Who owns MemoryStream objects placed on the clipboard? (Or should something other than MemoryStream be used?)

I think that if an object implements IDisposable is a good thing to dispose it when you don't need it anymore.

DataObject provides a basic implementation of the IDataObject interface so why don't you derive from it:

public sealed class HtmlDataObject : DataObject, IDisposable{    protected MemoryStream HtmlMemoryStream { get; set; }    public HtmlDataObject(MemoryStream memoryStream, string fallBackText)    {        HtmlMemoryStream = memoryStream;        SetText(fallBackText);        SetData(DataFormats.Html, false, HtmlMemoryStream );    }    public void Dispose()    {        HtmlMemoryStream .Dispose();    }}

So your method can be changed:

public HtmlDataObject GetClipboardData(){    return new HtmlDataObject(this.GenerateHtml(), this.fallbackText.ToString());}

And you can put it into an using statement or Dispose() it when you have finished using it.

Final thought: You should not worry about clipboard data because the DataObject will be destroyed anyway when you exit the application and your clipboard will lose what you put inside with it.http://msdn.microsoft.com/en-us/library/office/gg278673.aspx

If you want that the stream is persisted after disposing it and/or when the application exit you have to use Clipboard.SetDataObject with the copy parameter = true