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Using Statements vs Namespace path? C#
(6 answers)
Closed 8 years ago.
I want to use Console.WriteLine method. There are two ways
using System; and then Console.WriteLine(); this is one way
System.Console.WriteLine(). this is another way
What is the difference between these two?
Will all the classes include under system namespace if i use using System; ?
Will console class only include when i use System.Console.WriteLine statement?
using System is just a syntactic sugar that allows you to access all the classes under the namespace without specifying the namespace. So yes, using System imports all the classes that are defined in System namespace then you can access them by their name.
In case where you include two namespaces that has two identical class then you will have to use fully-qualified name of the class to avoid ambiguity.
Also it is worth noting that this is just a shortcut. The using directive doesn't add anything to your project.So if you need to use a class or a function from a third-party dll, first you have to add a reference to your assembly (*.dll file) then you can use using directive.
using System;
<CODEHERE>
<CODEHERE>
Console.WriteLine("Text");
<CODEHERE>
<CODEHERE>
This is the more generic, and usual way of doing it for two reasons:
It is more readable
And if you wanted to use another class of System, you would already have the import.
Using the other method:
System.Console.WriteLine("Text");
is a lot more rare to see, and you would have to write out the fully qualified name of any other System class, or just to use it again:
System.Console.WriteLine("Text");
System.Console.WriteLine("Text2");
If you look back at what the using statement actualy means:
'Using' is a statement used to stop the programmer from repeatedly having to type the fully qualified namespace for everything.
I would use the 'using System' version.
Related
This question already has answers here:
Importing nested namespaces automatically in C#
(7 answers)
Closed 4 years ago.
I am a new c# learner, that' why I have some of the dumbest question! But I want to know if we have already used "using System" statement in our app then why do we need to use statements like "using System.Data" or any other shown in the image. Haven't we already imported the main namespace, then why is there even need of adding using statements for sub-namespaces?
Because the Language Spec says so:
C# language specification section 9.4.2 Using namespace directives:
A using-namespace-directive imports the types contained in the given namespace, but specifically does not import nested namespaces.
A reason for this might be that if every nested namespace is imported, then there would be too many name conflicts. This reduces the benefits of using using directives.
Here are some name conflicts that I could think of if using directives imported nested namespaces:
System.Drawing.Path and System.IO.Path
System.Windows.Forms.Button and System.Web.UI.WebControls.Button
System.Timers.Timer, System.Threading.Timer and System.Windows.Forms.Timer
Because using is not recursive down the namespace tree.
When you import System you import only the definitions strictly inside System not the ones inside System.Collections.
That's why you need to specify them.
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 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)
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
Should Usings be inside or outside the namespace
So there are two approaches to where you have your using statements in regard to the namespace. You can either have them outside the namespace declaration or inside. What is the pro/con between the two approaches and which is generally preferred.
using System;
namespace MyNamespace
{
}
or:
namespace MyNamespace
{
using System;
}
I typically see the former in use. These using statements are typically at the very top of a source file, making it easy to see at a glance what a particular file makes use of. It also allows you to easily see the start of new code, as the namespace signals the new stuff.
The other way is a little bit less easy to follow from an organizational standpoint. The only benefit is that you could have different using statements in two different namespaces in the same file, but using two namespaces in the same place like that is bad coding practice, so it should be avoided.