这是我过去使用过的RichTextBox。它是从Stack Overflow和整个互联网上的代码拼接而成的：

public class RichBox : RichTextBox {
  private const UInt32 CFM_BOLD = 0x00000001;
  private const UInt32 CFM_ITALIC = 0x00000002;
  private const UInt32 CFM_UNDERLINE = 0x00000004;
  private const UInt32 CFM_STRIKE = 0x00000008;
  private const UInt32 CFM_FACE = 0x20000000;
  private const UInt32 CFM_SIZE = 0x80000000;

  private const int WM_PAINT = 0xF;
  private const int WM_SETREDRAW = 0xB;
  private const int WM_USER = 0x400;

  private const int EM_SETCHARFORMAT = (WM_USER + 68);
  private const int SCF_SELECTION = 0x0001;
  private const int EM_GETEVENTMASK = WM_USER + 59;
  private const int EM_SETEVENTMASK = WM_USER + 69;
  private const int EM_GETSCROLLPOS = WM_USER + 221;
  private const int EM_SETSCROLLPOS = WM_USER + 222;

  [StructLayout(LayoutKind.Sequential)]
  private struct CHARFORMAT {
    public int cbSize;
    public uint dwMask;
    public uint dwEffects;
    public int yHeight;
    public int yOffset;
    public int crTextColor;
    public byte bCharSet;
    public byte bPitchAndFamily;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 32)]
    public char[] szFaceName;
    public short wWeight;
    public short sSpacing;
    public int crBackColor;
    public int LCID;
    public uint dwReserved;
    public short sStyle;
    public short wKerning;
    public byte bUnderlineType;
    public byte bAnimation;
    public byte bRevAuthor;
  }

  [DllImport("kernel32.dll", CharSet = CharSet.Auto)]
  private static extern IntPtr LoadLibrary(string lpFileName);

  [DllImport("user32", CharSet = CharSet.Auto)]
  private static extern int SendMessage(IntPtr hWnd, int msg, int wParam, ref CHARFORMAT lParam);

  [DllImport("user32.dll")]
  private static extern IntPtr SendMessage(IntPtr hWnd, Int32 wMsg, Int32 wParam, ref Point lParam);

  [DllImport("user32.dll")]
  private static extern IntPtr SendMessage(IntPtr hWnd, Int32 wMsg, Int32 wParam, IntPtr lParam);

  private bool frozen = false;
  private Point lastScroll = Point.Empty;
  private IntPtr lastEvent = IntPtr.Zero;
  private int lastIndex = 0;
  private int lastWidth = 0;

  protected override CreateParams CreateParams {
    get {
      var cp = base.CreateParams;
      if (LoadLibrary("msftedit.dll") != IntPtr.Zero) {
        cp.ClassName = "RICHEDIT50W";
      }
      return cp;
    }
  }

  [Browsable(false)]
  [DefaultValue(typeof(bool), "False")]
  [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
  public bool FreezeDrawing {
    get { return frozen; }
    set {
      if (value != frozen) {
        frozen = value;
        if (frozen) {
          this.SuspendLayout();
          SendMessage(this.Handle, WM_SETREDRAW, 0, IntPtr.Zero);
          SendMessage(this.Handle, EM_GETSCROLLPOS, 0, ref lastScroll);
          lastEvent = SendMessage(this.Handle, EM_GETEVENTMASK, 0, IntPtr.Zero);
          lastIndex = this.SelectionStart;
          lastWidth = this.SelectionLength;
        } else {
          this.Select(lastIndex, lastWidth);
          SendMessage(this.Handle, EM_SETEVENTMASK, 0, lastEvent);
          SendMessage(this.Handle, EM_SETSCROLLPOS, 0, ref lastScroll);
          SendMessage(this.Handle, WM_SETREDRAW, 1, IntPtr.Zero);
          this.Invalidate();
          this.ResumeLayout();
        }
      }
    }
  }

  [Browsable(false)]
  [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
  public Font CurrentFont {
    get {
      Font result = this.Font;
      if (this.SelectionLength == 0) {
        result = SelectionFont;
      } else {
        using (RichBox rb = new RichBox()) {
          rb.FreezeDrawing = true;
          rb.SelectAll();
          rb.SelectedRtf = this.SelectedRtf;
          rb.Select(0, 1);
          result = rb.SelectionFont;
        }
      }
      return result;
    }
  }

  [Browsable(false)]
  [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
  public string SelectionFontName {
    get { return CurrentFont.FontFamily.Name; }
    set {
      CHARFORMAT cf = new CHARFORMAT();
      cf.cbSize = Marshal.SizeOf(cf);
      cf.szFaceName = new char[32];
      cf.dwMask = CFM_FACE;
      value.CopyTo(0, cf.szFaceName, 0, Math.Min(31, value.Length));
      IntPtr lParam = Marshal.AllocCoTaskMem(Marshal.SizeOf(cf));
      Marshal.StructureToPtr(cf, lParam, false);
      SendMessage(this.Handle, EM_SETCHARFORMAT, SCF_SELECTION, lParam);
    }
  }

  [Browsable(false)]
  [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
  public float SelectionFontSize {
    get { return CurrentFont.Size; }
    set {
      CHARFORMAT cf = new CHARFORMAT();
      cf.cbSize = Marshal.SizeOf(cf);
      cf.dwMask = CFM_SIZE;
      cf.yHeight = Convert.ToInt32(value * 20);
      IntPtr lParam = Marshal.AllocCoTaskMem(Marshal.SizeOf(cf));
      Marshal.StructureToPtr(cf, lParam, false);
      SendMessage(this.Handle, EM_SETCHARFORMAT, SCF_SELECTION, lParam);
    }
  }

  [Browsable(false)]
  [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
  public bool SelectionBold {
    get { return CurrentFont.Bold; }
    set {
      CHARFORMAT cf = new CHARFORMAT();
      cf.cbSize = Marshal.SizeOf(cf);
      cf.dwMask = CFM_BOLD;
      cf.dwEffects = value ? CFM_BOLD : 0;
      IntPtr lParam = Marshal.AllocCoTaskMem(Marshal.SizeOf(cf));
      Marshal.StructureToPtr(cf, lParam, false);
      SendMessage(this.Handle, EM_SETCHARFORMAT, SCF_SELECTION, lParam);
    }
  }

  [Browsable(false)]
  [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
  public bool SelectionItalic {
    get { return CurrentFont.Italic; }
    set {
      CHARFORMAT cf = new CHARFORMAT();
      cf.cbSize = Marshal.SizeOf(cf);
      cf.dwMask = CFM_ITALIC;
      cf.dwEffects = value ? CFM_ITALIC : 0;
      IntPtr lParam = Marshal.AllocCoTaskMem(Marshal.SizeOf(cf));
      Marshal.StructureToPtr(cf, lParam, false);
      SendMessage(this.Handle, EM_SETCHARFORMAT, SCF_SELECTION, lParam);
    }
  }

  [Browsable(false)]
  [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
  public bool SelectionStrikeout {
    get { return CurrentFont.Strikeout; }
    set {
      CHARFORMAT cf = new CHARFORMAT();
      cf.cbSize = Marshal.SizeOf(cf);
      cf.dwMask = CFM_STRIKE;
      cf.dwEffects = value ? CFM_STRIKE : 0;
      IntPtr lParam = Marshal.AllocCoTaskMem(Marshal.SizeOf(cf));
      Marshal.StructureToPtr(cf, lParam, false);
      SendMessage(this.Handle, EM_SETCHARFORMAT, SCF_SELECTION, lParam);
    }
  }

  [Browsable(false)]
  [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
  public bool SelectionUnderline {
    get { return CurrentFont.Underline; }
    set {
      CHARFORMAT cf = new CHARFORMAT();
      cf.cbSize = Marshal.SizeOf(cf);
      cf.dwMask = CFM_UNDERLINE;
      cf.dwEffects = value ? CFM_UNDERLINE : 0;
      IntPtr lParam = Marshal.AllocCoTaskMem(Marshal.SizeOf(cf));
      Marshal.StructureToPtr(cf, lParam, false);
      SendMessage(this.Handle, EM_SETCHARFORMAT, SCF_SELECTION, lParam);
    }
  }
}

它添加了新的属性，例如SelectionBold，SelectionItalic等，您可以在其中应用属性而不会丢失其他格式的文本。