When Create Dll

thankyou for you mr Madan 

thankyou for you Mr  Pritaeas

All of your answer great 

Usage of Access Specifiers

private: limits the accessibility of a member to within the defined type, for example if a variable or a functions is being created in a ClassA and declared as private then another ClassB can't access that.

public: has no limits, any members or types defined as public can be accessed within the class, assembly even outside the assembly. Most DLLs are known to be produced by public class and members written in a .cs file.

internal: internal plays an important role when you want your class members to be accessible within the assembly. An assembly is the produced .dll or .exe from your .NET Language code (C#). Hence, if you have a C# project that has ClassA, ClassB and ClassC then any internal type and members will become accessible across the classes with in the assembly.

protected: plays a role only when inheritance is used. In other words any protected type or member becomes accessible when a child is inherited by the parent. In other cases (when no inheritance) protected members and types are not visible.

Protected internal: is a combination of protected and internal both. A protected internal will be accessible within the assembly due to its internal flavor and also via inheritance due to its protected flavor.

2: You can only access public functions. Protected functions can only be called in descendant classes.

 

 3: You can obfuscate your code, but making a class invisible is next to impossible. What you can do is embed your dll with your executable. There are tools to help you with that. For example, dotNet Reactor, or dotFuscator.