I'm finding some frustration with the way C# handles namespaces when I want to reference a class whose class name happens to be identical to part of the current class's namespace. This will produce a compiler error, "'Class' is a namespace but is used like a type".
Consider the following code sample, which demonstrates the problem:
namespace A.B.C
{
public class Y
{
}
}
namespace X.Y.Z
{
public class Class1
{
public Y MyProp;
}
}
In this case, I want to use the class "Y" from the namespace "A.B.C". But because "Y" is also one of the parts of the namespace, C# treats "Y" as a namespace, not a type.
I can get around this by fully qualifying the class name, "A.B.C.Y", or using an alias, but my general preference would be for C# not to treat "Y" as a namespace in this context. Often I have code, such as test code, that contains similar namespaces as classes its testing, and this kind of class/namespace collision means having to be a lot more verbose in setting things up.
I'm not sure what the process is called, but it seems that in resolving classes and namespaces, C# will walk up the namespaces until it finds a part of the namespace that matches. In this case, it walks up and finds Y. Is there a way to tell C# not to walk up, not to allow this partial namespace matching?
If you write using A.B.C; (the namespace that Y is in) inside namespace X.Y.Z, the error goes away:
namespace X.Y.Z
{
using A.B.C;
public class Class1
{
public Y MyProp;
}
}
Related
namespace Y
{
class foo { }
}
namespace X.Z
{
class bar : Y.foo { }
} // fine
namespace X.Y.Z
{
class bar2 : Y.foo { }
} // error!, now foo can't be found because it looks for X.Y.foo
Is this a c# bug? I would have though being in a namespace just prefixed the names of items declared in the namespace. Apparently something else is happening and I'm not sure it is a good thing.
So
namespace X.Y.Z
{
class bar2 : Y.foo { }
}
Is just shorthand for:
namespace X
{
namespace Y
{
namespace Z
{
class bar2 : Y.foo { }
}
}
}
(Hopefully it's clear why the shorthand is actually valuable.)
So now it comes time to try to resolve the identifier Y.foo. The way C# does this is it always looks at the narrowest possible scope, and if it finds no suitable matches, it moves up a level. So first it'll look within the definition of bar2, and find no suitable candidates. Then it goes to the next level up to the namespace X.Y.Z. Then up a level again to the namespace X.Y. Now we have something Y can match, the namespace itself, and it looks to match the identifier foo inside of that, since it's a namespace, and again, finds that there is nothing.
Since there was a suitable match for Y, it's done. Anytime an identifier is able to be resolved, even if it errors, it doesn't continue searching for other things that the identifier could resolve to that might not error.
A way to resolve the identifier as you want to is to use the global:: prefix, which is a keyword that prevents the whole "work inside out" thing from happening and forces the identifier to start from the root and work its way down, allowing you to write global::Y.foo; and have it match the type that you want. You could also add a using to either import the namespace or create an alias for the type to resolve the type properly.
Of course the best resolution is to use different namespace names to avoid the ambiguity problem entirely, if you have control over that.
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
One article about namespaces says
Namespaces are C# program elements designed to help you organize your
programs. They also provide assistance in avoiding name clashes
between two sets of code. Implementing Namespaces in your own code is
a good habit because it is likely to save you from problems later when
you want to reuse some of your code. For example, if you created a
class named Console, you would need to put it in your own namespace to
ensure that there wasn't any confusion about when the System.Console
class should be used or when your class should be used.
Now imagine as in above statement I indeed created a Console class inside my namespace called: myNamespace.
Now at some point when someone wants to use say my library, he/she will have to do:
using myNamespace;
but likely she will need to do
using System;
too.
So we still have a clash. If I type Console, which console am I referring to?
How is this problem solved using namespaces?
PS. extra: also how namespaces work under the hood: I remember somewhere I read compiler just prepends functions for example with namespace to find their definitions?
Is this that? Is namespace really nothing just a means to group code?
If you use both of the namespaces, you will either have to fully qualify the usages(System.Console()), or use namespace/type aliases(Console2) to disambiguate the types.
using Console1 = myNamespace;
using Console2 = System;
public void MyMethod()
{
Console1.Console(); // or myNamespace.Console()
Console2.Console(); // or System.Console()
}
You would not include both using statements.
You choose, so your code looks cleaner:
using System;
public static void Main()
{
Console.WriteLine("I'm here!");
myNameSpace.Console.PopBeer("Mo beer!");
}
for practical purposes, if you have a really long namespace, you can use aliases too:
using System;
using sexyNs = myProject.AwesomeSolution.Helpers;
public static void Main()
{
Console.WriteLine("I'm here!");
sexyNs.Console.PopBeer("Mo beer!");
}
The program will not compile. You need to instantiate your classes like this:
System.Console.WriteLine("hi");
myNamespace.Console y = .... ;
a better solution would be to avoid using names which cause conflicts. Name your Console something else so it's clear which one you use when you need to.
#user200300,already many have given exact code solution to your problem description.But let me try to explain clearly.Namespaces also provide assistance in avoiding names clashes.
For example:Lets say there is a single project called Stackoverflow and it contains two teams say TeamA and TeamB.
namespace Stackoverflow
{
namespace TeamA
{
class classA
{
public static void Print()
{
Console.WriteLine("Print A");
}
}
}
}
namespace Stackoverflow
{
namespace TeamB
{
class classA
{
public static void Print()
{
Console.WriteLine("Print B");
}
}
}
}
Here both the team TeamA and TeamB are trying to modify same class classA.Namespaces have helped in organizing the code in efficient and in proper accessible way.
If you want to access the print method from any team you outside these namespaces or from any class for that matter you would write like
Stackoverflow.TeamA.classA.Print();
Stackoverflow.TeamB.classA.Print();
So above code is cumbersome and lengthy and its not clean.Hence aliases can be used .
using STAP=Stackoverflow.TeamA;
using STBP=Stackoverflow.TeamB();
Now you can access easily.
STAP.classA.print();//TeamA's print method.
Thus a namespace can contain Namespace,Class,Inteface,struct,enum,delegate
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.
I have a program that defines a class
namespace #default
{
public class Test
{
}
}
I then referance another library that has no namespaces with a class called Test. How can I hide this other libraries Test class so that my program does not see #default.Test as the other libraries Test class?
By default, inside your default namespace, the default.Test class is the one that will be used when you say Test.
Outside your default namespace, you will need to do something like this in your using statements:
using ExternalTest = global::Test;
using Test = default.Test;
The first line isn't strictly necessary, as you could use global::Test anywhere.
For those that haven't encountered global before: global (C# Reference).
Prefix the identifier name with global:: to indicate that it should be found starting at the global (unnamed) namespace. For example:
class Test { }
namespace Default {
public class Test {
global::Test theOtherOne;
}
}
You'll often see it used in auto-generated code to avoid accidental name collisions. Like Resources.Designer.cs