[Performance] ProjectTagViolationDetector reads and parses XML file for every reference × rule combination

[mfogliatto]

Description

ProjectTagViolationDetector.GetViolationsFrom() calls this.projectTagProvider.GetProjectTag(referenceContext.Reference) inside a nested loop (for each rule × each reference). ProjectTagProvider.GetProjectTag() loads and parses the XML project file from disk on every call.

Affected Files

• src/ReferenceCop/Detectors/ProjectTagViolationDetector.cs (line ~30, inside nested loop)
• src/ReferenceCop/Providers/ProjectTagProvider.cs (entire GetProjectTag method — XDocument.Load() per call)

Impact

For a solution with R rules and N project references:

• R × N file I/O operations and XML parses, even though the same project files are read repeatedly
• For a typical solution (e.g., 20 rules, 50 references), this means 1,000 XML file parses per build, most of them redundant
• Build-time analyzer, so this directly impacts developer inner loop

Suggested Optimization

Cache the project tag per file path:

public class ProjectTagProvider : IProjectTagProvider
{
    private readonly ConcurrentDictionary<string, string> _cache = new();

    public string GetProjectTag(string projectFilePath)
    {
        return _cache.GetOrAdd(projectFilePath, path =>
        {
            if (!File.Exists(path)) return UnknownProjectTag;
            var projectFile = XDocument.Load(path);
            return projectFile
                .Descendants(PropertyGroupNode)
                .Elements(ProjectTagNode)
                .FirstOrDefault()?.Value ?? UnknownProjectTag;
        });
    }
}

This reduces N×R file reads to at most N (one per unique project file), typically much fewer.