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C# - C# | Using Directive
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  Access Static Members of a Class

Allows you to import a specific type and use the type's static members without qualifying them with the type name. This shows an example using static methods:

using static System.Console;

// ...

string GetName()
{
    WriteLine("Enter your name.");
    return ReadLine();
}

And this shows an example using static properties and methods:

using static System.Math;

namespace Geometry
{
    public class Circle
    {
        public double Radius { get; set; };

        public double Area => PI * Pow(Radius, 2);
    }
}
  Associate an Alias to Resolve Conflicts

If you are using multiple namespaces that may have same-name classes(such as System.Random and UnityEngine.Random), you can use an alias to specify that Random comes from one or the other without having to use the entire namespace in the call.

For instance:

using UnityEngine;
using System;

Random rnd = new Random();

This will cause the compiler to be unsure which Random to evaluate the new variable as. Instead, you can do:

using UnityEngine;
using System;
using Random = System.Random;

Random rnd = new Random();

This doesn't preclude you from calling the other by it's fully qualified namespace, like this:

using UnityEngine;
using System;
using Random = System.Random;

Random rnd = new Random();
int unityRandom = UnityEngine.Random.Range(0,100);

rnd will be a System.Random variable and unityRandom will be a UnityEngine.Random variable.
  Using alias directives

You can use using in order to set an alias for a namespace or type. More detail can be found in here (opens new window).

Syntax:

using <identifier> = <namespace-or-type-name>;

Example:

using NewType = Dictionary<string, Dictionary<string,int>>;
NewType multiDictionary = new NewType();
//Use instances as you are using the original one
multiDictionary.Add("test", new Dictionary<string,int>());
  Basic Usage 
using System;
using BasicStuff = System;
using Sayer = System.Console;
using static System.Console;  //From C# 6

class Program
{
    public static void Main()
    {
        System.Console.WriteLine("Ignoring usings and specifying full type name");
        Console.WriteLine("Thanks to the 'using System' directive");
        BasicStuff.Console.WriteLine("Namespace aliasing");
        Sayer.WriteLine("Type aliasing");
        WriteLine("Thanks to the 'using static' directive (from C# 6)");
    }
}
  Reference a Namespace 
using System.Text;
//allows you to access classes within this namespace such as StringBuilder
//without prefixing them with the namespace.  i.e:

//...
var sb = new StringBuilder();
//instead of
var sb = new System.Text.StringBuilder();
  Associate an Alias with a Namespace 
using st = System.Text;
//allows you to access classes within this namespace such as StringBuilder
//prefixing them with only the defined alias and not the full namespace.  i.e:

//...
var sb = new st.StringBuilder();
//instead of
var sb = new System.Text.StringBuilder();

Remarks

The using keyword is both a directive (this topic) and a statement.

For the using statement (i.e. to encapsulate the scope of an IDisposable object, ensuring that outside of that scope the object becomes cleanly disposed) please see Using Statement (opens new window).

 

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