Nullable parameter type <T> for a generic method?

I'm using the pivot method presented at: http://www.extensionmethod.net/Details.aspx?ID=147

My question is: How can i use a nullable type as the second generic key?

public static Dictionary<TFirstKey, Dictionary<TSecondKey, TValue>> Pivot<TSource, TFirstKey, TSecondKey, TValue>(this IEnumerable<TSou开发者_运维问答rce> source, Func<TSource, TFirstKey> firstKeySelector, Func<TSource, TSecondKey> secondKeySelector, Func<IEnumerable<TSource>, TValue> aggregate) {
        var retVal = new Dictionary<TFirstKey, Dictionary<TSecondKey, TValue>>();

        var l = source.ToLookup(firstKeySelector);
        //l.Dump("Lookup first key");
        foreach (var item in l) {
            var dict = new Dictionary<TSecondKey, TValue>();
            retVal.Add(item.Key, dict);
            var subdict = item.ToLookup(secondKeySelector);
            foreach (var subitem in subdict) {
                dict.Add(subitem.Key, aggregate(subitem));
            }
        }

        return retVal;
    }

You should be able to use a nullable type just normally as long as the values aren't null - but you can't use null (or an empty Nullable<T>) as a key - it simply can't work.

It would make more sense to ensure (in the calling code) that secondKeySelector always returns something non-null; in the case of Nullable<T> perhaps by calling x => x.Foo.Value (if you see what I mean).

Another approach would be to declaratively preclude Nullable<T> here, by adding where TSecondKey : struct - but since string is a common key (and is a reference-type) this may be an undesirable approach.

You can be explicit on the use of the Nullable<T> on your generic declaration, which will limit you to use a Nullable on your method.

public static Dictionar<TFirstKey, Dictionary<Nullable<TSecondKey>, TValue>> Pivot<TSource, TFirstKey, TSecondKey, TValue>(this IEnumerable<TSource> source, Func<TSource, TFirstKey> firstKeySelector, Func<TSource, Nullable<TSecondKey>> secondKeySelector, Func<IEnumerable<TSource>, TValue> aggregate)
  where TSecondKey : struct
{}

As your using a Nullable<T>, you have to repeat the constraint for T to your implementation.

Please also note that Dictionary might not be happy with null key.