Dictionary returning a default value if the key does not exist

Question

I find myself using the current pattern quite often in my code nowadays

var dictionary = new Dictionary<type, IList<othertype>>();
// Add stuff to dictionary

var somethingElse = dictionary.ContainsKey(key) ? dictionary[key] : new List<othertype>();
// Do work with the somethingelse variable

Or sometimes

var dictionary = new Dictionary<type, IList<othertype>>();
// Add stuff to dictionary

IList<othertype> somethingElse;
if(!dictionary.TryGetValue(key, out somethingElse) {
    somethingElse = new List<othertype>();
}

Both of these ways feel quite roundabout. What I really would like is something like

dictionary.GetValueOrDefault(key)

Now, I could write an extension method for the dictionary class that does this for me, but I figured that I might be missing something that already exists. SO, is there a way to do this in a way that is more "easy on the eyes" without writing an extension method to dictionary?

Answer 1

TryGetValue will already assign the default value for the type to the dictionary, so you can just use:

dictionary.TryGetValue(key, out value);

and just ignore the return value. However, that really will just return default(TValue), not some custom default value (nor, more usefully, the result of executing a delegate). There's nothing more powerful built into the framework. I would suggest two extension methods:

public static TValue GetValueOrDefault<TKey, TValue>
    (this IDictionary<TKey, TValue> dictionary, 
     TKey key,
     TValue defaultValue)
{
    TValue value;
    return dictionary.TryGetValue(key, out value) ? value : defaultValue;
}

public static TValue GetValueOrDefault<TKey, TValue>
    (this IDictionary<TKey, TValue> dictionary,
     TKey key,
     Func<TValue> defaultValueProvider)
{
    TValue value;
    return dictionary.TryGetValue(key, out value) ? value
         : defaultValueProvider();
}

(You may want to put argument checking in, of course :)

Answer 2

No, nothing like that exists. The extension method is the way to go, and your name for it (GetValueOrDefault) is a pretty good choice.