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by Jiří {x2} Činčura

Exploring top-level statements in C# 9

11 Jun 2020 4 mins C#, Roslyn

I wanted to know how is the top-level statements feature in C# 9 handled and what’s actually produced. Although I originally planned to do just a quick test and the look at the IL, I kept testing the feature more and more.

Elementary

I started with the simplest code to see what’s what.

using System;

Console.WriteLine("Test");

This results in fairly uncomplicated code.

[CompilerGenerated]
internal static class $Program
{
    private static void $Main(string[] args)
    {
        Console.WriteLine("Test");
    }
}

Explore

Can I return an exit code?

using System;

Console.WriteLine("Test");
return 1;
[CompilerGenerated]
internal static class $Program
{
    private static int $Main(string[] args)
    {
        Console.WriteLine("Test");
        return 1;
    }
}

Looks like I can. Hmm. Can I use async/await?

using System.Threading.Tasks;

await Task.CompletedTask;
return 1;
[CompilerGenerated]
internal static class $Program
{
    // async state machine omitted
    
    [AsyncStateMachine(typeof(<$Main>d__0))]
    private static Task<int> $Main(string[] args)
    {
        <$Main>d__0 stateMachine = default(<$Main>d__0);
        stateMachine.<>t__builder = AsyncTaskMethodBuilder<int>.Create();
        stateMachine.<>1__state = -1;
        stateMachine.<>t__builder.Start(ref stateMachine);
        return stateMachine.<>t__builder.Task;
    }

    private static int <Main>(string[] args)
    {
        return $Main(args).GetAwaiter().GetResult();
    }
}

OK, OK, that’s expected. But still nice that I’m not limited (yet) in what I can do with this feature. Or maybe I am?

What about local functions?

using System;

Test();

void Test()
{
    void Test2()
    {
    }    
    Console.WriteLine("Test");
    Test2();
}
[CompilerGenerated]
internal static class $Program
{
    private static void $Main(string[] args)
    {
        <$Main>g__Test|0_0();
    }

    internal static void <$Main>g__Test|0_0()
    {
        Console.WriteLine("Test");
        <$Main>g__Test2|0_1();
    }

    internal static void <$Main>g__Test2|0_1()
    {
    }
}

Nothing special. As “normal” local functions, these are just expanded into the class. Aah, class! Is it going to be nested or not?

using System;

Console.WriteLine(new Test());

class Test
{ }
[CompilerGenerated]
internal static class $Program
{
    private static void $Main(string[] args)
    {
        Console.WriteLine(new Test());
    }
}

internal class Test
{
}

Ah. Makes sense, not nested. That would create unnecessary complication with naming. What about variables?

using System;

var i = DateTime.UtcNow.Add(TimeSpan.FromSeconds(1));
Console.WriteLine(i);
Test();

void Test()
{
    Console.WriteLine(i);
}
[CompilerGenerated]
internal static class $Program
{
    [StructLayout(LayoutKind.Auto)]
    private struct <>c__DisplayClass0_0
    {
        public DateTime i;
    }

    private static void $Main(string[] args)
    {
        <>c__DisplayClass0_0 <>c__DisplayClass0_ = default(<>c__DisplayClass0_0);
        <>c__DisplayClass0_.i = DateTime.UtcNow.Add(TimeSpan.FromSeconds(1.0));
        Console.WriteLine(<>c__DisplayClass0_.i);
        <$Main>g__Test|0_0(ref <>c__DisplayClass0_);
    }

    internal static void <$Main>g__Test|0_0(ref <>c__DisplayClass0_0 P_0)
    {
        Console.WriteLine(P_0.i);
    }
}

Finally! Something interesting. The compiler is not creating instance field as I was expecting (and trying to force it), but it’s creating a struct (a value type) instead and passing it around using ref.

What about some unsafe code? Let’s start with “safe”-ish Span<T>.

using System;

Span<long> longs = stackalloc long[10];
Console.WriteLine(longs.IsEmpty);
[CompilerGenerated]
internal static class $Program
{
    private static void $Main(string[] args)
    {
        Console.WriteLine(stackalloc long[10].IsEmpty);
    }
}

Let’s switch to really unsafe code.

using System;

long* longs = stackalloc long[10];

This fails with error Pointers and fixed size buffers may only be used in an unsafe context aka CS0214. OK, looks like the compiler will not generate unsafe Main. But will it be OK if I create my own unsafe method?

using System;

Test();

unsafe void Test()
{
    fixed (char* value = "safe")  
    {  
      char* ptr = value;  
      while (*ptr != '\0')  
      {  
         Console.WriteLine(*ptr);  
         ++ptr;  
      }  
    }   
}
[CompilerGenerated]
internal static class $Program
{
    private static void $Main(string[] args)
    {
        <$Main>g__Test|0_0();
    }

    internal unsafe static void <$Main>g__Test|0_0()
    {
        fixed (char* ptr = &"safe".GetPinnableReference())
        {
            for (char* ptr2 = ptr; *ptr2 != 0; ptr2++)
            {
                Console.WriteLine(*ptr2);
            }
        }
    }
}

It will. Nice.

I think that’s enough playing.

Summary

I like how the compiler uses struct and ref passing to handle “global”/“instance” variables. That’s smart.

Although it looks like a small feature with almost like a “just take this code and smack it into generated Main“, it’s not that easy. There’s a lot of features in C# one can use. Luckily most of the features one can seamlessly use while using top-level statements.

Profile Picture Jiří Činčura is an independent developer focusing on data and business layers, language constructs, parallelism and databases. Specifically Entity Framework, asynchronous and parallel programming, cloud and Azure. He's Microsoft Most Valuable Professional and you can read his articles, guides, tips and tricks at www.tabsoverspaces.com.