How to automatically run one CS file on Visual Studio Code using “CMD + B”?

I have a folder called z-lib structured as:

z-lib
   cpp
   java
   python
   csharp
         a.cs
         ...

I want to run a.cs file using CMD + B keyboard combo on VS Code (macbook).

Tried

In my settings.json, I have so far set up:

"csharp": "dotnet build $dir$fileName && dotnet run $dir$fileName",

which returns:

MSBuild version 17.9.8+b34f75857 for .NET
/Users/4a/Downloads/z-lib/csharp/a.cs(1,1): error MSB4025: The project file could not be loaded. Data at the root level is invalid. Line 1, position 1.

Build FAILED.

/Users/4a/Downloads/z-lib/csharp/a.cs(1,1): error MSB4025: The project file could not be loaded. Data at the root level is invalid. Line 1, position 1.
    0 Warning(s)
    1 Error(s)

Time Elapsed 00:00:00.04

also tried:

    // "csharp": "cd $dir && dotnet run --project $dir$fileName",
    // "csharp": "cd $dir && dotnet build $fileName && dotnet run $fileName",
    // "csharp": "dotnet build $fileName && dotnet run $fileNameWithoutExt",

a.cs

using System;

public class b {
    public static void Main() 
    {
        Console.WriteLine("Hello world!");
    }
}

How do I automatically run a cs file using CMD + B? What do I change in the settings.json?