// quite self-explanatory, isn't it?
if (!isKnown || valueChanged) 
{

}