Default visibility for C# classes and members (fields, methods, etc.)?

All of the information you are looking for can be found here and here (thanks Reed Copsey):

From the first link:

Classes and structs that are declared directly within a namespace (in other words, that are not nested within other classes or structs) can be either public or internal. Internal is the default if no access modifier is specified.

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The access level for class members and struct members, including nested classes and structs, is private by default.

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interfaces default to internal access.

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Delegates behave like classes and structs. By default, they have internal access when declared directly within a namespace, and private access when nested.

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From the second link:

Top-level types, which are not nested in other types, can only have internal or public accessibility. The default accessibility for these types is internal.

And for nested types:

Members of    Default member accessibility
----------    ----------------------------
enum          public
class         private
interface     public
struct        private

From MSDN:

Top-level types, which are not nested in other types, can only have internal or public accessibility. The default accessibility for these types is internal.

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Nested types, which are members of other types, can have declared accessibilities as indicated in the following table.

^{Source: Accessibility Levels (C# Reference) (December 6th, 2017)}

By default, the access modifier for a class is internal. That means to say, a class is accessible within the same assembly. But if we want the class to be accessed from other assemblies then it has to be made public.