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.
Related
I have the following situation:
A compiled library with the namespace Library which contains class Feauture.
Now there is another library in development, one which intends to utilize the feature, and that has been dubbed Library.Feature. Finally there is a third library: Library.Feature.UI.
When working in the Library.Feature.UI project, which has both other libraries referenced, VS is yelling a lot about trying to using the Feature class, because it is seeing it primarily as a namespace.
I've tried a few different using directives to get around this, as well as trying to qualify the class name, but nothing is working.
Assuming I don't have the ability to change any of the namespaces or existing class names, is there a way to circumvent this issue?
You can either use fully qualified names where you specify the namespace together with the type name or you can use a using directive to create an alias:
using MyFeature = Library.Feature;
You can use alias directives to give a different name to any namespace, and then use that alias to reference that namespace.
By doing this you can differentiate between the class and the namespace.
write the following on top while using namespaces.
using FeatureClass = Library.Feature;
For more knowledge on this, you can refer:
https://msdn.microsoft.com/en-us/library/aa664765%28v=vs.71%29.aspx
Let me know if you have any further issue...
This should do the trick (references)
using FeatureClass = Library.Feature;
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).
This question already has answers here:
Importing nested namespaces automatically in C#
(7 answers)
Closed 9 years ago.
so my program has these 2 lines at the beginning
using System;
using System.IO;
Question: Is the second statement actually necessary to include Sytem.IO methods and properties in my code?
It seems that 'System.IO' is a 'child' of the namespace 'System'. Shouldn't the first line grab all the child namespaces too? Or Do I not understand namespaces correctly?
System.IO namespace is used for Input Output operations.(Ex: File Operations)
System namespace does not include all child namespaces.
So if you want to perform IO Operations you should include System.IO namespace explicitly.
First Question : Is the second statement actually necessary to
include Sytem.IO methods and properties in my code?
Yes it is Necessary as System namespace does not include Child namespaces.
Second Question : It seems that 'System.IO' is a 'child' of the
namespace 'System'.
Yes System.IO is a Child of System namespace.
Note : though System.IO is a child namspace of System, it will not be included when you include System namspace
Third Question : Shouldn't the first line grab all the child
namespaces too? Or Do I not understand namespaces correctly?
No first line using System; does not grab all the Child namespaces as it is not java to import all child namspeaces using wild card character star *
C# is not like java, where you can use wildcards to import namespaces.
using System;
in C# is not the same as
import system.*;
in Java.
And that's really all there is to it. You need to explicitly include namespaces - not much more to say about it :)
Shouldn't the first line grab all the child namespaces too? Or Do I not understand namespaces correctly?
No - it will only make an alias for types in the System namespace directly. "Nested" namespaces (such as System.IO) are not made aliases automatically in C#. This is mentioned in the help for the using directive (italics added by me for emphasis):
allow the use of types in a namespace so that you do not have to qualify the use of a type in that namespace
You would still need to qualify IO for IO operations. For example, if you wanted to use the Path class, you'd need:
var filename = System.IO.Path.GetFilename(fullPath);
To expand on the other answers, there really aren't "child namespaces". There's nothing stopping you from declaring your own System.IO. You wouldn't want to of course, but you don't need a parent System namespace to do it.
You may have a project that by default has MyCompany.MyProject namespace, but you could just as easily declare XYZ.Some.Other.Random.Namespace in another .cs file of the same project. There's no inheritance to namespaces and XYZ.Some.Other doesn't exist. C# will let you use it, however, when you try to instantiate your class using the rest of it (Random.Namespace), it doesn't know how to find the class and won't resolve until you fully qualify the namespace in the using or fully qualify your variable declaration.
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.)
Do you use a global, catchall namespace for all of your extension methods, or do you put the extension methods in the same namespace as the class(es) they extend?
Or do you use some other method, like an application or library-specific namespace?
I ask because I have a need to extend System.Security.Principal.IIdentity, and putting the extension method in the System.Security.Principal namespace seems to make sense, but I've never seen it done this way.
Put your extensions in the same namespace as the classes they extend. That way, when you use the class, the extensions are available to you.
If you are writing a extension for Uri, put the extension in System.
If it's a extension for DataSet, put it in System.Data.
Also, Microsoft says this about extension methods:
In general, we recommend that you
implement extension methods sparingly
and only when you have to. Whenever
possible, client code that must extend
an existing type should do so by
creating a new type derived from the
existing type.
For more info about extension methods, see the MSDN page about extension methods.
If they're extension methods used throughout the solution (e.g. over 60% of classes), I put them in the base namespace of the solution (since they'll be automatically imported being in a parent namespace, no importing the common stuff every time).
In this category, things like:
.IsNullOrEmpty(this string value) and .HasValue(this string value)
However, if they're very specific and rarely used, I put them in a BaseNamepace.Extensions namespace so they must be manually imported and don't show in intellisense cluttering things up everywhere.
I would recommend putting all your extension methods in a single namespace (incidentally this is also what Microsoft did with Linq by putting them in a single class 'Extensions' inside the 'System.Linq' namespace).
Since Visual Studio provides no clues as to how to locate an extension method that you want to use, it reduces confusion by only having to remember one namespace.
I upvoted the answer from user276695 which seemed the simplest solution. However I found an even better solution: no namespace at all. You can put a static class with extension methods at the very root of a file, without wrapping any namespace declaration around it. Then the extension methods will be available without having to import/using any namespace.†
I'm slightly worried about being down-voted from namespace nazis. But I feel compelled to champion a slightly unorthodox position. At least in my line of work (IT) I find that most of my custom tools are easier to maintain without namespaces, and placing everything in one giant anonymous namespace. I'm sure there are other programming universes where this would not work as well. But perferences & circumstances being different, I just wanted to point out that you can in fact declare classes without namespaces, even classes with extension methods - for those who also find a giant anonymous namespace better.
† You also get to save one level of indentation. :-) And even with 3 giant monitors I'm always looking for ways to save screen real-estate.
My practice is to put the extensions in a namespace that is different from yet clearly identifies the source namespace that I'm extending.
Generally, I create a namespace by prefixing my "company name" part of the namespace path in front of the namespace I'm extending, and then suffix it with ".Extensions"
For example, if I'm extending (for no good reason) ::System.Text.StringBuilder because it's sealed, then I will put my extensions in the namespace ::CodeCharm.System.Text.Extensions.
That's a rule of thumb for when I make extensions that I suspect I could re-use those extensions in other solutions.
If you put them in a higher namespace than your current, they will be visible automatically.
So if you have namespaces for projects like:
AdventureWorksInc.Web
AdventureWorksInc.Logic
AdventureWorksInc.DataAccess
Then declare your extension directly in:
namespace AdventureWorksInc
{
public static class HtmlHelpers
{
public static string AutoCloseHtmlTags(this string html)
{
//...
}
}
}
This extension method will show up whenever you are writing code in any sub name space of AdventureWorksInc without the need for a using statement.
However, the above extension demonstrates a possible downside. Due to the fact that it operates on strings, it will now show up as an extension method for all strings, including those that aren't really HTML. This is actually not an issue with namespace scoping, but simply a misuse of an extension method. This should be a regular static that requires a standard parameter so the call is explicit.
Generally well designed extension methods with appropriately typed parameters will not show up on types that it would never apply.