Občas se hodí zobrazovat logy do vlastního okna, pokud nechceme řešit cmd v aplikaci Froms. Podařilo se mi pro tuto situaci najít výborný log4net appender pro richtextbox.

 
using log4net.Appender;
using log4net.Config;
using log4net.Core;
using log4net.Layout;
using log4net.Util;
using System;
using System.Collections.Generic;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

 /// <summary>
 /// Description of RichTextBoxAppender.
 /// http://stackoverflow.com/questions/2484822/richtextboxappender-using-log4net
 /// </summary>
 public class RichTextBoxAppender : AppenderSkeleton
 {
 #region Private Instance Fields
 private RichTextBox richTextBox = null;
 private Form containerForm = null;
 private LevelMapping levelMapping = new LevelMapping();
 private int maxTextLength = 100000;
 #endregion

 private delegate void UpdateControlDelegate(LoggingEvent loggingEvent);

 #region Constructor
 public RichTextBoxAppender(RichTextBox myRichTextBox) : base()
 {
 richTextBox = myRichTextBox;
 containerForm = (Form)richTextBox.Parent;
 }
 #endregion

 private void UpdateControl(LoggingEvent loggingEvent)
 {
 // There may be performance issues if the buffer gets too long
 // So periodically clear the buffer
 if (richTextBox.TextLength > maxTextLength)
 {
 richTextBox.Clear();
 richTextBox.AppendText(string.Format("(Cleared log length max: {0})\n", maxTextLength));
 }

 // look for a style mapping
 LevelTextStyle selectedStyle = levelMapping.Lookup(loggingEvent.Level) as LevelTextStyle;
 if (selectedStyle != null)
 {
 // set the colors of the text about to be appended
 richTextBox.SelectionBackColor = selectedStyle.BackColor;
 richTextBox.SelectionColor = selectedStyle.TextColor;

 // alter selection font as much as necessary
 // missing settings are replaced by the font settings on the control
 if (selectedStyle.Font != null)
 {
 // set Font Family, size and styles
 richTextBox.SelectionFont = selectedStyle.Font;
 }
 else if (selectedStyle.PointSize > 0 && richTextBox.Font.SizeInPoints != selectedStyle.PointSize)
 {
 // use control's font family, set size and styles
 float size = selectedStyle.PointSize > 0.0f ? selectedStyle.PointSize : richTextBox.Font.SizeInPoints;
 richTextBox.SelectionFont = new Font(richTextBox.Font.FontFamily.Name, size, selectedStyle.FontStyle);
 }
 else if (richTextBox.Font.Style != selectedStyle.FontStyle)
 {
 // use control's font family and size, set styles
 richTextBox.SelectionFont = new Font(richTextBox.Font, selectedStyle.FontStyle);
 }
 }
 richTextBox.AppendText(RenderLoggingEvent(loggingEvent));
 //scroll to end?
 richTextBox.ScrollToCaret();
 }

 protected override void Append(LoggingEvent LoggingEvent)
 {
 if (richTextBox.InvokeRequired)
 {
 richTextBox.BeginInvoke(
 new UpdateControlDelegate(UpdateControl),
 new object[] { LoggingEvent });
 }
 else
 {
 UpdateControl(LoggingEvent);
 }
 }

 public void AddMapping(LevelTextStyle mapping)
 {
 levelMapping.Add(mapping);
 }

 public override void ActivateOptions()
 {
 base.ActivateOptions();
 levelMapping.ActivateOptions();
 }

 protected override bool RequiresLayout { get { return true; } }
 }

 public class LevelTextStyle : LevelMappingEntry
 {
 private Color textColor;
 private Color backColor;
 private FontStyle fontStyle = FontStyle.Regular;
 private float pointSize = 0.0f;
 private bool bold = false;
 private bool italic = false;
 private string fontFamilyName = null;
 private Font font = null;

 public bool Bold { get { return bold; } set { bold = value; } }
 public bool Italic { get { return italic; } set { italic = value; } }
 public float PointSize { get { return pointSize; } set { pointSize = value; } }

 /// <summary>
 /// Initialize the options for the object
 /// </summary>
 /// <remarks>Parse the properties</remarks>
 public override void ActivateOptions()
 {
 base.ActivateOptions();
 if (bold) fontStyle |= FontStyle.Bold;
 if (italic) fontStyle |= FontStyle.Italic;

 if (fontFamilyName != null)
 {
 float size = pointSize > 0.0f ? pointSize : 8.25f;
 try
 {
 font = new Font(fontFamilyName, size, fontStyle);
 }
 catch (Exception)
 {
 font = new Font("Arial", 8.25f, FontStyle.Regular);
 }
 }
 }

 public Color TextColor { get { return textColor; } set { textColor = value; } }
 public Color BackColor { get { return backColor; } set { backColor = value; } }
 public FontStyle FontStyle { get { return fontStyle; } set { fontStyle = value; } }
 public Font Font { get { return font; } set { font = value; } }
 }