ListBoxTarget.cs

using NLog;
using NLog.Targets;
using System;
using System.Collections.Generic;
using System.Windows.Forms;

namespace LogViewer
{
    [Target("ListBoxTarget")]

    public sealed class ListBoxTarget : TargetWithLayout
    {
        public static ListBox ListBoxControl { get; set; }

        public static List<LogEventInfo> AllLogs { get; } = new List<LogEventInfo>();

        public static bool autoScroll { get; set; } = true;

        protected override void Write(LogEventInfo logEvent)
        {
            AllLogs.Add(logEvent);

            if (ListBoxControl != null)
            {
                string message = this.Layout.Render(logEvent);

                ListBoxControl.Invoke((MethodInvoker)(() =>
                {
                    ListBoxControl.Items.Add(message);
                    if (autoScroll)
                    {
                        ListBoxControl.TopIndex = ListBoxControl.Items.Count - 1;
                    }
                }));
            }
        }
    }
}