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Helper class for setting HTML and plain text formatting to clipboard (http://theartofdev.com/2014/06/12/setting-htmltext-to-clipboard-revisited/)
/// <summary>
/// Helper to encode and set HTML fragment to clipboard.<br/>
/// See http://theartofdev.com/2014/06/12/setting-htmltext-to-clipboard-revisited/.<br/>
/// <seealso cref="CreateDataObject"/>.
/// </summary>
/// <remarks>
/// The MIT License (MIT) Copyright (c) 2014 Arthur Teplitzki.
/// </remarks>
public static class ClipboardHelper
{
#region Fields and Consts
/// <summary>
/// The string contains index references to other spots in the string, so we need placeholders so we can compute the offsets. <br/>
/// The <![CDATA[<<<<<<<]]>_ strings are just placeholders. We'll back-patch them actual values afterwards. <br/>
/// The string layout (<![CDATA[<<<]]>) also ensures that it can't appear in the body of the html because the <![CDATA[<]]> <br/>
/// character must be escaped. <br/>
/// </summary>
private const string Header = @"Version:0.9
StartHTML:<<<<<<<<1
EndHTML:<<<<<<<<2
StartFragment:<<<<<<<<3
EndFragment:<<<<<<<<4
StartSelection:<<<<<<<<3
EndSelection:<<<<<<<<4";
/// <summary>
/// html comment to point the beginning of html fragment
/// </summary>
public const string StartFragment = "<!--StartFragment-->";
/// <summary>
/// html comment to point the end of html fragment
/// </summary>
public const string EndFragment = @"<!--EndFragment-->";
/// <summary>
/// Used to calculate characters byte count in UTF-8
/// </summary>
private static readonly char[] _byteCount = new char[1];
#endregion
/// <summary>
/// Create <see cref="DataObject"/> with given html and plain-text ready to be used for clipboard or drag and drop.<br/>
/// Handle missing <![CDATA[<html>]]> tags, specified start\end segments and Unicode characters.
/// </summary>
/// <remarks>
/// <para>
/// Windows Clipboard works with UTF-8 Unicode encoding while .NET strings use with UTF-16 so for clipboard to correctly
/// decode Unicode string added to it from .NET we needs to be re-encoded it using UTF-8 encoding.
/// </para>
/// <para>
/// Builds the CF_HTML header correctly for all possible HTMLs<br/>
/// If given html contains start/end fragments then it will use them in the header:
/// <code><![CDATA[<html><body><!--StartFragment-->hello <b>world</b><!--EndFragment--></body></html>]]></code>
/// If given html contains html/body tags then it will inject start/end fragments to exclude html/body tags:
/// <code><![CDATA[<html><body>hello <b>world</b></body></html>]]></code>
/// If given html doesn't contain html/body tags then it will inject the tags and start/end fragments properly:
/// <code><![CDATA[hello <b>world</b>]]></code>
/// In all cases creating a proper CF_HTML header:<br/>
/// <code>
/// <![CDATA[
/// Version:1.0
/// StartHTML:000000177
/// EndHTML:000000329
/// StartFragment:000000277
/// EndFragment:000000295
/// StartSelection:000000277
/// EndSelection:000000277
/// <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
/// <html><body><!--StartFragment-->hello <b>world</b><!--EndFragment--></body></html>
/// ]]>
/// </code>
/// See format specification here: http://msdn.microsoft.com/library/default.asp?url=/workshop/networking/clipboard/htmlclipboard.asp
/// </para>
/// </remarks>
/// <param name="html">a html fragment</param>
/// <param name="plainText">the plain text</param>
public static DataObject CreateDataObject(string html, string plainText)
{
html = html ?? String.Empty;
var htmlFragment = GetHtmlDataString(html);
// re-encode the string so it will work correctly (fixed in CLR 4.0)
if (Environment.Version.Major < 4 && html.Length != Encoding.UTF8.GetByteCount(html))
htmlFragment = Encoding.Default.GetString(Encoding.UTF8.GetBytes(htmlFragment));
var dataObject = new DataObject();
dataObject.SetData(DataFormats.Html, htmlFragment);
dataObject.SetData(DataFormats.Text, plainText);
dataObject.SetData(DataFormats.UnicodeText, plainText);
return dataObject;
}
/// <summary>
/// Clears clipboard and sets the given HTML and plain text fragment to the clipboard, providing additional meta-information for HTML.<br/>
/// See <see cref="CreateDataObject"/> for HTML fragment details.<br/>
/// </summary>
/// <example>
/// ClipboardHelper.CopyToClipboard("Hello <b>World</b>", "Hello World");
/// </example>
/// <param name="html">a html fragment</param>
/// <param name="plainText">the plain text</param>
public static void CopyToClipboard(string html, string plainText)
{
var dataObject = CreateDataObject(html, plainText);
Clipboard.SetDataObject(dataObject, true);
}
/// <summary>
/// Generate HTML fragment data string with header that is required for the clipboard.
/// </summary>
/// <param name="html">the html to generate for</param>
/// <returns>the resulted string</returns>
private static string GetHtmlDataString(string html)
{
var sb = new StringBuilder();
sb.AppendLine(Header);
sb.AppendLine(@"<!DOCTYPE HTML PUBLIC ""-//W3C//DTD HTML 4.0 Transitional//EN"">");
// if given html already provided the fragments we won't add them
int fragmentStart, fragmentEnd;
int fragmentStartIdx = html.IndexOf(StartFragment, StringComparison.OrdinalIgnoreCase);
int fragmentEndIdx = html.LastIndexOf(EndFragment, StringComparison.OrdinalIgnoreCase);
// if html tag is missing add it surrounding the given html (critical)
int htmlOpenIdx = html.IndexOf("<html", StringComparison.OrdinalIgnoreCase);
int htmlOpenEndIdx = htmlOpenIdx > -1 ? html.IndexOf('>', htmlOpenIdx) + 1 : -1;
int htmlCloseIdx = html.LastIndexOf("</html", StringComparison.OrdinalIgnoreCase);
if (fragmentStartIdx < 0 && fragmentEndIdx < 0)
{
int bodyOpenIdx = html.IndexOf("<body", StringComparison.OrdinalIgnoreCase);
int bodyOpenEndIdx = bodyOpenIdx > -1 ? html.IndexOf('>', bodyOpenIdx) + 1 : -1;
if (htmlOpenEndIdx < 0 && bodyOpenEndIdx < 0)
{
// the given html doesn't contain html or body tags so we need to add them and place start/end fragments around the given html only
sb.Append("<html><body>");
sb.Append(StartFragment);
fragmentStart = GetByteCount(sb);
sb.Append(html);
fragmentEnd = GetByteCount(sb);
sb.Append(EndFragment);
sb.Append("</body></html>");
}
else
{
// insert start/end fragments in the proper place (related to html/body tags if exists) so the paste will work correctly
int bodyCloseIdx = html.LastIndexOf("</body", StringComparison.OrdinalIgnoreCase);
if (htmlOpenEndIdx < 0)
sb.Append("<html>");
else
sb.Append(html, 0, htmlOpenEndIdx);
if (bodyOpenEndIdx > -1)
sb.Append(html, htmlOpenEndIdx > -1 ? htmlOpenEndIdx : 0, bodyOpenEndIdx - (htmlOpenEndIdx > -1 ? htmlOpenEndIdx : 0));
sb.Append(StartFragment);
fragmentStart = GetByteCount(sb);
var innerHtmlStart = bodyOpenEndIdx > -1 ? bodyOpenEndIdx : (htmlOpenEndIdx > -1 ? htmlOpenEndIdx : 0);
var innerHtmlEnd = bodyCloseIdx > -1 ? bodyCloseIdx : (htmlCloseIdx > -1 ? htmlCloseIdx : html.Length);
sb.Append(html, innerHtmlStart, innerHtmlEnd - innerHtmlStart);
fragmentEnd = GetByteCount(sb);
sb.Append(EndFragment);
if (innerHtmlEnd < html.Length)
sb.Append(html, innerHtmlEnd, html.Length - innerHtmlEnd);
if (htmlCloseIdx < 0)
sb.Append("</html>");
}
}
else
{
// handle html with existing start\end fragments just need to calculate the correct bytes offset (surround with html tag if missing)
if (htmlOpenEndIdx < 0)
sb.Append("<html>");
int start = GetByteCount(sb);
sb.Append(html);
fragmentStart = start + GetByteCount(sb, start, start + fragmentStartIdx) + StartFragment.Length;
fragmentEnd = start + GetByteCount(sb, start, start + fragmentEndIdx);
if (htmlCloseIdx < 0)
sb.Append("</html>");
}
// Back-patch offsets (scan only the header part for performance)
sb.Replace("<<<<<<<<1", Header.Length.ToString("D9"), 0, Header.Length);
sb.Replace("<<<<<<<<2", GetByteCount(sb).ToString("D9"), 0, Header.Length);
sb.Replace("<<<<<<<<3", fragmentStart.ToString("D9"), 0, Header.Length);
sb.Replace("<<<<<<<<4", fragmentEnd.ToString("D9"), 0, Header.Length);
return sb.ToString();
}
/// <summary>
/// Calculates the number of bytes produced by encoding the string in the string builder in UTF-8 and not .NET default string encoding.
/// </summary>
/// <param name="sb">the string builder to count its string</param>
/// <param name="start">optional: the start index to calculate from (default - start of string)</param>
/// <param name="end">optional: the end index to calculate to (default - end of string)</param>
/// <returns>the number of bytes required to encode the string in UTF-8</returns>
private static int GetByteCount(StringBuilder sb, int start = 0, int end = -1)
{
int count = 0;
end = end > -1 ? end : sb.Length;
for (int i = start; i < end; i++)
{
_byteCount[0] = sb[i];
count += Encoding.UTF8.GetByteCount(_byteCount);
}
return count;
}
}
@it3xl

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@it3xl it3xl commented Mar 26, 2021

Hello Arthur. First, I want to say many thanks.

I have to mention that you code do not work right in some situations.
I'll explain the source of troubles below. And I would recommend to overwrite your following code.

    // re-encode the string so it will work correctly (fixed in CLR 4.0)
    if (Environment.Version.Major < 4 && html.Length != Encoding.UTF8.GetByteCount(html))
        htmlFragment = Encoding.Default.GetString(Encoding.UTF8.GetBytes(htmlFragment));

It might look like this.

            var otherDotNetHostEncoding = Encoding.Default.CodePage != Encoding.UTF8.CodePage;

            // re-encode the string so it will work correctly (fixed in CLR 4.0)
            var oldDonNet = otherDotNetHostEncoding
                || Environment.Version.Major < 4
                    && html.Length != Encoding.UTF8.GetByteCount(html);

            if (otherDotNetHostEncoding || oldDonNet)
            {
                htmlFragment = Encoding.Default.GetString(Encoding.UTF8.GetBytes(htmlFragment));
            }

Yes, in many cases .NET processes have UTF8 encoding as the Default Encoding. But .NET could be hosted in many places and it could respect encoding of a hosting application. For example it could be hosted in MS Office Word, Excel, Outlook, etc., though VSTO Visual Studio projects.

Let's imagine that many of your customers have the "Language for non-Unicode programs" set to a non-standard value in their Windows 10 PC-s.
It could be an absolute burden to change this setting. Or it could broke other software.
Source of problem

In this case an insertion of English texts results to the following bugs
2021-03-26 22_03_12-screenshot-1 png - Windows Photo Viewer
2021-03-26 22_01_50-screenshot-1 png - Windows Photo Viewer
Insertions of texts in other languages may do not work, at all.

Here we have the following behavior in .NET

Ru-Ru Don Net

And my proposal completely fixes this trouble.

You can test everything by using VSTO Visual Studio projects.
Other encoding host

Actually they've used your code for many years but they were forced to override your logic and use Win32 API from .NET to solve described issue of Windows Clipboard insertion.
Now it is time to make everything simpler. :)

@it3xl

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@it3xl it3xl commented Apr 22, 2021

The second trouble is that you .NET process may be more fast than the Windows Clipboard.
I can't realyze why but it constantly a promlbes on some machines.

So, we just need to add a check.

Clipboard.Clear();
ClipboardHelper.CopyToClipboard(myHtmlPart);

then use somethign from the following to check

contains = Clipboard.ContainsData(DataFormats.Html);
// contains = Clipboard.ContainsText(TetxtDataFormat.Html);

And to deley

Task.Delay(TimeSpan.FromMillisecons(1))
    .Wait();

For example, for VSTO projects you'll get the following error

System.Runtime.InteropServices.COMException (0x800A11FD): 
This method or property is not available because the Clipboard is empty or not valid.

at Microsoft.Office.Interop.Word.Range.PasteSpecial(Object& IconIndex, Object& Link, Object& Placement, Object& DisplayAsIcon, Object& DataType, Object& IconFileName, Object& IconLabel)

I'll try to provide my entire checking code later after some testing.
In my code I also check that a pasting text is exactly the same as in the Clipboard.

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