Copying an HTML-fragment to the clipboard

Applications like Word and Excel let you copy some text or tabular data to the clipboard in HTML-format, so that the formatting and visual style doesn’t change when pasted. At work, I had to do something similar: our application uses a lot of grid-controls to display data, and we wanted the users to be able to select data from a grid, and copy it to the clipboard, in a format that would keep the tabular structure of the data. After some research, I found out that you can copy data to the clipboard in HTML-format, and I thought: this is it. Piece of cake. But it wasn’t. Apparently, you have to pass a bit more information than just the HTML-code. Googling around gave me some hints how it should be done, but nothing really specific. So I started experimenting, until it worked correctly.

Tip: use a StringBuilder to build the HTML-code, instead of concatenating strings. StringBuilders are very efficient for building long strings (hence the name StringBuilder): they are a lot faster than strings, and allocate much less memory.

Possible applications

There are a lot of situations where you could use this:

  • Copying tabular data (and pasting it in Excel, Word or Outlook).
  • Copying formatted, visually styled data.
  • …

The Code

using System;
using System.Globalization;
using System.IO;
using System.Text;
using System.Windows.Forms;

internal static class Utilities
{
  public static void CopyHtmlToClipBoard(string htmlFragment)
  {
    string headerFormat
      = "Version:0.9\r\nStartHTML:{0:000000}\r\nEndHTML:{1:000000}"
      + "\r\nStartFragment:{2:000000}\r\nEndFragment:{3:000000}\r\n";

    string htmlHeader
      = "<html>\r\n<head>\r\n"
      + "<meta http-equiv=\"Content-Type\""
      + " content=\"text/html; charset=utf-8\">\r\n"
      + "<title>HTML clipboard</title>\r\n</head>\r\n<body>\r\n"
      + "<!--StartFragment-->";

    string htmlFooter = "<!--EndFragment-->\r\n</body>\r\n</html>\r\n";
    string headerSample = String.Format(headerFormat, 0, 0, 0, 0);

    Encoding encoding = Encoding.UTF8;
    int headerSize = encoding.GetByteCount(headerSample);
    int htmlHeaderSize = encoding.GetByteCount(htmlHeader);
    int htmlFragmentSize = encoding.GetByteCount(htmlFragment);
    int htmlFooterSize = encoding.GetByteCount(htmlFooter);

    string htmlResult
      = String.Format(
          CultureInfo.InvariantCulture,
          headerFormat,
          /* StartHTML     */ headerSize,
          /* EndHTML       */ headerSize + htmlHeaderSize + htmlFragmentSize + htmlFooterSize,
          /* StartFragment */ headerSize + htmlHeaderSize,
          /* EndFragment   */ headerSize + htmlHeaderSize + htmlFragmentSize)
      + htmlHeader
      + htmlFragment
      + htmlFooter;

    DataObject obj = new DataObject();
    obj.SetData(DataFormats.Html, new MemoryStream(encoding.GetBytes(htmlResult)));
    Clipboard.SetDataObject(obj, true);
  }
}

Example: copying a DataTable

StringBuilder html = new StringBuilder();
html.Append("<table>");

foreach(DataRow row in table.Rows)
{
  html.Append("<tr>");
  foreach(object o in row)
  {
    html.AppendFormat("<td>{0}</td>", o);
  }
    
  html.Append("</tr>");
}

html.Append("</table>");
Utilities.CopyHtmlToClipboard(html.ToString());