When same-named namespaces exist (in current scope), how to refer any of them?

Question

I have:

namespace Book {
   ...
}

    ...
    ...



namespace Company 
{ 
    public class Book {

    }
    ...
    ...
    ...
    ...
    ...
    ...
    public class MyBook : Book.smth
    {
    }
}

How I make it, as MyBook to inherit the top-level Book namespace, instead of Company's ?

Answer 1

First off, ideally you do not get into this bad situation in the first place. Try to name your namespaces and classes so that they don't conflict with those of your dependencies! But sometimes it is unavoidable, particularly when a dependency adds a feature that you already implemented.

---

Technique #1:

You can do that with the global:: modifier.

namespace Company 
{ 
  namespace Xyz 
  { 
    public class DefaultClass {} 
  }
  public class myClass: global::Xyz.DefaultClass

Now Xyz.DefaultClass refers to the top level Xyz.DefaultClass, not Company.Xyz.DefaultClass

---

Technique #2:

Use a using alias directive, either on the namespace:

using TheOtherXyz = Xyz;
namespace Company {
  namespace Xyz 
  { 
    public class DefaultClass {} 
  }
  public class MyClass : TheOtherXyz.DefaultClass

or on the class:

using TheOtherDefault = Xyz.DefaultClass;
namespace Company {
  namespace Xyz 
  { 
    public class DefaultClass {} 
  }
  public class MyClass : TheOtherDefault

Note that the second variation does not work on generic classes. You can say

using StringList = System.Collections.Generic.List<String>

but you cannot say

using MyList<T> = System.Collections.Generic.List<T>

to make a generic alias. It's a perfectly sensible feature, it's just never been high priority enough to add it.

---

A question you didn't ask, but I'll answer it anyways:

What if I have namespace N with class C in two different assemblies that I am referencing? N.C is now ambiguous and global:: doesn't help, and neither does a using alias directive.

In that unfortunate case you use an extern alias directive, which you can read about here.

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/extern-alias

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If you are interested in other things that can go horribly wrong when you have a name collision like this, see my articles on that subject here:

https://blogs.msdn.microsoft.com/ericlippert/tag/namespaces/