Any reason to write the “private” keyword in C#?

Question

As far as I know, private is the default everywhere in C# (meaning that if I don\'t write public, protected, internal

Answer 1

AFAIK, private is the default everywhere in C#

Not quite - the default is "the most restricted access available for this declaration". So for example, with a top-level type the default is internal; for a nested type the default is private.

So, what's the reason to write that keyword, or why does it even exist?

It makes it explicit, which is good for two reasons:

• It makes it clearer for those who don't know the defaults, as per your question (I've never liked this argument, personally, but I figured it's worth mentioning)
• It gives an impression that you've deliberately decided to make it private, rather than just gone with the defaults.

As for your last part:

Moreover is there a case where writing "private" (alone) will change the accessibility of the member?

Yes, for making half of a property more restrictive than the other:

// Public getter, public setter
public int Foo { get; set; }

// Public getter, private setter
public int Bar { get; private set; }

I used to go with defaults everywhere I could, but I've been convinced (partly by Eric Lippert) that making it clear that you've thought about it and decided to make something private is a good idea.

Personally I wish there were a way of doing that for sealed / unsealed, too, for type declarations - possibly not even have a default. I suspect that many developers (myself included if I'm not careful) leave classes unsealed just because it's less effort than making them sealed.