Why would this code complain about "the arity of the generic type definition"?

Question

I've got a generic type:

class DictionaryComparer<TKey, TValue> : IEqualityComparer<IDictionary<TKey, TValue>>

And a factory method that will (should) create an instance of this class for a given dictionary type.

    private static IEqualityComparer<T> CreateDictionaryComparer<T>()
    {
        Type def = typeof(DictionaryComparer<,>);
        Debug.Assert(typeof(T).IsGenericType);
        Debug.Assert(typeof(T).GetGenericArguments().Length == 2);

        Type t = def.MakeGenericType(typeof(T).GetGenericArguments());

        return (IEqualityComparer<T>)Activator.CreateInstance(t);
    }

Stripping away all of the extraneous stuff - even this code throws the same exception.

private static object CreateDictionaryComparer()
{
    Type def = typeof(DictionaryComparer<,>);

    Type t = def.MakeGenericType(new Type[] { typeof(String), typeof(object) });

    return Activator.CreateInstance(t);
}

The Asserts pass so I know that T is generic and has two generic arguments. The line with MakeGenericType however excepts with:

The number of generic arguments provided doesn't equal the arity of the generic type definition.

Parameter name: instantiation

I've done this sort of thing in the past and for the life of me can't figure out why this isn't working in this case. (plus I had to Google arity).

Answer 1

Figured it out.

I had DictionaryComparer declared as an inner class. I can only assume that MakeGenericType wanted to make a Query<T>.DictionaryComparer<string,object> and was not provided T.

Failing code

class Program
{
    static void Main(string[] args)
    {
        var q = new Query<int>();
        q.CreateError();
    }
}

public class Query<TSource>
{
    public Query()
    {    
    }

    public object CreateError()
    {
        Type def = typeof(DictionaryComparer<,>);

        Type t = def.MakeGenericType(new Type[] { typeof(String), typeof(object) });

        return Activator.CreateInstance(t);
    }

    class DictionaryComparer<TKey, TValue> : IEqualityComparer<IDictionary<TKey, TValue>>
    {
        public DictionaryComparer()
        {
        }

        public bool Equals(IDictionary<TKey, TValue> x, IDictionary<TKey, TValue> y)
        {
            if (x.Count != y.Count)
                return false;

            return GetHashCode(x) == GetHashCode(y);
        }

        public int GetHashCode(IDictionary<TKey, TValue> obj)
        {
            int hash = 0;
            unchecked
            {
                foreach (KeyValuePair<TKey, TValue> pair in obj)
                {
                    int key = pair.Key.GetHashCode();
                    int value = pair.Value != null ? pair.Value.GetHashCode() : 0;
                    hash ^= key ^ value;
                }
            }
            return hash;
        }
    }
}