Top-Level Programs in C# 9

Earlier I blogged about two new features in C# 9: C# 9 Init-Only Properties and C# 9 Record Type. Another feature in C# 9 that I didn't originally plan on writing about is top-level programs. However, top-level programs do simplify sharing sample code on a blog by removing boilerplate code and nesting. Therefore, I'll probably be using it quite a bit on my blog.

What are Top-Level Programs in C# 9?

As mentioned above, top-level programs in C# 9 reduce the boilerplate code that you see in C# programs. For example, when you create a C# Console Application it comes with the following "hello world" code.

using System;

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

With the new top-level programs feature in C# 9, this code can be reduced to the following.

using System;

Console.WriteLine("Hello World!");

That's a heck of a lot easier to read and share on my blog!

Although it's not as obvious, you still have access to any arguments passed to the application.

using System;

string name = args.Length > 0 ? args[0] : "World!";

Console.WriteLine($"Hello {name}!");

And, of course, you can still create methods (which look more like functions now) and access them in top-level programs in C# 9.

using System;

Console.WriteLine(SayHello("World"));

string SayHello(string name)
{
    return $"Hello {name}!";
}

When using top-level programs, the top-level statements must come after any using statements and before any namespace and type declarations. Therefore, in the example below, the type declaration for Person must come after the top-level statements.

using System;

var person = new Person("John Doe", 18);

Console.WriteLine(person);

record Person(string Name, int Age) { }

As you can see, the new top-level programs feature in C# 9 is great for sharing code on a blog when you want to reduce the noise of boilerplate code and the amount of nesting. I'll be using this feature quite a bit in the future to share sample code on my blog.