Question

All I want is a dictionary which tells me which key it couldn’t find, rather than just saying The given key was not present in the dictionary.

I briefly considered doing a subclass with override new this[TKey key], but felt it was a bit hacky, so I’ve gone with implementing the IDictionary interface, and passing everything through directly to an inner Dictionary, with the only additional logic being in the indexer:

public TValue this[TKey key]
{
    get
    {
        ThrowIfKeyNotFound(key);
        return _dic[key];
    }
    set
    {
        ThrowIfKeyNotFound(key);
        _dic[key] = value;
    }
}
private void ThrowIfKeyNotFound(TKey key)
{
    if(!_dic.ContainsKey(key))
        throw new ArgumentOutOfRangeException("Can't find key [" + key + "] in dictionary");
}

Is this the right/only way to go? Would newing over the this[] really be that bad?

Answer 1

Sounds like a good fit for an extension method:

public static class SomeUtilClass {
    public static TValue VerboseGetValue<TKey, TValue>(
        this IDictionary<TKey, TValue> data, TKey key)
    {
        TValue result;
        if (!data.TryGetValue(key, out result)) {
            throw new KeyNotFoundException(
                "Key not found: " + Convert.ToString(key));
        }
        return result;
    }
}

This will then work on all your existing dictionaries whenever you call VerboseGetValue, for example:

    var data = new Dictionary<int, string> { { 123, "abc" } };
    Console.WriteLine(data.VerboseGetValue(123));
    Console.WriteLine(data.VerboseGetValue(456));