How to explicitly cast an object using the user-defined operator in the derived class

Question

Consider the following code:

  class A {
    public static explicit operator int(A a) {
      Console.WriteLine("User-defined explicit cast A");
      return 42;
    }
  }
  
  class B : A {
    public static explicit operator int(B a) {
      Console.WriteLine("User-defined explicit cast B");
      return 1;
    }
  }

  public void TestCast() {
    A b = new B();
    Console.WriteLine($"The result is: {(int) b)}");
  }

This prints:

User-defined explicit cast A
The result is: 42

Is there a way for me to cast using the instance's type's user-defined cast operator at runtime, without knowing what the instance type actually is at compile time?

Answer 1

While overload resolution usually happens at compile time, casting the object to dynamic forces it to be resolved at runtime.

class A {
    public static explicit operator int(A a) {
      Console.WriteLine("User-defined explicit cast A");
      return 42;
    }
  }
  
  class B : A {
    public static explicit operator int(B a) {
      Console.WriteLine("User-defined explicit cast B");
      return 1;
    }
  }

  public void TestCast() {
    A b = new B();
    Console.WriteLine($"The result is: {(int) (dynamic) b)}");
  }

This now prints:

User-defined explicit cast B
The result is: 1