C# Only one compilation unit can have top-level statements

Question

I just started learning c#, I created C# console application. To understand the concepts, I watched videos of how to setup vs code for c#

When I run the dotnet new console command in VS code terminal, it creates a new project including Program.cs file.

In the video, the Program.cs file appears like that

// Program.cs
using System;
namespace HelloWorld
{
  class Program
  {
    static string Main(string[] args)
    {
      Console.WriteLine("Hello, World!");
    }
  }
}

Program.cs in my IDE appears like,

// Program.cs
// See https://aka.ms/new-console-template for more information
Console.WriteLine("Hello, World!");

When I run the code using terminal dotnet run it runs perfectly on my computer.

when I create a new cs file, it contains

// hello.cs
Console.WriteLine("hello world");

after running it says Only one compilation unit can have top-level statements.

when I use class method and namespace like

// hello.cs
namespace helloworld
{
    class hello
    {
        static void Main()
        {
            Console.WriteLine("hello world");

        }
    }
}

it runs THE Program.cs file not the new file and shows this warning

PS C:\Users\User\C#projects> dotnet run hello.cs C:\Users\User\C#projects\hello.cs(5,21): warning CS7022: The entry point of the program is global code; ignoring 'hello.Main()' entry point. [C:\Users\User\C#projects\C#projects.csproj] Hello, World!

Project structure:

I tried another method by pressing run and debug and show nothing.

When I click on Generate c# Assets for Build and Debug button it shows this

Could not locate .NET Core project. Assets were not generated.

Answer 1

C# 9 feature: Top-level statements

This is a newly introduced feature in C# 9 called Top-level statements

The video to which you are referring might be using a lower version of C#(lower than C# 9). Where we use to get

namespace helloworld
{
    class hello
    {
        static void Main()
        {
            Console.WriteLine("hello world");  
        }
    }
}

as a default structure of Main program.

If you look closely you will find, only one line of code which prints a string to the console i.e.

Console.WriteLine("hello world");  

Top-level statements were introduced to remove unnecessary ceremony from this console application.

As you are using C#9 or higher, dot net run with top-level statement compiling your code successfully, but when you are replacing one-liner code to legacy structure, then compiler warns you regarding the global entry of Main function and Main() function which you added by replacing top-level statement.

To Get more clarity you can go through MSDN documentation: Top-level statements

---

Why you are getting an error "Only one compilation unit can have top-level statements."?

• As per the MSDN documentation, An application must have only one entry point.
• A project can have only one file with top-level statements.
• When you created a new file, you added a new top-level statement which result in below compile time error:

CS8802 Only one compilation unit can have top-level statements.

---

how to fix it?

• As per the above explanation, your project should not contain two or more top-level statements. To fix this error, you can delete a file that was added later on.

Answer 2

Please check that inside your project folder there aren't any other worker projects that have the same program.cs

Answer 3

Microsoft Documentation

Look into the documentation, it clearly says you cannot have top-level statements in multiple source files. Maybe you created a Program.cs file previously, deleted it and created another. The previous file would have been cached by VS Code. Close your VS Code and open it again. If this doesn't work, check the other .cs files.