Is this piece of code where I lock a part of the function correct? Or can it have use drawbacks when multiple sessions ask concurrently for the same Exam?

Purpose is that client that first asks for the Exam will assemble it, all next clients will get the cached version.

public Exam GetExamByExamDto(ExamDTO examDto, int languageId)
{
    Log.Warn("GetExamByExamDto");
    lock (LockString)
    {
        if (!ContainsExam(examDto.id, languageId))
        {
            Log.Warn("Assembling ExamDto");
            var examAssembler = new ExamAssembler();
            var exam = examAssembler.createExam(examDto);

            if (AddToCache(exam))
            {
                _examDictionary.Add(examDto.id + "_" + languageId, exam);
            }
            Log.Warn("Returning non cached ExamDto");
            return exam;
        }
    }
    Log.Warn("Returning cached ExamDto");
    return _examDictionary[examDto.id + "_" + languageId];
}

I have a feeling that this isn't the way to do it.