Okay, there's System and System.Web. Am I correct in that the structure this suggests is:
namespace System
{
// all of the outer namespace members
namespace Web
{
// all of the inner members
}
}
And that when a namespace is nested within another, having a using directive with the parent/outer namespace only doesn't automatically bring in the child/nested namespace? In other words:
using System;
public class Example
{
public Example()
{
context1 = new HttpContext(); // won't work
context2 = new System.Web.HttpContext(); // will work
}
}
Just trying to see if I actually understand this correctly.
System.Web is declared as:
namespace System.Web
{
public class HttpContext {}
}
However, it would be possible to actually declare a child namespace:
namespace System
{
namespace Web
{
public class HttpContext {}
}
}
I have never seen something like this but the syntax allows it and the effect is the same. In both cases, the namespace of HttpContext is System.Web.HttpContext.
Even with the second example, using System; wouldn't import the child namespace, only the types defined in that namespace are imported.
You could nest namespaces and any using directive would only grant access to the members defined within the specific namespace you are referencing.
So from your example:
namespace System
{
// all of the outer namespace members
namespace Web
{
// all of the inner members
}
}
Referencing System would grant you access to the outer namespace members and referencing System.Web would grant you access to all of the inner namespace members.
But this is atypical and usually namespaces are defined only once within a file. The dot-notation typically follows a folder or project structure, so files that were nested as such:
WebApplication
- Models
- MyModel.cs
- Controllers
- MyController.cs
Might use namespaces of WebApplication.Models and WebApplication.Controllers.
I can't think of a great example off the top of my head where you would want to nest namespaces, but there may be a good reason to. However, it would be considered an exception to the rule, in my opinion.
Yes, a using directive only allows types declared in that namespace to be used without namespace qualifier. Nested namespaces are not automatically included.
As Daniel said, System.Web is not declared separately. System and System.Web are two separate namespaces which are technically unrelated.
That's why your code example of new HttpContext() won't work - because HttpContext is not in the System namespace at all.
this is a common confusion regarding composite namespaces. Heres a great article of microsoft about it: http://msdn.microsoft.com/en-us/library/ms973231.aspx
Related
I was wondering Why I can access class when I defined it outside of namespace scope?
I am not very familiar with namespaces, I am just learning, but I thought that namespace should like wrap all my classes to one 'box' and they can access each other just inside that 'box' (scope).
Code example:
class Point
{
public int X;
}
namespace ConsoleApplication12
{
class Program
{
static void Main(string[] args)
{
Point p = new Point();
p.X = 50;
Console.WriteLine(p.X);
Console.ReadLine();
}
}
}
Thank you for answers.
Namespaces have nothing to do with access. It's important to differentiate between namespaces and assemblies - they're often closely related, in that types in a namespace Foo.Bar would be likely to be in assembly Foo.Bar.dll, but that's a convention - it's not something the compiler or language cares about.
A namespace is primarily "a space in which names have to be unique". In other words, while it's fine to have two types called Point in different namespaces, you can't have two types called Point in the same namespace. Indeed, the primary reason for namespaces existing (IMO) is so that we don't all have to use completely unique names across every piece of .NET code in the world.
You can use your Point class which is implicitly internal so long as it's declared within the same assembly. If it's in the global namespace (i.e. not declared in a namespace at all) then you don't need any using directives to refer to it - the purpose of a using directive is to allow you to refer to members of a different namespace just by their short name.
So if you have:
namespace Foo.Bar
{
public class Baz {}
}
then you could access that as either:
namespace Other
{
class Test
{
Foo.Bar.Baz baz = new Foo.Bar.Baz();
}
}
or
// Allow anything in namespace Foo.Bar to be accessed by its
// short name
using Foo.Bar;
namespace Other
{
class Test
{
Baz baz = new Baz();
}
}
If the type is defined in the "global" namespace, then you just don't need the using directive. From the C# 5 language specification section 3.8 (namespace and type names) where it's describing the type name lookup procedure:
If the previous steps were unsuccessful then, for each namespace N, starting with the namespace in which the namespace-or-type-name occurs, continuing with each enclosing namespace (if any), and ending with the global namespace, the following steps are evaluated until an entity is located
So in your case, looking for Point from your Main method, first ConsoleApplication12 would be checked for a Point type, then the global namespace.
To access class, outside namespace, your class should be public
public class Point
and in your namespace, where you want to see this class, you need to add namespace in top of usings
using mynamespace.external;
Than you can access your class
I am just looking at a project and I'm seeing something working which I didn't think should work.
If you look at the snippet below I'm in the namespace CustomFields.DateTimeField.Drivers, and the only other using statements in that file are other namespaces below CustomFields.DateTimeField.
But if you look on the public class DateTimeFieldDriver line it is using a type Fields.DateTimeField.
Looking at the definition of DateTimeField thats in the namespace CustomFields.DateTimeField.Fields but I have only set usings for its sister namespaces.
So the question is twofold - why does this work? is this considered a bad practice?
Snippet in question:
using System;
using JetBrains.Annotations;
using Orchard;
using Orchard.ContentManagement;
using Orchard.ContentManagement.Drivers;
using Orchard.Localization;
using CustomFields.DateTimeField.Settings;
using CustomFields.DateTimeField.ViewModels;
namespace CustomFields.DateTimeField.Drivers {
[UsedImplicitly]
public class DateTimeFieldDriver : ContentFieldDriver<Fields.DateTimeField> {
public IOrchardServices Services { get; set; }
private const string TemplateName = "Fields/Custom.DateTime"; // EditorTemplates/Fields/Custom.DateTime.cshtml
public DateTimeFieldDriver(IOrchardServices services) {
Services = services;
T = NullLocalizer.Instance;
}
The project can be downloaded here if you want to investigate it (MVC2 project).
When you declare code to be inside a particular namespace, for example, CustomFields.DateTimeField.Drivers, you are implicitly importing all the "parent" namespaces as well.
In this case, you have an implied using for CustomFields and CustomFields.DateTimeField. This is why you do not have to specify an explicit using statement for types in CustomFields.DateTimeField and furthermore, for subnamespaces contained therein.
Thus Fields.DateTimeField is found by combining CustomFields.DateTimeField (the implied namespace) with Fields.DateTimeField (the explicit namespace) to resolve to CustomFields.DateTimeField.Fields.DateTimeField.
And no, as far as I know this is not considered bad practice.
In C#, when you're within a namespace, you have implicit using for each of the parent namespaces. For example in this file:
using System;
namespace Foo.Bar.Baz {
class Qux {
}
}
\EOF
Is the same as this file:
using System;
using Foo;
using Foo.Bar;
namespace Foo.Bar.Baz {
class Qux {
}
}
\EOF
If you have a namespace imported, then you don't need to specify the full namespace to access members of a sibling namespace tree provided it has the same prefix as an imported namespace:
using Foo;
using Foo.Bar;
namespace Foo.Bar.Baz {
class Qux {
}
}
namespace Foo.Bar.Norf {
class Guf : Bar.Baz.Qux { // namespace `Foo.` is implicitly assumed to prefix the namespace reference `Bar.Baz`
}
}
\EOF
You're experiencing these two effects combined: implicit namespace prefix usage to an implicitly imported namespace.
As for "bad practice" - experienced C# developers will be familiar with this nuance, but generally it isn't a problem unless you have ambiguous names.
As a tip, try to minimize the number of namespaces and their depths. For example, in your case I see you're storing DateTime-specific stuff in its own DateTimeField namespace. This has a bad "code smell" to me. I would just put everything in the CustomFields namespace, especially as you've given each typename the prefix DateTime anyway, so it's just double-typing.
FxCop will complain if you have fewer than 5 types in a namespace too, btw.
Update: I took a look at the C# specification ( http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-334.pdf ), section 16, but I can't find any part of it which states that parent namespaces are implicitly imported, which is strange.
This question already has answers here:
Importing nested namespaces automatically in C#
(7 answers)
Closed 9 years ago.
in c# if I want to add other System namespaces why do I need to call each namespace?
For example
if I want to call the System.Text namespace I have to use:
using System;
using System.Text;
why cant I just use using System?
Why do I also have to call using System.Text?
Why cant I use all System namespaces by just using System?
Why we use System for all namespace ?and what the meanings of system ?
the System namespace refers only to the files directly found under System. It does not automatically include all System.* namespaces. This is intended, since these are specialized namespaces which aren't needed in every class. Not every project automatically needs the System.Web namespace, for example.
You actually just need
using System.Text;
if you only want to use the System.Text namespace.
Within the System namespace, there are various types, eg System.DateTime. System.Text is not in System though, it is a separate namespace.
The confusion is caused by there being types at each "level" of a namespace. This means eg System.DateTime and System.Text both appear to be in System, when in reality the former is a type in System and the latter is a completely different namespace.
Here's a visual example that may help you out a little.
Basically, you can't access the namespace .Text from namespace .System because the methods contained within the .Text namespace do not exist in the .System namespace.
namespace OuterNamespace
{
public class DoStuff
{
public DoStuff()
{
//This DoStuff is different...
}
}
namespace InnerNamespace
{
public class DoStuff
{
public DoStuff()
{
//than this DoStuff.
}
}
}
}
public class Test
{
public Test()
{
//This "DoStuff" class
OuterNamespace.DoStuff outerStuff = new OuterNamespace.DoStuff();
//Is different than this "DoStuff" class
OuterNamespace.InnerNamespace.DoStuff innerStuff = new OuterNamespace.InnerNamespace.DoStuff();
}
}
Because they're different namespaces.
This article explains how to use Namespaces. Especially the example regarding Fully Qualified Names might be interesting to you.
System and System.Text are two different namespaces and that's why you had to refer both.
The System namespace contains fundamental classes and base classes that define commonly-used value and reference data types, events and event handlers, interfaces, attributes, and processing exceptions
The System.Text namespaces contain types for character encoding and string manipulation. A child namespace enables you to process text using regular expressions.
Now if you are confused about naming style then I would stick to Microsoft naming convention since it clearly isolate relating types and easy to iterate. This make stepped type iteration than listing all CLR types within one System namespace.
More about namespaces
I am writing code for a game, and wanted to include my main method in two different namespaces so that it could easily access all the classes from both the 'Engine' and 'Core' Namespaces.
namespace Engine Core
{
class ExampleClass
{
}
}
Although I just put a space between Engine and Core, I know that this syntax is incorrect, I would like to know how to make a class a member of multiple namespaces. If this is not possible, is there anything that I could do that would act the same? (Classes in neither these two namespaces having to refer to this class by 'Engine.' or 'Core.'
A class can not belong to two different namespaces.
If you want to refer to a class of the Engine or Core namespaces without explicitly writing the namespace each time you reference a type of those namespaces, just use using at the beginning of the file. The using directive allows the use of types in a namespace so that you do not have to qualify the use of a type in that namespace:
using Engine;
or
using Core;
Check the documentation: using Directive
So you want someone to be able to access ExampleClass by using Engine.ExampleClass and Core.ExampleClass? I'm not sure why you would (I'm sure you have your reasons) but two ways to expose something like this are:
namespace Foo
{
abstract class ExampleClass
{
//Only implement the class here
}
}
namespace Engine
{
class ExampleClass : Foo.ExampleClass
{
//Don't implement anything here (other than constructors to call base constructors)
}
}
namespace Core
{
class ExampleClass : Foo.ExampleClass
{
//Don't implement anything here (other than constructors to call base constructors)
}
}
Or you could use namespace aliasing but every cs file using the class would require the alias to be defined.
using Engine = Core;
I have a webservice project with a class (let's refer to it as webservice.classA).
I have another class project producing a dll which references that class in its own namespace and instantiates an instance of it (lets call the dlls namespace dllnamespace).
In another project I want to access the member in the dll
e.g.
using webservice;
namespace other_project
{
class B
{
classA copy = null;
//....
dllnamespace.dostuff(); // amongst other things instantiates a classA object
//....
copy = dllnamespace.getclassA(); // method to return classA member
The compiler error I get is cannot convert type from dllnamespace.webservice.classA to other_project.webservice.classA
I guess I have a fundamental design flaw but I figure there must be (?) a way to declare/use "webservice.classA" in more than one namespace.
You have a name clash. The supported way of avoiding this (short of not naming your classes the same), is to define a using alias for one of the classes:
using webservice.classA = myWebserviceClassA;
You are right...the design flaw does exist in terms of naming.
Let us assume:
you have a class named
MyClass
the class exists both in namespace- abc.xyz.qwe.tyu.MyClass
and in namespace - sed.qwe.dfg.ert.MyClass
The workaround is -
using NS1 = abc.xyz.qwe.tyu.MyClass;
using NS2 = sed.qwe.dfg.ert.MyClass;
This way you avoid the clash.
Also, helpful to use if you have very long namespaces.
FURTHER REFERENCE : (From MSDN article on using Directive )
The scope of a using directive is
limited to the file in which it
appears.
Create a using alias to make it easier to qualify an identifier to a
namespace or type.
Create a using directive to use the types in a namespace without having to specify the namespace. A using directive does not give you access to any namespaces that are nested in the namespace you specify.
Change the copy definition line to:
dllnamespace.webservice.classA copy = null;
That's just the problem - you cannot have a class in more than one namespace. This is what namespaces were designed for - to prevent classes with the same name written by different people from aliasing. You'll need to decide for one of your namespaces to own that class and in the other one to import it. Alternatively if the dll and the web service are part of the same distributed app then they should use the same namespace.