Fix file-existence validation bypassed when --file is provided explicitly

[Copilot]

DefaultValueFactory only runs when no value is provided, so placing file-existence validation there silently bypasses it when the user supplies --file nofile, causing an unhandled FileNotFoundException instead of the expected error message.

Changes

• Stage3/Program.cs — Uses two properties together to correctly separate concerns:

  • DefaultValueFactory supplies only the default sampleQuotes.txt value when --file is omitted
  • CustomParser handles token→FileInfo conversion and file-existence validation when --file is explicitly provided; Required = true is removed since DefaultValueFactory provides the fallback

• get-started-tutorial.md — Updates the step explanation to accurately describe each mechanism using second-person voice: DefaultValueFactory supplies the default when you don't provide --file (and doesn't run when you do); CustomParser handles conversion and validation when you do provide it

Option<FileInfo> fileOption = new("--file")
{
    DefaultValueFactory = _ =&gt; new FileInfo("sampleQuotes.txt"),
    CustomParser = result =&gt;
    {
        var file = new FileInfo(result.Tokens.Single().Value);
        if (!file.Exists)
            result.AddError("File does not exist");
        return file;
    }
};

Fixes #52979.

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Internal previews

📄 File	🔗 Preview link
docs/standard/commandline/get-started-tutorial.md	docs/standard/commandline/get-started-tutorial

[Copilot]

Pull request overview

Fixes a validation gap in the System.CommandLine “get started” tutorial where file-existence checks placed in DefaultValueFactory were bypassed when --file is explicitly provided, resulting in an unhandled FileNotFoundException instead of a parse error.

Changes:

• Moves file-existence validation to fileOption.Validators.Add(...) so it runs for both default and user-supplied values.
• Updates the tutorial text to describe the split between defaulting/parsing and validation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
docs/standard/commandline/snippets/get-started-tutorial/csharp/Stage3/Program.cs	Refactors option setup by separating default value creation from file-existence validation via validators.
docs/standard/commandline/get-started-tutorial.md	Revises the tutorial explanation to match the intended validation mechanism.

[Vladimir-Bosnjak]

Checked this PR. This resolves the issue I reported. Validation is no longer tied to DefaultValueFactory, so it no longer gets bypassed when --file is provided. We now have working parse-time error.

Execution methods should still handle I/O failures independently but for tutorial illustration this fixes it.