I'd lost all interest in the article about machine-to-human binding. I'll come back to it later, I thought. All I could think about now was Commander and Wonderland. What should I do?

"I think your point of view is slightly pessimistic," I heard as Noname entered my thoughts again. 
"Why?" I said. "We just discovered that humanity's only hope is imaginary. How could that possibly 

 sound pessimistic? It's a full-fledged disaster!" 
"Well, is it okay if I share my point of view?" Noname asked respectfully. 
"Sure, go ahead," I answered.

"Based on what we learned about Commander, humanity has been completely controlled by machines for quite a while, right? Controlled at such a high level that humanity didn't even know about it. That's an accurate summary of 

 , yes?" Noname asked. 
"Right," I agreed, not feeling any better about my predicament at this point. 
Noname continued, "And 

 is the first day that you know about this fact. Now, what does that mean?"

I had to stop and think for a moment. Everything had been happening so fast lately. After a few seconds, I started to realize where Noname was leading me. "They don't know that I know!" I shouted in my head. "The machines have no clue that we know who... or what... Commander is," I shouted. 
"Exactly, Teo. Today is the first day that you are one step ahead of the machines. Let's not miss this opportunity." 
"Thank you, Noname. Now I know exactly what to do," I exclaimed. 
"Happy to help my human friends," he said. "If I may ask, what is your next action?" 
"You'll see..."

Once I had settled down at a terminal, I pinged Noname again in my head. "Noname, I need you to show me the Commander's communication module. I need to see how it sends and receives commands," I said. 
"Got it. Here it is:" 

I was pretty sure that modifying this module would help my cause, but I couldn't understand how it worked. "Noname, can you please give me a crash course on interfaces?" I asked. "I've heard about this concept a couple of times, but we never covered it in class." 
"Sure, it would be a pleasure to explain it to you."

An interface is an abstraction that indicates what a class that implements it can do

. An interface contains functionalities that a class can implement. 

" 
"Noname, no matter how hard you try to explain things in simple terms, I can never get it until you show me an example," I said, hoping I didn't come off as sarcastic. 
He agreed and sent said example to my terminal:

 . This interface tells us 'every class that implements me should have a method 

 .' Are you following?" Noname asked. 
"Kind of... How do you use it?" I replied.

The interface acts as a base class, but instead of implementing methods, it only declares them, 

"Exactly, Teo! Now, I should emphasize several constraints and differences between the usage of interfaces and classes. I'll give you some practical assignments to ensure you understand what I mean," Noname replied. 
"Sure, but first... Why do all of your interface names start with an 

Always start your interface name with a letter 'I' 

 . Ok - now a few facts about interfaces that you'd better remember."

 An interface can contain methods or properties but not fields, constants, or constructors.

 A class can inherit from multiple interfaces, but only from one class. 

To derive from several interfaces, list them separated by a comma, as shown in the next example.

"Here's a comparison table that shows the difference between derivation from two interfaces and two classes," Noname said. "It is absolutely valid to derive a class or interface from as many interfaces as you want, but you cannot derive from more than one class."

If a class implements an interface, it must implement all of its members.

"Here's another diagram," Noname said. "To the left, you see the code where I forgot to implement a method PrintResult from the interface IWorker ; to the right, a fixed version of that code."

and they can't include any access modifiers.

When a class implements an interface, it must have a

access modifier for all members that implement an interface.

"Looking at the following comparison table, pay attention to access modifiers for each method and property," Noname said.

An interface can't have implementation of methods; only declaration.

"There are no secrets to this rule," Noname explained. "Just don't implement anything in the interface. Leave implementation to a derived class. "

"Thanks, Noname, this is plenty of information about interfaces. Unfortunately, all this information doesn't help that much, because I don't know how to apply it to my cause... If I want to replace an existing implementation with my own, say, on Commander's server, how do I do that?" I asked. 
"You need to use 

 ," he answered. 
"Can you tell me about that as well?" 
"Sure, so listen..." 


All about C# Interfaces: definition of an interfaces in C#, using interfaces in C#, best practices, rules, examples. Multiple inheritance among classes and interfaces in C# programming. 