Why elements defined in a namespace cannot be explicitly declared?

Solution 1:

Elements defined in a namespace may be explicitly declared public or internal.

They may not be explicitly declared private or protected (or protected internal) because these modifiers only make sense for members of a class.

Your protected class GetDataBL, for example, makes no sense, because "protected" means "accessible to classes that inherit from the containing class" -- but there is no containing class for GetDataBL.

Solution 2:

private protected means they will be accessible to this class or to the derived classes.
In the Namespace level there is no class to derived from so it useless.

You can use only public or internal in the Namespace level

MSDN docs

Solution 3:

(I believe you'll actually get a compile-time error; if you're only seeing this at execution time, then chances are your code is being compiled at execution time too, e.g. as part of a web app. Logically it's a compile-time error, not an exception.)

The protected access modifier (loosely) makes a member accessible to a derived containing type; but in the case of a namespace member there is no containing type.

Likewise a private member's accessibility domain is the program text of the containing type - and again, there is no containing type.

What are you actually trying to achieve by making GetDataBL protected?

Solution 4:

It's the scoping of the elements that is causing the error, as explained by the error - and the C# specification (ECMA section 10.5.1):

• Types declared in compilation units or namespaces can have public or internal declared accessibility and default to internal declared accessibility.
• Class members can have any of the five kinds of declared accessibility and default to private declared accessibility.
• Struct members can have public, internal, or private declared accessibility and default to private declared accessibility because structs are implicitly sealed.