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ReadOnly ComboBox

, 23 Oct 2008 CPOL
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An extended ComboBox that adds a ReadOnly property to it.



I decided to create a new combobox based on requests from the users of MSDN Windows Forms forum. It adds a new property to the regular ComboBox so it can be set to read-only, much like the regular TextBoxes.

Using the code

I made this code pretty straightforward so all you have to do is download the C# file enclosed in the ZIP file above and add it to your project. Build it, and the ExComboBox should be available on the Toolbox.

You can change the way the disabled button looks by commenting the ComboBoxRenderer line and uncommenting the commented portion of the code. You can set the color of the gradient that makes the button of the read-only mode by changing the colors of the LinearGradientBrush variable lgb on the OnPaint method:

protected class DblPanel : Panel
    protected override void OnPaint(PaintEventArgs e)
        if (this.Visible == true)
            ComboBoxRenderer.DrawDropDownButton(e.Graphics, e.ClipRectangle, 
            /*Pen pen = new Pen(Color.DarkGray);
            Pen penBR = new Pen(Color.LightGray);
            Pen penArrow = new Pen(Color.LightGray);
            penArrow.Width = 2;
            penArrow.EndCap = LineCap.Square;
            Graphics g = e.Graphics;
            LinearGradientBrush lgb = new LinearGradientBrush(new Point(0, 0), 
                                      new Point(0, this.Height + 1), 
                                      Color.LightGray, Color.Gray);

Points of interest

I learned a bit while trying to add the read-only feature to the regular ComboBox. It is interesting that the combobox does not have any sub-controls in it (check the ComboBox.Controls.Count property and you will see it is 0); all is done by custom drawing. And, I also learned that there is no way to change this by overriding its Paint methods as it doesn't happen there; and, I couldn't figure out where it draws itself, so I used a Panel to draw over the regular dropdown button.

So basically, all the functionality is kept from the original combo box except the ability for the user to change the selected item without disabling the control (many complained that setting the Enabled property to false made it difficult to read and didn't allow the use of custom colors).


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Fabio Franco
Founder Actus Sistemas
Brazil Brazil
Fábio is a party boy that for some reason fell into the geek world. Perhaps his taste for coding came as a consequence of his taste for games, his first program was a batch file for MS-DOS at the age of 14. The need to make it easier to start the games had him do a games menu, with password to blackmail his younger brothers.

After that his taste for programming only grew, at the age of 16 Fábio accidentaly stumbled in a program that makes programs, a.k.a. Visual Basic 5.
From there he self taught, through books, how to code in VB 5, 6 and later C, C++ and finally C#.

Currently Fábio specializes in C# Desktop and Web Development, that he's been working with since 2004. When he has time, he helps users on MSDN forums and tries to write articles on codeproject.

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Comments and Discussions

GeneralError when Visual Styles not active PinmemberFrazer J10-Dec-09 9:29 
GeneralRe: Error when Visual Styles not active PinmemberFabio Franco11-Dec-09 3:13 
GeneralThankyou Pinmemberger0nim016-Jun-09 8:23 
GeneralRe: Thankyou PinmemberFabio Franco16-Jun-09 8:28 
Questionuse of panel [modified] PinmemberMember 197457029-Apr-09 21:21 
AnswerRe: use of panel PinmemberFabio Franco30-Apr-09 2:55 
Hi, I inserted a panel in the inherited control, so it will be on top of the original DropDown button.
The size of the panel is just big enough to cover the whole DropDown button.
Then I draw on this panel to look like a disabled DropDown Button.

I hope this answers your question.
QuestionRe: use of panel PinmemberMember 197457030-Apr-09 3:22 
AnswerRe: use of panel PinmemberFabio Franco30-Apr-09 6:14 
GeneralSimply useful PinmemberGio Bejarasco23-Oct-08 10:44 
GeneralRe: Simply useful PinmemberDotDue23-Oct-08 22:33 
GeneralRe: Simply useful PinmemberFabio Franco24-Oct-08 2:05 
GeneralA couple of suggestions PinmemberDotDue22-Oct-08 23:01 
GeneralRe: A couple of suggestions PinmemberFabio Franco23-Oct-08 1:55 
GeneralRe: A couple of suggestions PinmemberDotDue23-Oct-08 22:31 
Generalgood Pinmemberjosephlawrencecarpio29-Jul-08 17:53 
GeneralRe: good PinmemberFabio Franco30-Jul-08 5:09 
GeneralI might be stupid PinmemberJohnny J.29-Jan-08 6:41 
GeneralRe: I might be stupid PinmemberFabio Franco29-Jan-08 7:14 
GeneralRe: I might be stupid PinmemberFabio Franco29-Jan-08 8:05 
GeneralUsing property Pinmemberdfjohn0054-Jan-08 0:56 
GeneralRe: Using property PinmemberFabio Franco4-Jan-08 1:03 
GeneralRe: Using property PinmemberThomas Stockwell22-Jan-08 9:33 
GeneralRe: Using property [modified] PinmemberFabio Franco24-Jan-08 4:16 

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