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C# Using Directive - Including All Child Libraries [duplicate]

Sorry if this question was asked already.
I started studying C# and noticed that C# doesn't automatically import nested namespaces.
I don't understand:
using System;
should automatically import all classes contained in the System namespace right?
So there should be no need for me to write
using System.Windows.Form;
I would understand if using Windows.Form even worked. But the compiler could not resolve it! What is the point of the using System; before it then?
So why does using System; not import System.Windows automatically as well as System.Windows.Forms - sorry if the word import is wrong here.. maybe move to global namespace is the right terminology.

C# is not Java.
A using directive is used so you don't have to type in the fully qualified name of a type. It also helps with disambiguating type names (using aliases for instance).
In the case of Console, for example, you don't need to type System.Console.
It is important to understand the difference between a namespace and an assembly - a namespace is a logical grouping of types. An assembly is a physical grouping of types. Namespaces can span assemblies.
When you reference an assembly (this is more like importing a package in Java), you gain access to all of the public types in it. In order to use a type you need to uniquely identify it. This is done through the namespace - the using directive simply means you don't have to type the fully qualified name of the type.

The using directive has two uses:
To allow the use of types in a namespace so that you do not have to
qualify the use of a type in that namespace:
using System.Text;
To create an alias for a namespace or a type. This
is called a using alias directive.
using Project = PC.MyCompany.Project;
http://msdn.microsoft.com/en-us/library/sf0df423.aspx
However, you have to note that System and System.Windows.Form are not connected through name itself in anyway. If you import (using) System that means you will use the System assembly types in this class. Actual reference you specify in references section in Visual Studio project which you can really use (even without using statement, as this is just a shortcut for types).

C# doesn't import nested namespaces and this is by design.
Namespace scope lets you organize code and gives you a way to create
globally unique types.
Nested namespaces are used to group related functionality, but use parts of it on-demand.
I guess you wouldn't want to have all the types from such a big namespace like System if the only thing you need is System.Windows.
So probably the question is why C# doesn't have something like using System.*; like java does. I don't know the answer, but I guess this is because of KISS principle. It's something like using
select *
you will never know what types you will add and how they will affect existing code.

Even in Java you'd have to explicitly write
import System.*;
Much of the time you don't want all of the nested namespaces. These would simply clutter IntelliSense.

The "using" syntax allows you shorthand access to namespaces that are already listed as References in the project settings. If the namespace is listed as a reference you already have access to it by it's full name without the "using" directive. Just saves keystrokes.

"Using" a given namespace means that you will get access to all definitions implemented directly in it, not that it will recursively look up the embedded namespaces; doing otherwise would defeat the purpose of the "Using" statement.
Namespaces exist to avoid class name ambiguity. The "Using" statement is here to avoid the use of fully qualified types nested in namespaces, when you know no (or little) ambiguity may occur.

No, this is not how it works.
And I will give a good argument against what you said: intellisnse would go crazy and finding the what you want would be hell.
You do have access to everything on every namespace available (with dots), the using keyword simplifies this because you don't have to specify from which namespace a class or struct is "coming from" (I mean, defined).

System.Linq.Queryable is not an attribute class error?

I am getting an error when adding [Queryable] the error is similar like this 'System.Linq.Queryable' is not an attribute class
when I tried to add namespace before [System.Web.Http.Queryable] I got another error
The type System.Web.Http.QueryableAttribute exists in both
System.Web.Http.OData.dll and System.Web.Http.dll
How to resolve this?

As Soner Gönül said.
Most probably you have
two using directives in your cs file:
using System.Web;
using System.Web.Http;
both define the attribute. Either remove one of those namespaces or fully qualify your attribute either as
[System.Web.Queryable]
// or
[System.Web.Http.Queryable]
Depending what the difference is of both and what you need. You will find more help in the corresponding MSDN documentation of both.

You have two DLL references that contain exactly the same namespace, containing a class with exactly the same name. The simplest solution is to remove one of these references from your project. If you need them both for some other reasons then the only solution I can think of is to create another project which will wrap the classes you need within other classes in a different namespace.

exclude a class from a used namespace

The first statement of all my C# files is "using System;".
Now with framework version 4 this namespace contains a class called "Action". This is also the name for a class im my own code in a regularly used namespace. Now there is of course a conflict. Ofcourse I can resolve this conflict by using explicit "MyNamespace.Action" where ever I was using "Action" before. This are several hundreds if not thousands of lines. Or I could not use the System namespace, which of course leads to many other problems. I would like to write something like "using System but not System.Action", but I cannot find a statement to do this.
Any ideas?

No, you can't.
But you can add using Action = MyNamespace.Action. This will be highly confusing for new developers, though, as Action is a fundamental part of .net since 3.5 so I strongly suggest you rename your class.

The using directive has two uses:
To permit the use of types in a namespace so you do not have to qualify the use of a type in that namespace:
using System.Text;
To create an alias for a namespace or a type (you could go for this one).
using Project = PC.MyCompany.Project;
The using keyword can also be used to create using statements, which define when an object will be disposed. See using statement for more information.
using directive (C# Reference)

c# - Use alias in another file

Is it possible to use an alias defined in one file throughout an assembly?
For eg. in Foo.cs I have
using IO = System.IO;
namespace Foo
{}
How can I use the alias "IO" in FooBar.cs
namespace Foo.Bar
{}

According to MSDN
The scope of a using directive is limited to the file in which it appears.
So the answer is No. You cannot define an alias in one file and use it throughout assembly

It would be nice to define global/shared using directive aliases. I recently considered this topic while working with a rather long class name across multiple files and methods.
One possibility to consider is using a code-generation tool that will add the alias to new files. This seems a bit pedantic however as it is pretty easy to just copy+paste the alias for every file that needs it.
Another option that one might consider is mentioned in hrzafer's comment:
A work around would be inheriting class IO : System.IO
Ignoring the bad syntax -- System.IO is a namespace and cannot be inherited by a class -- what hrzafer may have meant is something more like:
class Dir : System.IO.DirectoryInfo
Although this could work as a makeshift alias with simple classes, it could very well cause confusion since the point of inheritance is to reuse, extend, and modify the behavior defined in the base class.
Also, this approach doesn't work for sealed or static classes.

explicitly refer to a class without a namespace in C#

The code I'm working with has a class called Environment that is not in any namespace. Unfortunately if I am in a class that imports the System namespace, there is no way to refer to the custom class called Environment. I know this was an unfortunate choice and should be refactored, but is there any way I can explicitly refer to the conflicting class?
In C++ it seems the way to do this is by using ::, and in Java there is something called global:: How do I do it in C#?

C# also has a global (or unnamed) namespace - you can use global:: to access your class:
global::Environment
See more on MSDN. Also see the :: operator.
You can create an alias for it as well:
using myEnv = global::Environment;
using sysEnv = System.Environment;

Should be global::Environment just like in Java

The code I'm working with has a class called Environment that is not in any namespace
You should absolutely change that. Or if it’s not your code, file a bug report and defer usage until the bug is fixed. Not using a namespace – that’s an absolute no-go.
(Notwithstanding the well-working solution posted by #Oded.)