Using SemaphoreSlim for async locking scenarios.

program.cs

private static readonly SemaphoreSlim __questionCloseLock = new SemaphoreSlim(initialCount: 1);

public async Task<OperationResult<QuestionCloseRecord>> CloseQuestionAsync(int questionId, QuestionCloseType closeType)
{
    try
    {
        await __questionCloseLock.WaitAsync().ConfigureAwait(false);

        Question question = await GetQuestionAsync(questionId).ConfigureAwait(false);
        if (question == null || question.CloseRecord != null)
        {
	return new OperationResult<QuestionCloseRecord>(false);
        }

        question.CloseRecord = new QuestionCloseRecord
        {
            ClosedOn = DateTimeOffset.Now,
            ClosedBy = closeType
        };
        
        _questionRepository.Edit(question);
         int result = await _questionRepository.SaveAsync().ConfigureAwait(false);

         return result > 0
	 ? new OperationResult<QuestionCloseRecord>(true) { Entity = question.CloseRecord }
	 : new OperationResult<QuestionCloseRecord>(false);
    }
    finally
    {
        __questionCloseLock.Release();
    }
}