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).
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;
}
}
}