解决C＃中方法重载的优先规则是什么？

Question

I'm writing a serializer in which I want to make use of method overloads extensively, to serialize objects of types deriving from IEnumerable<T> , IDictionary<K,V> and so on.

I also intend to use dynamic keyword to let CLR choose the correct overload based on the runtime type of the object to be serialized.

Have a look at this code snippet:

void Serialize<TKey, TValue>(IDictionary<TKey, TValue> dictionary)
{
  Console.WriteLine("IDictionary<TKey, TValue>");
}

void Serialize<TKey, TValue>(IEnumerable<KeyValuePair<TKey, TValue>> items)
{
  Console.WriteLine("IEnumerable<KeyValuePair<TKey, TValue>>");
}

void Serialize<T>(IEnumerable<T> items)
{
  Console.WriteLine("IEnumerable<T>");
}

And I want to do this:

void CallSerialize(object obj)
{
   Serialize(obj as dynamic); //let the CLR resolve it at runtime.
}

Now based on the runtime-type of obj , the correct overload will be called. For example,

//Test code
CallSerialize(new List<int>()); //prints IEnumerable<T>

In this case, the third overload is called and the rationale is pretty much straightforward : that is only the viable option.

However, if I do this:

CallSerialize(new Dictionary<int,int>()); //prints IDictionary<TKey, TValue>

It calls the first overload. I don't exactly understand this. Why does it resolve to the first overload when all three overloads are viable options ?

In fact, if I remove the first one, the second overload is called, and if I remove the first and second overload, then the third overload is called.

What are the rules of precedence in resolving the method overloading?

Answer 1

The rules for resolving method overloads will try to pick the method header with the most specific type match. Here you can read more about overload resolution and here I think is your case.

From MSDN:

Given an argument list A with a set of argument types {A1, A2, ..., AN} and two applicable function members MP and MQ with parameter types {P1, P2, ..., PN} and {Q1, Q2, ..., QN}, MP is defined to be a better function member than MQ if

• for each argument, the implicit conversion from AX to PX is not worse than the implicit conversion from AX to QX, and

• for at least one argument, the conversion from AX to PX is better than the conversion from >AX to QX.

When performing this evaluation, if MP or MQ is applicable in its expanded form, then PX or QX refers to a parameter in the expanded form of the parameter list.