The declaration problem of mixed inheritance between base class and interface

1. The problem is at first

Today, looking at the feedback from the article "Interface inheritance declaration issues", I found that ninputer left such a question:

If there is

Class A:interface1

So

Class B:a, Inteface1

And

Class B:a

What would be the difference? What does the compiler do with the IL level to achieve this functionality?

2. Exploring Questions & Understanding issues

The process of solving a problem is both a process of exploration and a process of reasoning and argumentation. OK, I'm going to try to use contradiction to explore this problem.

First, I assume that the two types of inheritance of Class B in the question have the same effect and try to find them differently. To understand the effects of both approaches, I added the above code to complete:

interface IC { }
class A : IC { }
class B1 : A { }
class B2 : A, IC { }
class Program
{
　　static void Main()
　　{
　　　　A a = new A();
　　　　B1 b1 = new B1();
　　　　B2 b2 = new B2();
　　　　Console.WriteLine(a is IC);
　　　　Console.WriteLine(b1 is A);
　　　　Console.WriteLine(b1 is IC);
　　　　Console.WriteLine(b2 is A);
　　　　Console.WriteLine(b2 is IC);
　　}
}

The results of the code run are:

True
True
True
True
True