WCF web method with mutex

Question

I have a WCF service that processes transactions. It first generates a reference number based on the last reference number in the database + 1, then assists the reference number to the wcf request message it received, and then saves all the information in the request to the database.

I had an issue where duplicate reference numbers were generated, which was a typical race condition.

I implemented the following code to insure that only one request can generate a new reference number and save that to the database.

Will I be guaranteed that only ONE request will be able to execute the block of code within the mutex at a time?

    private static Mutex waitForDBInsert = new Mutex();

    protected override PostPaymentResponse PostPayment(PostPaymentRequest request)
    {
        var response = new PostPaymentResponse();
        try
        {
            // Save to transaction and payee to database
            if (request.PaymentDetails.PaymentID == 0)
            {
                waitForDBInsert.WaitOne();

                // Generate next available transactions reference number.
                response.PaymentDetails.ReferenceNumber = GenerateReferenceNumber(null);

                // Save transaction to database before processing.
                var saveToDbResponse = SaveFXPaymentToDatabase(request, response);
                waitForDBInsert.ReleaseMutex();
            }
        }
        catch (Exception ex)
        {
            if (!waitForDBInsert.SafeWaitHandle.IsClosed)
                waitForDBInsert.ReleaseMutex();
        }

        return response;
    }

Answer 1

Yes, if it's a global mutex. See the code for class SingleGlobalInstance posted in this SO thread for a great example: What is a good pattern for using a Global Mutex in C#?

Also, as already commented, you should also strongly consider using an auto-incremented column instead.