erica1.cs

namespace EricaRepro
{
    using System;
    using System.Text;
    using System.Threading.Tasks;

    public class MessageForwarder
    {
        public bool AlreadyForwardedHeader { get; private set; } = false;
        public Func<byte[], Task> Forwader { get; private set; }
        private readonly Func<string, Task> _logger;

        public MessageForwarder(Func<byte[], Task> forwader, Func<string, Task> logger = null)
        {
            this.Forwader = forwader;
            this._logger = logger;
        }

        private Task log(string message)
        {
            if (_logger == null)
            {
                return Task.CompletedTask;
            }

            return _logger(message);
        }

        public async Task ReceiveAsync(byte[] msg)
        {
            var msgSTring = Encoding.UTF8.GetString(msg);
            await log($"Received {msgSTring}");
            if (msgSTring.Split(',')[0].Contains("header"))
            {
                if (this.AlreadyForwardedHeader)
                {
                    return;
                }
                else
                {
                    this.AlreadyForwardedHeader = true;
                }
            }
            await this.Forwader(msg);
            await log($"Forwarded message {msgSTring}");
        }
    }
}

namespace UnitTestProject1
{
    using System;
    using System.Text;
    using System.Threading.Tasks;
    using Microsoft.VisualStudio.TestTools.UnitTesting;
    using EricaRepro;

    [TestClass]
    public class UnitTest1
    {
        [TestMethod]
        public async Task TestMethod1Async()
        {
            byte[] msg = null;
            var fw1 = new MessageForwarder(forwader: async (bytes) =>
            {
                await Task.Delay(0);
                msg = bytes;
            });

            Assert.IsFalse(fw1.AlreadyForwardedHeader);

            await fw1.ReceiveAsync(Encoding.UTF8.GetBytes("Hallo"));
            Assert.IsFalse(fw1.AlreadyForwardedHeader);

            await fw1.ReceiveAsync(Encoding.UTF8.GetBytes("header: 123"));
            Assert.IsTrue(fw1.AlreadyForwardedHeader);
        }

        [TestMethod]
        public async Task TestMethodWithLoggingAsync()
        {
            byte[] msg = null;
            var fw1 = new MessageForwarder(
                forwader: async (bytes) =>
                    {
                        await Task.Delay(0);
                        msg = bytes;
                    }, 
                logger: Console.Out.WriteLineAsync);

            Assert.IsFalse(fw1.AlreadyForwardedHeader);

            await fw1.ReceiveAsync(Encoding.UTF8.GetBytes("Hallo"));
            Assert.IsFalse(fw1.AlreadyForwardedHeader);

            await fw1.ReceiveAsync(Encoding.UTF8.GetBytes("header: 123"));
            Assert.IsTrue(fw1.AlreadyForwardedHeader);
        }
    }
}