Dataflow_Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllersWithViews();

    // Setup the DataFlow

    // CSV File Setup (do/check this one time at application start)
    if (!File.Exists("entries.csv"))
    {
        // If it doesn't exist, write the header line
        File.AppendAllLines("entries.csv", new[] { $"Email,Answer,IPAddress,Created(UTC)" });
    }

    // The Consumer Block - this is the block that receives an entry and write to the file
    var fileWriter = new ActionBlock<CompetitionEntry>((entry) =>
    {
        // When the object is received, put the new csv line together
        string newCsvLine = $"{entry.Email},{entry.Answer},{entry.IPAddress},{entry.Created:s}";
        File.AppendAllLines("entries.csv", new[] { newCsvLine });
    });

    // A simple buffer (one at a time), link this to the consumer above to create a "pipeline"
    var producer = new BufferBlock<CompetitionEntry>();
    producer.LinkTo(fileWriter);

    // Allow the controller to have the buffer injected
    services.AddSingleton(producer);
}