c# - 为什么这段代码会提示 "the arity of the generic type definition"？

Question

我有一个通用类型:

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

还有一个工厂方法，它将(应该)为给定的字典类型创建此类的实例。

    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);
    }

剥离所有无关的东西——甚至这段代码也会抛出相同的异常。

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

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

    return Activator.CreateInstance(t);
}

断言通过，所以我知道 T 是通用的并且有两个通用参数。 MakeGenericType 行除外:

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

Parameter name: instantiation

我过去做过这种事情，但我终究无法弄清楚为什么这在这种情况下不起作用。 (另外我还得用谷歌搜索 arity)。

Answer 1

想通了。

我有 DictionaryComparer声明为内部类。我只能假设 MakeGenericType想做一个Query<T>.DictionaryComparer<string,object>并且没有提供T .

失败代码

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;
        }
    }
}