Question

Not sure if this is C# 4+ specific, but just noticed this.

Consider the following classes:

class Base
{
  protected void Foo(object bar, DayOfWeek day)
  {
  }
}

class Program : Base
{
  protected void Foo(object bar, object baz)
  {
  }

  void Bar(DayOfWeek day)
  {
    Foo(new { day }, day);
  }
}

The call to Foo in Bar, resolves to Foo(object, object).

While changing it to:

class Base
{

}

class Program : Base
{
  protected void Foo(object bar, object baz)
  {
  }

  protected void Foo(object bar, DayOfWeek day)
  {
  }

  void Bar(DayOfWeek day)
  {
    Foo(new { day }, day);
  }
}

The call to Foo in Bar, resolves to Foo(object, DayOfWeek).

My understanding is that it should always resolve as in the second example.

Is this a ‘bug’ or just my lack of understanding (or ignorance)?

Update:

Thanks for the answers. As I have found out, one can use base. to call the method in the base class. The problem comes back however when adding another derived class in the mix.

class Base
{
  protected void Foo(object bar, DayOfWeek day)
  {
  }
}

class Program : Base
{
  protected void Foo(object bar, object baz)
  {
  }

  void Bar(DayOfWeek day)
  {
    base.Foo(new { day }, day);
  }
}

class Derived : Program
{
  void Baz(DayOfWeek day)
  {
    base.Foo(new { day }, day);
  }
}

The base. call works in Program, but then resolves to Foo(object, object) in Derived.

How would one call Foo(object,DayOfWeek) from Derived then without having to create ‘redundant’ methods in Program ?

Answer 1

I think for resolving method call it looks in its class first, since DayOfWeek can be passed as object type, it calls class own method, not the one from the base class.

In the second case, the method call resolves to a the more particular type parameter, therefore Foo(object bar, DayOfWeek day) gets called.

From MSDN – Method resolution.

methods in a base class are not candidates if any method in a derived
class is applicable (Section 7.5.5.1).

• Given the set of applicable candidate function members, the best function member in that set is located.
• If the set contains only one function member, then that function member is the best function member.
• Otherwise, the best function member is the one function member that is better than all other function members with respect to the given
  argument list, provided that each function member is compared to all
  other function members using the rules in Section 7.4.2.2.
• If there is not exactly one function member that is better than all other function members, then the function member invocation is
  ambiguous and a compile-time error occurs.