swapping out stdin with a provided file

reader.cs

using System.CommandLine;
using System.CommandLine.Parsing;
using System.Text;

var fileContent = new Argument<Stream?>("file")
{
    Arity = ArgumentArity.ZeroOrOne,
    DefaultValueFactory = OpenStream,
    Description = "The file to read from. Defaults to stdin if provided",
    CustomParser = OpenStream,
};

fileContent.Validators.Add(FileMustExist);
fileContent.Validators.Add(MustBeRedirected);

var encoding = new Option<System.Text.Encoding>("--encoding")
{
    Description = "The encoding to use when reading the file",
    DefaultValueFactory = (ar) => Encoding.UTF8,
    CustomParser = (ar) => Encoding.GetEncoding(ar.Tokens[0].Value),
};
encoding.AcceptOnlyFromAmong(Encoding.GetEncodings().Select(e => e.Name).ToArray());

var c = new RootCommand("read a file in a given encoding")
{ fileContent,
  encoding };

c.SetAction(async (ParseResult pr, CancellationToken ctok) =>
{
    var fileStream = pr.GetValue(fileContent)!; // will be enforced non-null by validations
    var enc = pr.GetValue(encoding)!;
    await WriteFileToOutput(fileStream, enc, pr.Configuration.Output, ctok);
    return 0;
});

var config = new CommandLineConfiguration(c);
config.ThrowIfInvalid();
config.EnablePosixBundling = false;
return await config.Parse(args).InvokeAsync();


async Task WriteFileToOutput(Stream contents, Encoding encoding, TextWriter output, CancellationToken ctok)
{
    using var inputReader = new StreamReader(contents, encoding);
    var input = await inputReader.ReadToEndAsync(ctok);
    await output.WriteAsync(input);

}

static void FileMustExist(ArgumentResult ar)
{
    if (ar.Tokens.Count == 1)
    {
        var filePath = ar.Tokens[0].Value;
        if (filePath == "-")
        {
            return;
        }
        else if (!File.Exists(filePath))
        {
            ar.AddError($"The file {filePath} does not exist");
        }
    }
}
static void MustBeRedirected(ArgumentResult ar)
{
    if (ar.Tokens.Count == 0 && !Console.IsInputRedirected)
    {
        ar.AddError($"When no file is provided, input must be redirected from stdin");
    }
}
static Stream? OpenStream(ArgumentResult ar)
{
    return ar.Tokens switch
    {
        [] when Console.IsInputRedirected => Console.OpenStandardInput(),
        [] => null,
        [{ Value: "-" }] => Console.OpenStandardInput(),
        [{ Value: var file }] => File.OpenRead(file),
        _ => throw new InvalidOperationException("Too many arguments"),
    };
}