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.
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