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Top-level Statements in C#

akshay03 profile image Akshay Khot Originally published at akshaykhot.com ・2 min read

C# 9.0 introduced top-level statements that allow you to skip all the boilerplate code and get to the point quickly. No need to declare a namespace, class, or even a Main() method. Just open your editor and start writing beautiful C# code.

Writing a simple console program in C# has always involved a ton of ceremony. You have to create a class named Program. Then declare the Main method that is public, static, void, and takes an array of strings as arguments. Here is an example.

using System;

class Program
{
    static void Main(string[] args)
    {
        int sum = 45 + 50;
        Console.WriteLine(sum);
    }
}
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In the above example, there is too much clutter. There are only two lines that are actually doing something. When a beginner reads the above program, it can feel quite overwhelming.

  • What is System? Why are we using it?
  • What is a class? Where are the students?
  • What is static, why is it void?
  • What is args?

The essential part in the above program is the code that adds and displays the sum of two numbers. Rest is just ceremony.

In C# 9.0, you can simply write this:

int sum = 45 + 50;
System.Console.WriteLine(sum);
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Using sharplab, we can see how the C# compiler transforms this code before it's compiled. Notice that the Program class and Main method are still getting generated behind the scenes, but you don't have to write them.

using System;
using System.Diagnostics;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Security;
using System.Security.Permissions;

[CompilerGenerated]
internal static class <Program>$
{
    private static void <Main>$(string[] args)
    {
        int value = 95;
        Console.WriteLine(value);
    }
}
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What happens to the command-line arguments?

They are still accessible using the args array. You can run the following code using dotnet run hello, and it will print hello to the console.

System.Console.WriteLine(args[0]);
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How can I return a status code?

Just return it, as you normally would. For example,

System.Console.WriteLine("Hello");
return 0;
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Can I have multiple files with top-level statements?

No. Only one file in your application can use top-level statements. The compiler throws an error when it finds multiple files containing top-level statements.

Can I create classes in the same file?

Yes, the only requirement is that the top-level statements should come before any type declaration. For example,

using System;

var animal = new Animal { Name = "Dog" };
Console.WriteLine(animal.Name);

class Animal
{
    public string Name { get; set; }
}
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You can't create a class and then write your statements. For example, this code is invalid.

using System;

class Animal
{
    public string Name { get; set; }
}

var animal = new Animal { Name = "Dog" };
Console.WriteLine(animal.Name);
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It throws the following error.

error CS8803: Top-level statements must precede namespace and type declarations. 
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Top-level statements make C# beginner-friendly, giving it a scripty feel, like Ruby or Python. They also make simple programs clear and expressive. If you just want to understand a concept in C# or try out a base class library, you can quickly do so using the top-level statements. They are also great for small tools and utilities.

Did I miss anything? Let me know.

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