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Posted 29 May 2005


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Syntax highlighting textbox written in C#

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4.78/5 (81 votes)
13 Jul 20053 min read
This article describes the code of a RichTextBox that knows how to highlight parts of the text and provide auto-completion.

Sample Image


Hey. This code is a class derived from .Net's RichTextBox. It offers syntax highlighting and auto-completion features (like Intellisense, only a little dumber).


Have you ever written an application that had its own script language? Ever needed to edit SQL/code in you application? Didn't you just want a control that was a little better than a TextBox with a fixed width font? Now you can have it :-)

This control was written when I needed to edit SQLs in an application I wrote. Then I thought:

"Even if I could find a control that knows to edit SQLs well, it still wouldn't highlight names of tables or stored procedures that I wrote. And I know I'll need a control that can highlight my own script language sometime..."

So I sat down and wrote it. Later I realized that syntax highlighting without an auto-completion is not worth much, so I added support for that too.

Using the code

First of all, I must apologise. I'm not much of a GUI programmer, so all of the manipulation is done using code, as I lack the knowledge to write custom editors for a PropertyGrid. With that out of the way, we may continue! The control has two inputs for highlighting:

  1. Separators: Accessed using the Seperators property.

    Each separator is a char, and what is later refferd to as "token" is a not-empty string between separators.

  2. Highlight Descriptor: You can use the (how surprising) HighlightDescriptor method to add a HighlightDescriptor.

    A HighlightDescriptor is an instance of a class describing a highlighting rule, which can be divided into token identification info and design info.

Highlighting Rules

A highlighting rule has 6 fields it's constructed with:

  • Token: A string that is later compared to the text.
  • DescriptorType: Sets the highlighting type. The options are in a single word, to the end of the line, and to the corresponding closing token.
  • DescriptorRecognition: Determines how the token is compared to the token from the text. The options are: the text is equal to the token, the text begins with the token, and the text contains the token.
  • Color: Sets the color of the token when highlighted.
  • Font: Sets the font on the token when highlighted. This field is optional.
  • UseForAutoComplete: Determines if the token will be used for auto completion.

Due to laziness, a HighlightDescriptor's values can only be set in the constructor. <SideNote> I really think that the readonly keyword is under appreciated.</SideNote>.

The HighlightDescriptor is as follows:

public class HighlightDescriptor
    public HighlightDescriptor(string token,
           string closeToken, Color color, Font font,
           DescriptorType descriptorType, DescriptorRecognition dr,
           bool useForAutoComplete)
        Color = color;
        Font = font;
        Token = token;
        DescriptorType = descriptorType;
        CloseToken = closeToken;
        DescriptorRecognition = dr;
        UseForAutoComplete = useForAutoComplete;

    public readonly Color Color;
    public readonly Font Font;
    public readonly string Token;
    public readonly string CloseToken;
    public readonly DescriptorType DescriptorType;
    public readonly DescriptorRecognition DescriptorRecognition;
    public readonly bool UseForAutoComplete;

Points of Interest

There were a few interesting things during the writing:

  1. Scrollbars: Since every time the text changes, I actually change it again (as far as the base textbox knows), the scrollbars reset themselves to top and left most positions. To resolve this, I used the EM_GETSCROLLPOS and EM_GETSCROLLPOS Windows messages:
    #region Scrollbar positions functions
    /// <summary>
    /// Sends a win32 message to get the scrollbars' position.
    /// </summary>
    /// <returns>a POINT structre containing horizontal
    ///       and vertical scrollbar position.</returns>
    private unsafe Win32.POINT GetScrollPos()
        Win32.POINT res = new Win32.POINT();
        IntPtr ptr = new IntPtr(&res);
        Win32.SendMessage(Handle, Win32.EM_GETSCROLLPOS, 0, ptr);
        return res;
    /// <summary>
    /// Sends a win32 message to set scrollbars position.
    /// </summary>
    /// <param name="point">a POINT
    ///        conatining H/Vscrollbar scrollpos.</param>
    private unsafe void SetScrollPos(Win32.POINT point)
        IntPtr ptr = new IntPtr(&point);
        Win32.SendMessage(Handle, Win32.EM_SETSCROLLPOS, 0, ptr);
  2. Keyboard strokes: During my work on the auto complete feature, I found out that even if you override the OnKeyDown method and not call the base method, keystrokes of keys containing actual characters are still handled (they arrive using the WM_CHAR message). Due to that I decided to override the WndProc method and filter unwanted keystrokes at that level.
  3. RTF: I didn't really learn RTF for this project. I just took a regular RichTextBox and understood what I have to build for my needs. Hence my resulting RTF is probably not optimal. It probably has language limitations that can be solved fairly easily by manipulating the RTF header.


  • 05/29/2005: First release.
  • 04/06/2005: Fixed a few bugs when using the ToCloseToken DescriptorType.
  • 07/13/2005: Fixed a few bugs, added Undo/Redo functionality.


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here


About the Author

Uri Guy
Web Developer
Israel Israel
I've been programming since I first understood english at age 10, starting with QBASIC.
I've been developing production C# applications since .Net's first beta, and I enjoy every second of it, as I enjoy sailing, kitesurfing, and playing guitar.

Comments and Discussions

QuestionShowing AutoComplete Pin
Member 140448974-Sep-19 15:41
MemberMember 140448974-Sep-19 15:41 
AnswerRe: Showing AutoComplete Pin
Wrangly12-Mar-20 12:35
MemberWrangly12-Mar-20 12:35 
QuestionMy vote of 5 Pin
Jason Newland16-Apr-19 19:37
MemberJason Newland16-Apr-19 19:37 
BugFilterAutoComplete Pin
tayoufabrice28-Oct-15 23:34
professionaltayoufabrice28-Oct-15 23:34 
BugFont Pin
MarcinJXXXX17-Apr-15 22:36
MemberMarcinJXXXX17-Apr-15 22:36 
QuestionLicense Terms for Use of Code Pin
dbhatt7911-Mar-15 6:10
Memberdbhatt7911-Mar-15 6:10 
QuestionLicense Type used Pin
dbhatt7923-Feb-15 8:50
Memberdbhatt7923-Feb-15 8:50 
QuestionProblem when it has line more Pin
diemtrang7-May-14 23:47
Memberdiemtrang7-May-14 23:47 
QuestionThanks Pin
diemtrang7-May-14 18:08
Memberdiemtrang7-May-14 18:08 
BugHighlight Color Don't Match Glitch + Can set font\Highlight Pin
Globin21-Dec-12 6:11
MemberGlobin21-Dec-12 6:11 
QuestionLicense Pin
Member 877392130-Mar-12 3:16
MemberMember 877392130-Mar-12 3:16 
AnswerMe Too! Pin
Globin21-Dec-12 5:34
MemberGlobin21-Dec-12 5:34 
QuestionHighlight between ' and ' Pin
Pedro Mendonça5-Jan-12 6:31
MemberPedro Mendonça5-Jan-12 6:31 
AnswerRe: Highlight between ' and ' Pin
lordofm22-Aug-12 23:58
Memberlordofm22-Aug-12 23:58 
GeneralRe: Highlight between ' and ' Pin
Globin21-Dec-12 6:19
MemberGlobin21-Dec-12 6:19 
QuestionThanks Pin
Ross Canning3-Nov-11 14:23
MemberRoss Canning3-Nov-11 14:23 
Questionabout licenses Pin
Member 828710218-Oct-11 2:31
MemberMember 828710218-Oct-11 2:31 
Good morning.
I wonder what the licenses imposed on the artifact produced in this article. It is opensource, free or simply does not have license? I am wanting to use it in a small project at my university and I need to explain and cite properly.

Could you help me?

Victon Malcolm Rodrigues
QuestionPls help Pin
Member 824009118-Sep-11 3:06
MemberMember 824009118-Sep-11 3:06 
AnswerRe: Pls help Pin
wizard.ubit25-Sep-11 4:14
Memberwizard.ubit25-Sep-11 4:14 
Questionsloved this problem Length < 0 on SetAutoCompleteItems() Method Error Pin
dubbakakishore4-Aug-11 23:18
Memberdubbakakishore4-Aug-11 23:18 
GeneralMy vote of 4 Pin
ymmt7-Jun-11 15:26
Memberymmt7-Jun-11 15:26 
GeneralLength < 0 on SetAutoCompleteItems() Method Error Pin
Karl_Mauzoul27-Apr-11 5:13
MemberKarl_Mauzoul27-Apr-11 5:13 
QuestionWeb version of this? Pin
Rob Achmann18-Jan-11 8:07
MemberRob Achmann18-Jan-11 8:07 
QuestionToCloseToken problem Pin
Rex 12412-Jun-10 7:16
MemberRex 12412-Jun-10 7:16 
QuestionComments / Multi-Comments with seperators? Pin
DJB MASTER9-May-10 13:09
MemberDJB MASTER9-May-10 13:09 

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