misleading validation and inconsistent behavior for Option<FileInfo>

[Vladimir-Bosnjak]

Type of issue

Code doesn't work

Description

Command (passed as argument as per example):
quotes read --file nofile
Expected (according tutorial):
_File does not exist_ message in console
Actual Result:
FileNotFoundException thrown at runtime

Root cause:
DefaultValueFactory is used for validation in the sample but it is not consistently invoked for provided values.
FileInfo binding always succeeds new FileInfo(...) does not validate existence.
So validation is effectively bypassed.

The tutorial implies:

• custom parsing + validation happens in DefaultValueFactory
• parser will surface errors via result.AddError(...)

But in practice:

• validation is skipped for normal input
• errors appear only later during file access

return new FileInfo("sampleQuotes.txt"); does not validate existence, while the other branch does.
Is DefaultValueFactory really the proper place for validation then? Right now it appears that validation has to be moved out to a separate validation mechanism.

Page URL

https://learn.microsoft.com/en-us/dotnet/standard/commandline/get-started-tutorial

Content source URL

https://github.com/dotnet/docs/blob/main/docs/standard/commandline/get-started-tutorial.md

Document Version Independent Id

fa160b1d-66ed-8c5d-37cd-b7e02443e9f1

Platform Id

0f144444-1ce3-cdf7-87a6-c659d44e08d3

Article author

@gewarren

Metadata

• ID: 7705dc41-7297-0807-7e9c-2e6548c26b84
• PlatformId: 0f144444-1ce3-cdf7-87a6-c659d44e08d3
• Service: dotnet-fundamentals

Related Issues

[Vladimir-Bosnjak]

Solved, but the code in the tutorial and the tutorial itself has to be updated.
Reason:
Migration document states that the factory is only called when the option is not passed (i.e. --file is absent from the command line).
if --file is absent then: default FileInfo object will be passed (i.e. FileInfo("sampleQuotes.txt").
if --file is passed then: the entire factory action won't be triggered, effectively rendering the file-checking code dead code.

Solution:
Remove from the factory this:

       string filePath = result.Tokens.Single().Value;
       if (!File.Exists(filePath))
       {
           result.AddError("File does not exist"); 
           return null;
       }
       else
       {
           return new FileInfo(filePath);
       }

That's the dead code.

... and optionally implement validation here.

    internal static void ReadFile(FileInfo file)
    {
        foreach (string line in File.ReadLines(file.FullName))
        {
            Console.WriteLine(line);
        }
    }

So the Option definition becomes:

        Option<FileInfo> fileOption = new("--file")
        {
            Description = "An option whose argument is parsed as a FileInfo",
            Required = true,
            DefaultValueFactory = _ => new FileInfo("sampleQuotes.txt")            
        };

Required is defined within the scope of Option<T> yet its context is about the value bound to it (in this case a filepath following --file). Better would be to have something like this:
PresenceRequired -> the option token must appear
ValueRequired -> the option must resolve to a value

[gewarren]

This should be addressed by an in-progress PR: #52974. Can you take a look at that PR @Vladimir-Bosnjak?

[Vladimir-Bosnjak]

This should be addressed by an in-progress PR: #52974. Can you take a look at that PR @Vladimir-Bosnjak?

@gewarren PR #52974 fixes it.