Masui Masanori
Posted on December 18, 2021
Intro
From .NET 6, Program.cs is made with "Top-level statements".
In this time, I will try using "Top-level statements".
Environments
- .NET ver.6.0.101
Trying "Top-level statements" with default console project
First, I create a console project by "dotnet new console".
Program.cs
// See https://aka.ms/new-console-template for more information
Console.WriteLine("Hello, World!");
I decompile it by ilspy-vscode.
Program.cs
using System;
using System.Runtime.CompilerServices;
[CompilerGenerated]
internal class Program
{
private static void <Main>$(string[] args)
{
Console.WriteLine("Hello, World!");
}
}
From the result, a class and a main method were generated.
I can get command-line arguments from "args".
Program.cs
foreach(var a in args)
{
Console.WriteLine($"arcuments: {a}");
}
Console.WriteLine("Hello, World!");
namespace ?
Can I put the class in namespaces? No.
Program.cs
namespace TopLevelStatementSample;
// Compiling error
Console.WriteLine("Hello, World!");
Program.cs
namespace TopLevelStatementSample
{
// Compiling error
Console.WriteLine("Hello, World!");
}
error CS0116: A namespace cannot directly contain members such as fields, methods or statements
Of course I also can't put it into namespaces with curly braces.
Multiple files
Can I add one more files what is written by "Top-level statements"? No.
SampleClass.cs
// Compiling error
void Greet(string message)
{
Console.WriteLine("Hello sample class");
}
error CS8802: Only one compilation unit can have top-level statements.
Adding another main class
Can I add one more files what has static "Main" method? Yes.
SampleClass.cs
namespace TopLevelStatementSample;
public class SampleClass
{
public static void Main(string[] args)
{
Console.WriteLine("Hello sample class");
}
}
But I get a warning and "SampleClass" isn't treated as the main class.
warning CS7022: The entry point of the program is global code; ignoring 'SampleClass.Main(string[])' entry point.
Result
Hello, World!
Add classes, methods, and variables
I can add classes into Program.cs.
But I only can write it at the bottom of the file.
classes
Program.cs
using TopLevelStatementSample;
class Sample1
{
// can't write classes at the top of the file.
}
// Compiling error
Console.WriteLine("Hello, World!");
class Sample2
{
// can't write classes at the middle of the file.
}
// Compiling error
SampleClass.Main(new string[0]);
var s3 = new Sample3();
class Sample3
{
// OK.
}
methods and variables
I can write methods and variables at everywhere in the file.
Program.cs
using TopLevelStatementSample;
void Greet(string message)
{
Console.WriteLine(message);
}
// OK
var message = "Hello message";
// OK
Greet(message);
SampleClass.Main(new string[0]);
Because they are treated as global, other classes also can look them.
But they can't call any methods and variables in Program.cs.
SampleClass.cs
namespace TopLevelStatementSample;
public class SampleClass
{
public static void Main(string[] args)
{
// Compiling error
Greet("Hello sample class");
}
}
error CS8801: Cannot use local variable or local function 'Greet' declared in a top-level statement in this context.
Outro
Because "Top-level statements" are only for the entry point, they will only be created by "dotnet new" command.
I'm worried about implicit processing(ex. command-line arguments), but I think that the problem will disappear as you get used to it.
Resources
Posted on December 18, 2021
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