Why use system.something

Scott Stewart

If you have a directive "using System" don't you already inherit all the System methods etc? Why do you need the directive "using System.Windows.Forms?"

I might have missed something in my reading. But this is just sticking in my head like a thorn.

Thanks for putting up with these newbie questions.

4 Replies

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Vulpes14y ago
Accepted answer
The 'using' directive (in its usual from) imports a namespace into a file, rather than an individual class.

The following are examples of namespaces in the .NET Framework though you can also create your own:

System
System.Windows.Forms
System.IO
System.Text

Namespaces can be nested; so System is a 'top-level' namespace and the others listed above are nested namespaces.

However, each one of them is treated separately for the purposes of the 'using' directive. So this directive:

using System;

only imports classes and other types within the System namespace itself, not any nested namespaces. Similarly, the following directive:

using System.Windows.Forms;

only imports types within the System.Windows.Forms namespace.

In this context, 'importing' a type means being able to use it without the need to qualify it with its namespace.

So, when you include the last 'using' directive in your file, you can just refer to a Form rather than a System.Windows.Forms.Form in your code. The compiler figures out which class you mean.
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Scott Stewart14y ago
Thank you again, Vulpes. That cleared it up for me. And thank you, Satyapriya for your response.
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Scott Stewart14y ago
OK. But by referencing the System class don't I get all of the methods, etc.?

This came up because I wanted to add a split window to my MDI. On the left of the screen I will have a tree view based on data in my DB. So I looked into this and came across this http://msdn.microsoft.com/en-us/library/system.windows.forms.splitcontainer.aspx. Well, I see these directives and I ask myself "Why?" Of course there are a bunch of questions following the 'Why' but OMG! what a lot of stuff to follow and (as is normal with Microsoft) it is not clear to me. So I am trying to get a grip but the more I try to understand the more I get into stuff I can't figure out.

I appreciate any clarity anyone can provide. This is, obviously, the place I can get it.
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Satyapriya Nayak14y ago
Hi Scott,

The System.Windows.Forms namespace contains classes for creating Windows-based applications that take full advantage of the rich user interface features available in the Microsoft Windows operating system.

Refer

http://msdn.microsoft.com/en-us/library/system.windows.forms.aspx

Thanks
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