不能像其他任何元素一样引用堆栈名称空间吗？

Question

So here is a snippet I made where one namespace in inside another (B is inside A). Usually when you use 'using SpaceA;' you can access all elements without typing SpaceA. This is the case for class A, but I cannot see SpaceB without having to type SpaceA.SpaceB. Why is this? Here is the code:

using System;

using SpaceA;

namespace SpaceA
{
    class A
    {
        public A()
        {
            Console.WriteLine("A");
        }
    }

    namespace SpaceB
    {
        public class B
        {
            public B()
            {
                Console.WriteLine("B");
            }
        }
    }
}

namespace TestingCSharp
{
    class Program
    {
        static void Main(string[] args)
        {
            //This does not work
            SpaceB.B x = new SpaceB.B();

            //This does work
            SpaceA.SpaceB.B y = new SpaceA.SpaceB.B();
        }
    }
}

Answer 1

Usually when you use 'using SpaceA;' you can access all elements without typing SpaceA.

Only the direct types which are members of SpaceA . A namespace-or-type-name is never resolved using a namespace in a normal using directive and then another "subnamespace" in the name. Note that this has nothing to do with how the types are declared (in terms of having a nested namespace declaration or just namespace SpaceA.SpaceB ) - another example would be:

using System;

...
Xml.Linq.XElement x = null; // Invalid

See section 3.8 of the C# 5 specification for the precise details of how names are resolved.

One slight difference to this is if you have a using alias directive for a namespace, at which point that can be used to look up other namespaces

using X = System.Xml;
...
X.Linq.XElement x = null; // Valid