BinaryReader BinaryWriter Console DataTable interface namespace StreamReader StreamWriter System using Alias

Using

In C# the using-keyword has 2 purposes: it includes a namespace, or manages the underlying resources of a type. Most C# files use the using-keyword.

The using-statement is combined with a type that implements the IDisposable interface. Things like StreamReader or StreamWriter implement IDisposable.

Tip Using protects the system's resources by specifying the scope of the usage of the resource.

Using-statement

This program defines a class called SystemResource. The class implements the IDisposable interface and the required Dispose method.

Note In the Main method, we wrap the SystemResource instance inside a using-statement.

Start The SystemResource instance is allocated upon the managed heap. Next the "1" is written.

And The "0" is written because the Dispose method is invoked. And finally, the "2" is printed.

Info As demonstrated, the Dispose method is called immediately when control flow exits the using block.

using System;
using System.Text;

class Program
{
    static void Main()
    {
        // Use using statement with class that implements Dispose.
        using (SystemResource resource = new SystemResource())
        {
            Console.WriteLine(1);
        }
        Console.WriteLine(2);
    }
}

class SystemResource : IDisposable
{
    public void Dispose()
    {
        // The implementation of this method not described here.
        // ... For now, just report the call.
        Console.WriteLine(0);
    }
}1
0
2

Nested statements

It is possible to nest multiple using-statements one after another. You do not need to use any curly brackets in this case.

Tip In the second or further using-statements, you can use the variables declared in previous using statements as well.

using System;

class Program
{
    static void Main()
    {
        using (SystemResource resource1 = new SystemResource())
        using (SystemResource resource2 = new SystemResource())
        {
            Console.WriteLine(1);
        }
    }
}

class SystemResource : IDisposable
{
    public void Dispose()
    {
        Console.WriteLine(0);
    }
}

Using directive

The using-keyword in C# is also used to include a namespace during compilation. This is what "using System" lines do at the top of files.

using System;

class Program
{
    static void Main()
    {
        // Console is in the System namespace.
        Console.WriteLine("Hello");
    }
}
Hello

Global using

We can place the keyword "global" before a using directive to indicate that the entire project should include the using directive. This can reduce source file size.

Here The Program.cs file has a global using for the System namespace, so Test.cs can access Console without a using System directive.

// Program.cs
global using System;

class Program
{
    static void Main()
    {
        Test.Print();
    }
}
// Test.cs
static class Test
{
    public static void Print()
    {
        Console.WriteLine("Hello");
    }
}
Hello

Discussion

It is most common to use "using" with types already defined in .NET. Some of these types include BinaryReader, BinaryWriter, DataTable.

Info Use a resource acquisition expression inside the using-statement. You do not need to implement any interfaces.

Using alias

For the using namespace feature, we can specify an alias for a namespace. This can reduce redundant text in source files.

A using-block invokes the Dispose method found in the IDisposable interface implementation. Meanwhile a using-statement includes a namespace during compilation.