strange method overload resolution with constrained generic arguments: overload with base class arg always called

Question

I have some - probably really stupid - question on overload resolution. Suppose there is a constrained generic method Greet accepting parameter person of type T which must derive from Person. This Greet method then calls some non-generic method in another class passing that person as parameter. That non-generic method has several overloads - for the base class and for a derived class. Why is the overload for the base class always called?

Example:

public static class Test 
{
  public static void Greet() 
  {
    new SomeClass().Greet(new Person()); // Hi, I am a Person
    new SomeClass().Greet(new Manager()); // Hi, I am a Person -- Why?
  }
}
public class SomeClass 
{    
   public void Greet<T>(T person) where T : Person 
   {
     new Greeter().SayHi(person);
   }
}
public class Person { /* some props and methods */ }
public class Manager : Person { /* some props and methods */ }
public class Greeter 
{  
   public SayHi(Person person) { Console.WriteLine("Hi, I am a person"); }
   public SayHi(Manager person) { Console.WriteLine("Hi, I am a manager"); }
}

However, only the SayHi accepting a Person is always called. Why?

outside of using dynamic, overload resolution is performed at compile time, not runtime. It has to pick one method to compile a call to. — Damien_The_Unbeliever

Jon Skeet Jon Skeet · Accepted Answer · 2016-09-21T10:18:19

The compiler has to perform overload resolution for the call of

 new Greeter().SayHi(person);

At that point, the type of person is just T, and all it knows is that T is implicitly convertible to Person. Therefore, the only applicable method in Greeter is SayHi(Person).

It's important to understand that this happens at compile-time, once - it doesn't occur at execution time separately for each type argument that's used for T.

strange method overload resolution with constrained generic arguments: overload with base class arg always called

1 Answers