$CSharpCode = @"
    using System;
    
    public class HelloWorld
    {
        public static void SayHello()
        {
            Console.WriteLine("Hello, World!");
        }
    }
    "@

    Add-Type -TypeDefinition $CSharpCode -Language CSharp
    [HelloWorld]::SayHello()

There's your scriptable-in-PowerShell C# ;-)