I’ve had to work on some C# features for work lately, so I wanted to take some time to write down some of the weird things that I’ve learned. C# allows you to develop an application to be run in .NET.
I’ve got a mild case of Python brain-rot that made it really difficult to wrap my head around the types in C#. But, once we’ve got a Program.cs file going, we can create a variety of
public static functions to handle command-line arguments. This is based off of the “task-based asynchronous pattern” that’s explained in further detail here.
├── program.cs
├── inputs.cs
├── service.cs
Let’s go through these one-by-one.
Inputs
This file we can use to define an interface and a class for the file input that we want. For me, I want to use a CSV file to parse inputs from in order to access a bucket on S3.
namespace FileDownload;
public class FileInput
{
public string InputCSV
{
get;
set;
}
}
If we run this program
dotnet run FileDownload Example.csv
The “Example.csv” portion of this string will be captured with this class declaration, which we can then use in the next file we want, service.cs
Service
namespace FileDownload;
public class FileService
{
private readonly FileInput _input;
public FileService(FileInput input)
{
_input = input;
}
public async Task StartAsync()
{
if (!File.Exists(_input.InputCSV))
{
await _consoleWriter.WriteLine("Please pass a CSV file.");
return;
}
}
public Task StopAsync()
{
return Task.CompletedTask;
}
}
So, now we can do something with the new FileInput class and define the logic around what we want to do with the command-line argument.
Program
Finally, this brings us back to the Program.cs file and our public static async Task Main(string[] args) function.
We want to define a couple things within this main function, in accordance to the task-based pattern.
- An object that utilizes a function for the program inputs
- Another object that utilizes a function that “builds” the program
- A call to StartAsync within FileService
public static async Task Main(string[] args)
{
var settings = CreateApplicationSettings(args, null, null);
var builder = CreateHostApplicationBuilder(settings);
var host = builder.Build();
await host.StartAsync();
}
public static HostApplicationBuilderSettings CreateApplicationSettings(string[] args)
{
// This allows for faster start up, but you needs to setup more things.
var settings = new HostApplicationBuilderSettings
{
DisableDefaults = true,
Args = args,
ApplicationName = ApplicationName
};
return settings;
}
public static HostApplicationBuilder CreateHostApplicationBuilder(HostApplicationBuilderSettings settings)
{
var builder = Host.CreateApplicationBuilder(settings);
var applicationInputs = CreateApplicationInputs(settings.Args);
builder.Services.AddLogging();
builder.Configuration.AddEnvironmentVariables();
builder.Services.AddSingleton(applicationInputs);
builder.Services.AddHostedService<FileService>();
return builder;
}
private static FileDownloaderInput CreateApplicationInputs(string[]? args)
{
var fileArgument = new Argument<string>("file", "CSV Input File");
var inputs = new FileInput();
return inputs;
}