Which mechanism is a better way to extend Dictionary to deal with missing keys and why?

Question

There is a minor annoyance I find myself with a lot - I have a Dictionary that contains values that may or may not be there.

So norm

Answer 1

When naming an extension method intended to replace an existing method, I tend to add to the method name for specificity rather than shortening it:

GetValueOrDefault(...)

As for the ForgivingDictionary, you can constrain TKey so that it can't be a value type. However, if you must deal with value types in it, you're going to return something for a value type and the best option is to return default(TKey) since you can't return null.

Honestly, I'd go with the extension method.

Edit: GetValueOrDefault(), of course, wouldn't add to the dictionary if it didn't find the key. I would just return a default value if it wasn't found, because that's how it's named. If one wanted it to insert as well, a good name would be GetValueOrInsertDefault().

Answer 2

I'm unable to infer from your question what should be done when a key is not found. I can imagine nothing should be done in that case, but I can also imagine the opposite. Anyway, an elegant alternative for a series of these TryGetValue-statements you describe, is using one of the following extension methods. I have provided two options, depending on whether something should be done or not when the dictionary does not contain the key:

/// <summary> Iterates over all values corresponding to the specified keys, 
///for which the key is found in the dictionary. </summary>
public static IEnumerable<TValue> TryGetValues<TKey, TValue>(this Dictionary<TKey, TValue> dictionary, IEnumerable<TKey> keys)
{
    TValue value;
    foreach (TKey key in keys)
        if (dictionary.TryGetValue(key, out value))
            yield return value;
}

/// <summary> Iterates over all values corresponding to the specified keys, 
///for which the key is found in the dictionary. A function can be specified to handle not finding a key. </summary>
public static IEnumerable<TValue> TryGetValues<TKey, TValue>(this Dictionary<TKey, TValue> dictionary, IEnumerable<TKey> keys, Action<TKey> notFoundHandler)
{
    TValue value;
    foreach (TKey key in keys)
        if (dictionary.TryGetValue(key, out value))
            yield return value;
        else
            notFoundHandler(key);                        
}

Example code on how to use this is:

TKey[] keys = new TKey{...};
foreach(TValue value in dictionary.TryGetValues(keys))
{
    //some action on all values here
}

Answer 3

Or perhaps

public static TValue TryGet<TKey, TValue>(this Dictionary<TKey, TValue> input, 
                                                   TKey key)
{

     return input.ContainsKey(key) ? input[key] : *some default value*;

}