I've come across countless articles in the past explaining how to implement a transparent background in a RichTextBox control. I was disappointed to find that they're all terribly complex, and incredibly slow.
While researching the problem I stumbled across a far easier way to implement this in just a few lines of code.
The real problem isn't so much that the RichTextBox doesn't support transparency, it's that .Net uses an old version of the RichEdit control.
It seems there are 4 versions of the RichEdit control out there - when I'm talking about the RichEdit control, I'm talking about the C DLL that either comes with Windows or some version of Office. The files are named either RICHEDXX.DLL (XX is the version number), or MSFTEDIT.DLL and they're in the System32 folder.
.Net RichTextBox control is bound to version 2. The biggest problem with this version (at least for me) is that it does not render properly if you try to make the window transparent. Later versions, however, do.
We can fix that. If you create a control deriving from the original RichTextBox control, but overriding the CreateParams property, you can put in a new Windows class name (this is the window class name, nothing to do with classes in the C# sense). This effectively gives us a free upgrade. When the .Net RichTextBox control instantiates, it will now use the latest RichEdit control and not the old, archaic, version 2.
There are other benefits too - version 3 and beyond of the RichEdit control support quite an extensive array of layout features, such as tables and full text justification. This is the version of the RichEdit that WordPad uses in Windows XP. To really see what it's capable of displaying you can create documents in Word and save them in RTF, load these into the new RichEdit and in a lot of cases it'll look identical, it's that powerful. A full list of features can be found here:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/shellcc/platform/commctls/richedit/richeditcontrols/aboutricheditcontrols.asp
There are a couple of caveats.
1. The control that this is bound to was shipped with Windows XP, and so this code won't work in Windows 2000 or earlier.
2. The RichTextBox control in C# only knows about version 2, so the interface doesn't include all the new features. You can wrap a few of the features yourself through new methods on the RichEdit50 class. I may implement a few and post them at a future date. nb - I've noticed that SelectedFont also doesn't seem to report the correct values in this version.
The code is as follows:
public class RichEdit50 : RichTextBox
{
[DllImport("kernel32.dll", CharSet=CharSet.Auto)]
static extern IntPtr LoadLibrary(string lpFileName);
protected override CreateParams CreateParams
{
get
{
CreateParams prams = base.CreateParams;
if (LoadLibrary("msftedit.dll")!=IntPtr.Zero)
{
prams.ExStyle |= 0x020; // transparent
prams.ClassName = "RICHEDIT50W";
}
return prams;
}
}
}
Just replace you references to RichTextBox with RichEdit50 and you'll be rolling. Because it's transparent you will need to create some kind of background on the control behind the RichTextBox - draw a gradient or something to get some cool effects. Of course if you just want the features of the RichEdit without transparency, comment out the line that ends with '// transparent'.
Have fun.
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