C# namespace vs directives

Question

I have years of experience in C++ and Java, but I am new to C#.

Recently when I was reading a book on C#, they address the following as name spaces, which I found it wierd. According to my knowledge in C++, these are actually pre-processor directives.

using System; //A book called these as namespace using System.Collections.Generic; //A book called these as namespace using System.Text; //A book called these as namespace

And according to the msdn website, the above are known as directives.

If the above is call namespace

What about the following?

namespace My_First_Program //This is also call namespace { class Program { static void Main() } }

So my first question is: Which exactly is the namespace? Am I correct to call using System; as pre-processor directive?

My second question is: Can I assumenamespace My_First_Program works similar to using namespace std in C++ ?

I think you need to see using as the same as import in java. — Stijn Bernards
– Stijn Bernards, Commented May 25, 2014 at 12:42
@StijnBernards That's what I thought, but I was later confused by the book's way of addressing it as a namespace. — user3437460
– user3437460, Commented May 25, 2014 at 12:44
They called it namespace because using actually uses a namespace. — Stijn Bernards
– Stijn Bernards, Commented May 25, 2014 at 12:46
namespace My_First_Program{} is a namespace definition and using System; imports the namespace. — Sriram Sakthivel
– Sriram Sakthivel, Commented May 25, 2014 at 12:46

ColinE · Accepted Answer · 2014-05-25 12:45:56Z

The following statement defines the namespace:

namespace My_First_Program { ... }

Any code within the braces is within the (wonderfully named) My_First_Program namespace.

The using statements 'import' namespaces so that you do not have to use their fully qualified name.

The 'using' statement is not a pre-processor directive. You can find a list of them here.

StuartLC · Accepted Answer · 2014-05-25 16:30:49Z

No, C#'s using is not a preprocessor directive.

In C#, these are both almost identical to C++

using System;

would have similar intent to

using namespace std;

or in Java

import java.util.*;

viz that symbols in the System namespace are now in scope, provided that the requisite assemblies are also referenced by this project. Without using, all code which access System would need to qualify the full namespace as a prefix, e.g. System.DateTime. You can also alias a namespace with using, e.g. if the result of adding multiple using statements brings 2 or more symbols with the same name into scope, e.g. (using m=My.Very.Long.NameSpace)

Similarly:

namespace foo { }

also has the same effect as it does in C++ - this places all symbols defined in the block as in the namespace provided, foo. As with other languages, this is done to prevent cluttering the global namespace. This is quite similar to the package keyword in Java files. In theory in C# you can have multiple namespace blocks in a file, although this would be unusual and likely poor practice to do so.

Stijn Bernards · Accepted Answer · 2014-05-25 12:48:14Z

Using is the same as import in java. Your book refered to it as namespace because using imports a namespace.

And yes to my knowledge using namespace std in c++ is the same as in c# using system;

Selman Genç · Accepted Answer · 2014-05-25 13:10:12Z

Which exactly is the namespace? Am I correct to call using System; as pre-processor directive?

No, using System; is just includes the namespace so you can access classes of System without specifying the namescape.this has nothing to do with preprocessor directives in C#.

In this case, System is the namespace but before you include System namespace with a using directive, you have to add a reference to System.dll. DLL files are assemblies which can contain one or more namespaces.

Can I assume namespace My_First_Program works similar to using namespace std in C++ ?

No, namespace My_First_Program is declaration of a namespace. using namespace std is similar to using directive in C#

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