Organize code with namespaces

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Once your program has more than a handful of methods, keeping everything in a single file becomes hard to navigate. C# helps you organize code with namespaces, and reuse code from other files and libraries with the using directive.

Using code from the .NET class library

Many of the methods you've already used, like Console.WriteLine(), come from the .NET Class Library, organized into namespaces. The System namespace, for example, contains fundamental types like Console and Random.

Some namespaces are available without an explicit using directive, but others need to be brought in before you can use their types.

using System.Text;

StringBuilder builder = new StringBuilder();
builder.Append("Hello, ");
builder.Append("world!");
Console.WriteLine(builder.ToString()); // Output: Hello, world!

The using System.Text; directive brings the StringBuilder type into scope, so you can refer to it by its short name instead of its full name, System.Text.StringBuilder.

Organizing your own code with namespaces

A namespace groups related classes and methods under a shared name, which helps you avoid naming collisions as your project grows.

namespace MathUtilities
{
    class Calculator
    {
        public static int Add(int a, int b)
        {
            return a + b;
        }
    }
}

From another file in the same project, you bring that namespace into scope with using, then call its method through the class name.

using MathUtilities;

Console.WriteLine(Calculator.Add(2, 3)); // Output: 5

Using an alias

If a namespace or type name is long, or clashes with another name, you can give it a shorter alias with using and =.

using Calc = MathUtilities.Calculator;

Console.WriteLine(Calc.Add(2, 3)); // Output: 5

Why namespaces matter

• They keep related code grouped under one name, so large projects stay organized.
• They prevent naming collisions between your code and code from other libraries.
• They make it clear where a type or method comes from, just by reading a file's using directives.