C# using

In C#, we use the using keyword to import external resources (namespaces, classes, etc) inside a program. For example,

// using System namespace
using System;

namespace Program {

  class Program1 {
    static void Main(string[] args) {
      Console.WriteLine("Hello World!");   
    }
  }
}

Output

Hello World!

In the above example, notice the line

using System;

Here, we are importing the System namespace inside our program. This helps us to directly use the classes present in the System namespace.

Also, because of this, we don't have to write the fully qualified name of the print statement.

// full print statement
System.Console.WriteLine("Hello World!");

// print statement with using System;
Console.WriteLine("Hello World!");

To learn more about the namespace, visit C# namespaces.

C# using to create an alias

We can also create aliases with the help of using in C#. For example,

// creating alias for System.Console
using Programiz = System.Console;

namespace HelloWorld {

  class Program {
    static void Main(string[] args) {

      // using Programiz alias instead of System.Console
      Programiz.WriteLine("Hello World!");    
    }
  }
}

Output

Hello World!

In the above program, we have created an alias for System.Console.

using Programiz = System.Console;

This allows us to use the alias Programiz instead of System.Console.

Programiz.WriteLine("Hello World!");

Here, Programiz will work just like System.Console.

C# using static directive

In C#, we can also import classes in our program. Once we import these classes, we can use the static members (fields, methods) of the class.

We use the using static directive to import classes in our program.

Example: C# using static with System.Math

using System;

// using static directive
using static System.Math;

namespace Program {  

  class Program1  {  
    public static void Main(string[] args)  {  
       	 
      double n  = Sqrt(9);
      Console.WriteLine("Square root of 9 is " + n);  
      	 
    }  
  }  
}

Output

Square root of 9 is 3

In the above example, notice the line,

using static System.Math;

Here, this line helps us to directly access the methods of the Math class.

double n = Sqrt(9);

We have used the Sqrt() method directly without specifying the Math class.

If we don't use the using static System.Math in our program, we have to include the class name Math while using Sqrt(). For example,

using System; 
 
namespace Program {  

  class Program1 {  
    public static void Main(string[] args) {  

      // using the class name Math
      double n  = Math.Sqrt(9);
      Console.WriteLine("Square root of 9 is " + n);  
    }  
  }  
} 

Output

Square root of 9 is 3

In the above example, notice the line,

double n = Math.Sqrt(9);

Here, we are using Math.Sqrt() to compute the square root of 9. This is because we haven't imported the System.Math in this program.