How to prevent Concurrency issues in ASP.NET Core Web API

Question

I tried Mutex but getting exception 'wait completed due to an abandoned mutex'.

How to prevent concurrency issues in ASP.NET Core Web API ?

In our API, we are fetching code after some calculations in other method but we have seen when there are multiple request at same time then the same code gets stored in db.How to overcome issues.

How to achieve that each request that comes at API will have unique code even if multiple request are their at same time? I tried mutex - here's the snippet

 `private static  readonly Mutex codeMutex = new Mutex();

        string code;
        codeMutex.WaitOne();
        try
        {
            code = await _repository.GetCode(id);
        }
        finally
        {
            codeMutex.ReleaseMutex();
        }
       
    }`