怎么样给c#控件RichTextBox扩展？

以下是关于扩展插入图片的代码：由于其他功能和本身提供的一样故在这里就不再贴代码了，只是把其中的关于插入图片的封装贴出来。

#region Insert Image

/// <summary>
/// Inserts an image into the RichTextBox. The image is wrapped in a Windows
/// Format Metafile, because although Microsoft discourages the use of a WMF,
/// the RichTextBox (and even MS Word), wraps an image in a WMF before inserting
/// the image into a document. The WMF is attached in HEX format (a string of
/// HEX numbers).
///
/// The RTF Specification v1.6 says that you should be able to insert bitmaps,
/// .jpegs, .gifs, .pngs, and Enhanced Metafiles (.emf) directly into an RTF
/// document without the WMF wrapper. This works fine with MS Word,
/// however, when you don't wrap images in a WMF, WordPad and
/// RichTextBoxes simply ignore them. Both use the riched20.dll or msfted.dll.
/// </summary>
/// <remarks>
/// NOTE: The image is inserted wherever the caret is at the time of the call,
/// and if any text is selected, that text is replaced.
/// </remarks>
/// <param name="_image"></param>
public void InsertImage(Image _image)
{

StringBuilder _rtf = new StringBuilder();

// Append the RTF header
_rtf.Append(RTF_HEADER);

// Create the font table using the RichTextBox's current font and append
// it to the RTF string
_rtf.Append(GetFontTable(this.Font));

// Create the image control string and append it to the RTF string
_rtf.Append(GetImagePrefix(_image));

// Create the Windows Metafile and append its bytes in HEX format
_rtf.Append(GetRtfImage(_image));

// Close the RTF image control string
_rtf.Append(RTF_IMAGE_POST);

this.SelectedRtf = _rtf.ToString();
}

/// <summary>
/// Creates the RTF control string that describes the image being inserted.
/// This des cription (in this case) specifies that the image is an
/// MM_ANISOTROPIC metafile, meaning that both X and Y axes can be scaled
/// independently. The control string also gives the images current dimensions,
/// and its target dimensions, so if you want to control the size of the
/// image being inserted, this would be the place to do it. The prefix should
/// have the form ...
///
/// {/pict/wmetafile8/picw[A]/pich[B]/picwgoal[C]/pichgoal[D]
///
/// where ...
///
/// A = current width of the metafile in hundredths of millimeters (0.01mm)
/// = Image Width in Inches * Number of (0.01mm) per inch
/// = (Image Width in Pixels / Graphics Context's Horizontal Resolution) * 2540
/// = (Image Width in Pixels / Graphics.DpiX) * 2540
///
/// B = current height of the metafile in hundredths of millimeters (0.01mm)
/// = Image Height in Inches * Number of (0.01mm) per inch
/// = (Image Height in Pixels / Graphics Context's Vertical Resolution) * 2540
/// = (Image Height in Pixels / Graphics.DpiX) * 2540
///
/// C = target width of the metafile in twips
/// = Image Width in Inches * Number of twips per inch
/// = (Image Width in Pixels / Graphics Context's Horizontal Resolution) * 1440
/// = (Image Width in Pixels / Graphics.DpiX) * 1440
///
/// D = target height of the metafile in twips
/// = Image Height in Inches * Number of twips per inch
/// = (Image Height in Pixels / Graphics Context's Horizontal Resolution) * 1440
/// = (Image Height in Pixels / Graphics.DpiX) * 1440
///
/// </summary>
/// <remarks>
/// The Graphics Context's resolution is simply the current resolution at which
/// windows is being displayed. Normally it's 96 dpi, but instead of assuming
/// I just added the code.
///
/// According to Ken Howe at pbdr.com, "Twips are screen-independent units
/// used to ensure that the placement and proportion of screen elements in
/// your screen application are the same on all display systems."
///
/// Units Used
/// ----------
/// 1 Twip = 1/20 Point
/// 1 Point = 1/72 Inch
/// 1 Twip = 1/1440 Inch
///
/// 1 Inch = 2.54 cm
/// 1 Inch = 25.4 mm
/// 1 Inch = 2540 (0.01)mm
/// </remarks>
/// <param name="_image"></param>
/// <returns></returns>
private string GetImagePrefix(Image _image)
{

StringBuilder _rtf = new StringBuilder();

// Calculate the current width of the image in (0.01)mm
int picw = (int)Math.Round((_image.Width / xDpi) * HMM_PER_INCH);

// Calculate the current height of the image in (0.01)mm
int pich = (int)Math.Round((_image.Height / yDpi) * HMM_PER_INCH);

// Calculate the target width of the image in twips
int picwgoal = (int)Math.Round((_image.Width / xDpi) * TWIPS_PER_INCH);

// Calculate the target height of the image in twips
int pichgoal = (int)Math.Round((_image.Height / yDpi) * TWIPS_PER_INCH);

// Append values to RTF string
_rtf.Append(@"{/pict/wmetafile8");
_rtf.Append(@"/picw");
_rtf.Append(picw);
_rtf.Append(@"/pich");
_rtf.Append(pich);
_rtf.Append(@"/picwgoal");
_rtf.Append(picwgoal);
_rtf.Append(@"/pichgoal");
_rtf.Append(pichgoal);
_rtf.Append(" ");

return _rtf.ToString();
}

/// <summary>
/// Use the EmfToWmfBits function in the GDI+ specification to convert a
/// Enhanced Metafile to a Windows Metafile
/// </summary>
/// <param name="_hEmf">
/// A handle to the Enhanced Metafile to be converted
/// </param>
/// <param name="_bufferSize">
/// The size of the buffer used to store the Windows Metafile bits returned
/// </param>
/// <param name="_buffer">
/// An array of bytes used to hold the Windows Metafile bits returned
/// </param>
/// <param name="_mappingMode">
/// The mapping mode of the image. This control uses MM_ANISOTROPIC.
/// </param>
/// <param name="_flags">
/// Flags used to specify the format of the Windows Metafile returned
/// </param>
[DllImportAttribute("gdiplus.dll")]
private static extern uint GdipEmfToWmfBits(IntPtr _hEmf, uint _bufferSize,
byte[] _buffer, int _mappingMode, EmfToWmfBitsFlags _flags);

/// <summary>
/// Wraps the image in an Enhanced Metafile by drawing the image onto the
/// graphics context, then converts the Enhanced Metafile to a Windows
/// Metafile, and finally appends the bits of the Windows Metafile in HEX
/// to a string and returns the string.
/// </summary>
/// <param name="_image"></param>
/// <returns>
/// A string containing the bits of a Windows Metafile in HEX
/// </returns>
private string GetRtfImage(Image _image)
{

StringBuilder _rtf = null;

// Used to store the enhanced metafile
MemoryStream _stream = null;

// Used to create the metafile and draw the image
Graphics _graphics = null;

// The enhanced metafile
Metafile _metaFile = null;

// Handle to the device context used to create the metafile
IntPtr _hdc;

try
{
_rtf = new StringBuilder();
_stream = new MemoryStream();

// Get a graphics context from the RichTextBox
using (_graphics = this.CreateGraphics())
{

// Get the device context from the graphics context
_hdc = _graphics.GetHdc();

// Create a new Enhanced Metafile from the device context
_metaFile = new Metafile(_stream, _hdc);

// Release the device context
_graphics.ReleaseHdc(_hdc);
}

// Get a graphics context from the Enhanced Metafile
using (_graphics = Graphics.FromImage(_metaFile))
{

// Draw the image on the Enhanced Metafile
_graphics.DrawImage(_image, new Rectangle(0, 0, _image.Width, _image.Height));

}

// Get the handle of the Enhanced Metafile
IntPtr _hEmf = _metaFile.GetHenhmetafile();

// A call to EmfToWmfBits with a null buffer return the size of the
// buffer need to store the WMF bits. Use this to get the buffer
// size.
uint _bufferSize = GdipEmfToWmfBits(_hEmf, 0, null, MM_ANISOTROPIC,
EmfToWmfBitsFlags.EmfToWmfBitsFlagsDefault);

// Create an array to hold the bits
byte[] _buffer = new byte[_bufferSize];

// A call to EmfToWmfBits with a valid buffer copies the bits into the
// buffer an returns the number of bits in the WMF.
uint _convertedSize = GdipEmfToWmfBits(_hEmf, _bufferSize, _buffer, MM_ANISOTROPIC,
EmfToWmfBitsFlags.EmfToWmfBitsFlagsDefault);

// Append the bits to the RTF string
for (int i = 0; i < _buffer.Length; ++i)
{
_rtf.Append(String.Format("{0:X2}", _buffer[i]));
}

return _rtf.ToString();
}
finally
{
if (_graphics != null)
_graphics.Dispose();
if (_metaFile != null)
_metaFile.Dispose();
if (_stream != null)
_stream.Close();
}
}

#endregion