using System;
using System.Threading;
using System.Threading.Tasks;
using NServiceBus;

class SerializedHandler : IHandleMessages<MyMessage>
{
    static readonly SemaphoreSlim sequentialAccess = new SemaphoreSlim(1);
    static readonly TimeSpan waitDuration = TimeSpan.FromSeconds(5);

    public async Task Handle(OrderReceived message, IMessageHandlerContext context)
    {
        try
        {
            var success = await sequentialAccess.WaitAsync(waitDuration)
                .ConfigureAwait(false);

            if (!success) throw new InvalidOperationException($"Could not acquire access within {waitDuration}");

            // Do my stuff here
        }
        finally
        {
            sequentialAccess.Release();
        }
    }
}