Reporting an error from Roslyn analyzer

AnalyzeSymbol.cs

private static void AnalyzeSymbol(SymbolAnalysisContext context)
{
    var classSymbol = context.Symbol as INamedTypeSymbol;
    if (classSymbol?.TypeKind != TypeKind.Class)
        return; //we only need classes, not other types like enum, struct, etc.
    var members = GetMembersFromClass(classSymbol); //retrieve only methods and properties
    var unorderedMember = GetUnorderedMember(members); //then find the first member that is out of order
    if (unorderedMember != null)
    {
        var rule = new DiagnosticDescriptor(DiagnosticId, Title, $"Move {unorderedMember.Name} lower in the file", Category,
            DiagnosticSeverity.Error, isEnabledByDefault: true, description: Description);
        //now we're creating a rule that tells the user to move the member lower in the file
        var diagnostic = Diagnostic.Create(rule, unorderedMember.Locations[0], classSymbol);
        context.ReportDiagnostic(diagnostic);
    }
}