The using Keyword

Sometimes you'll want to use the fully qualified name for a given type in the form namespace.type. However, this can be rather tedious and sometimes isn't necessary. In the following example, we're using the Console object that exists in the System namespace.

class Using1
{
    public static void Main()
    {
        System.Console.WriteLine("test");
    }
}

However, what if we know that the Console object exists only in the System namespace? The using keyword allows us to specify a search order of namespaces so that when the compiler encounters a nonqualified type, it can look at the listed namespaces to search for the type. In the following example, the compiler locates the Console object in the System namespace without the developer having to specify it each time.

using System;
class Using2
{
    public static void Main()
    {
        Console.WriteLine("test");
    }
}

As you start building real-world applications with a few hundreds of calls to various System objects, you'll quickly see the advantage of not having to prepend each one of these object references with the name of the namespace.-

You cannot specify a class name with the using keyword. Therefore, the following would be invalid: -

using System.Console; // Invalid.
class Using3
{
    public static void Main()
    {
        WriteLine("test");
    }
}

What you can do, however, is use a variation of the using keyword to create a using alias: -

using console = System.Console;
class Using4
{
    public static void Main()
    {
        console.WriteLine("test");
    }
}

This is especially advantageous in cases where nested namespaces combined with long class names make it tedious to write code.