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.
Related
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 have a question about the using statement for multiple files at once.
I have created an overload for a class that I want to use in my program.
To do so in one of my files I have added the following using statement.
using ClassName = CustomClassName;
This works, but only for that particular file.
Is there a way to get this to work for my entire project?
No.
using directives are per file.
You can always create a template that includes it.
No; C# does not have any such feature.
This is called type aliasing, re-utilizing the using keyword may be confusing, but it is not an import.
The purpose of this statement is to make a certain class accessible via a different name. This is useful in cases when you have different assemblies linked to your project which accidentally have classes with the same name.
If you for instance have A.dll that defines class Foo under the A namespace, and a B.dll assembly that also defines a Foo class under the B namespace, you can use:
using FooA = A.Foo;
using FooB = B.Foo;
to make distinctions between both.
The scope of this is usually the current file, although, if you happen to define multiple namespaces in the same file, you can scope that to the namespace within the file:
using FooA = A.Foo;
namespace N1
{
// knows about FooA;
using FooB = B.Foo;
}
namespace N2
{
// knows about FooA
// does not know about FooB
}
Practically you can make this aliasing more defined but no broader than the file's scope.
A simple solution for this is to create a template and use it into you project or class. this will give you a way to use the desire usings and directive as you wish.
here is a good sample to create a template .
http://www.rhyous.com/2010/02/17/how-to-modify-the-default-new-class-template-in-visual-studio-2008/
http://visualstudiomagazine.com/articles/2008/09/01/define-your-own-item-templates.aspx
Is it better to use a class of another C# project by its class name. Like this
Buss_Logic.Class1 myClass1 = new Buss_Logic.Class1();
Buss_Logic.Class2 myClass2 = new Buss_Logic.Class2();
OR by using keyword on top of the file
using Buss_Logic;
As long as there is no namespace collision, it makes for more compact / easier to read code to include namespaces with the using keyword.
If there is ever any doubt as to where the class is defined, hover the mouse over the class name. The tool tip will show the fully qualified class name.
Use explicit namespaces if there is a namespace conflict (e.g. two classes called File in two different namespaces, where both classes are needed in the current source document).
Use a using at the top of the file as long as there are no conflicts in the names.
Given that the class themselves are clear enough, it makes the code easier to read.
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
I am creating a little Math library for myself contained within a single project and am running into some issues with namespaces. I have the project MyMathLib and the top level namespace:
namespace MyMathLib
{ ... }
and in a separate file...
namespace MyMathLib.Addition
{ ... }
and...
namespace MyMathLib.Subtraction
{ ... }
In the MyMathLib.Subtraction namespace I have a method that needs to use a static method SomeClass.Work() defined in MyMathLib.Addition so I included using MyMathLib.Addition at the beginning of the Subtraction file. But when I try to use the method it would like me to first qualify it with Addition.SomeClass.Work() and I want to be able to just type SomeClass.Work(). What am I doing wrong?
Thanks!
EDIT
Thanks for the suggestions! In each file, I actually named the class after the namespace (i.e. in the namespace MyMathLib.Addition is a static class Addition and in MyMathLib.Subtraction there is a static class Subtraction). Apparently this is what caused the issue (looking back, I should have stated this instead of using SomeClass). If I change the namespace to MyMathLib.MyAddition while keeping the static class as Addition, the using MyMathLib.MyAddition works as I want; that is, I can now just type Addition.Work() in my static Subtraction class. I've seen classes named the same as it's containing namespace before, could someone maybe explain why this is causing an issue? Shouldn't the compiler be able to determine whether I want to use the namespace or the class from the context of the code?
I'm guessing that you either have two classes called SomeClass that are both in namespaces you reference, or you have a variable or property named SomeClass. Either of these situations would make it impossible for the compiler to know that you're trying to call the static MyMathLib.Addition.SomeClass.Work() method, but the specific solution the compiler is suggesting makes it seem more likely to be the former.
Update
Seeing your edit, that makes sense. If you were using these in a namespace outside of MyMathLib, then you would still be able to avoid this namespace conflict. However, because you are inside the MyMathLib.Subtraction namespace, the compiler will implicitly consider any portion of the namespace "above" you to take precedence over class names. In this case, when you say "Addition", the compiler will look for the following items to resolve the name:
A class explicitly identified by a using ... = ... directive.
MyMathLib.Subtraction.Addition namespace.
MyMathLib.Addition namespace.
Addition namespace.
Any classes in the namespaces identified by using statements.
In this case, you're hitting #3 before #4, so you should be able to work around it either by renaming the class or namespace, or by using Yahia's suggestion (#1):
using Addition = MyMathLib.Addition.Addition;
Update 2
After looking at the article you linked to, it sounds like the explicit using statement still won't work. I guess item #1 actually gets evaluated down around item #4 instead. Bummer. You can use an alias to give the class a different name locally:
using Add = MyMathLib.Addition.Addition;
...
var add = new Add();
But the best solution is still probably just to avoid the namespace collision entirely by changing your namespace or class name.
try putting additionally the floowing line into your substraction source
using SomeClass = Addition.SomeClass;
http://msdn.microsoft.com/en-us/library/dfb3cx8s.aspx
http://www.blackwasp.co.uk/NamespaceAliasQualifier.aspx
Sounds like you're in the Subtraction namespace...add this to the top, inside the namespace declaration:
using Addition;
That should do the trick.