Register, grab usings, sort them, create diagnostic reports

UsingsAnalyzer.cs

public override void Initialize(AnalysisContext context)
{
    context.RegisterSyntaxTreeAction(AnalyzeTree);
}

private void AnalyzeTree(SyntaxTreeAnalysisContext obj)
{
    var fileName = obj.Tree.FilePath.Split('\\').Last();
    var syntaxNode = obj.Tree.GetRoot();
    var descendantNodes = syntaxNode.DescendantNodes();
    var usings = descendantNodes.OfType<UsingDirectiveSyntax>().ToList();
    if (usings.Count == 0)
        return;
    var sortedUsings = usings.OrderBy(o => new string(o.Name.ToString().TakeWhile(c => c != '.').ToArray()))
                                .ThenBy(o => o.Name.ToString().Length)
                                .ToList();
    for (var index = 0; index < usings.Count; index++)
    {
        var usingDirectiveSyntax = usings[index];
        if (usingDirectiveSyntax.ToString() != sortedUsings[index].ToString())
        {
            var diagnostic = Diagnostic.Create(Rule, sortedUsings[index].GetLocation(), fileName);
            obj.ReportDiagnostic(diagnostic);
        }
    }
}