"does reflection breaks the idea of private methods, because private methods can be access outside of the class?" Code Answer

do we use private methods only for program logic and not for program security?

It is not clear what you mean by "program security". Security cannot be discussed in a vacuum; what resources are you thinking of protecting against what threats?

The CLR code access security system is intended to protect resources of user data from the threat of hostile partially trusted code running on the user's machine.

The relationship between reflection, access control and security in the CLR is therefore complicated. Briefly and not entirely accurately, the rules are these:

• full trust means full trust. Fully trusted code can access every single bit of memory in the process. That includes private fields.

• The ability to reflect on privates in partial trust is controlled by a permission; if it is not granted then partial trust code may not do reflection on privates.

See http://blogs.msdn.com/b/shawnfa/archive/2006/09/29/777047.aspx for details.

• The desktop CLR supports a mode called "restricted skip visibility" in which the rules for how reflection and the security system interact are slightly different. Basically, partially trusted code that has the right to use private reflection may access a private field via reflection if the partially trusted code is accessing a private field from a type that comes from an assembly with equal or less trust.

See

http://blogs.msdn.com/b/shawnfa/archive/2006/10/05/using-lightweight-codegen-from-partial-trust.aspx

for details

The executive summary is: you can lock partially trusted code down sufficiently that it is not able to use reflection to look at private stuff. You cannot lock down full trust code; that's why it's called "full trust". If you want to restrict it then don't trust it.

So: does making a field private protect it from the threat of low trust code attempting to read it, and thereby steal user's data? Yes. Does it protect it from the threat of high trust code reading it? No. If the code is both trusted by the user and hostile to the user then the user has a big problem. They should not have trusted that code.

Note that for example, making a field private does not protect a secret in your code from a user who has your code and is hostile to you. The security system protects good users from evil code. It doesn't protect good code from evil users. If you want to make something private to keep it from a user then you are on a fool's errand. If you want to make it private to keep a secret from evil hackers who have lured the user into running hostile low-trust code then that is a good technique.