While working on another multi-media program, I found myself in need of a transparent edit control (
TextBox) for .NET. I soon discovered that if there is one out there, then it must be so transparent that I just can't find it. In my searches, I did find that there is an AlphaBlend filter available for web buttons hosted by IE, but I could find none for Win Forms. So I set out to write one.
I present here version 1.0 of AlphaBlendTextBox written in C#. Be warned that it is not fully tested, not fully optimized and is not yet in use in any production system. However, I wanted to submit it here, hoping that some of you will find the control and it's source code useful.
A big note: I have used Visual Studio .NET 2003 and .NET framework 1.1 to create this control and the sample projects. I will repost a VS2002 .NET 1.0 version as soon as I can.
While designing this control, I decided that the best method would be to subclass the existing
TextBox control, so that the new control could be used anywhere that the original
TextBox control could. A quick discovery was that the
TextBox control is a real pain to subclass. Several programmers out there have noted that it can be an all-or-nothing process.
This is because in the current versions of .NET, the
TextBox control is just a wrapper for the old Win32 control. This makes it difficult to modify. If you override the
OnPaint method for the control, it is ignored unless you change the control's style to
UserPaint. However, doing this keeps the control from drawing itself at all, until the user selects some text, and even then painting is erratic.
Back to my original problem. I wanted a
TextBox control that was transparent to the background. While looking through the help files, I noticed that the .NET controls have a new style called
SupportsTransparentBackColor that would seem to do the trick. However, this style requires that the
UserPaint style be set, and as noted above this keeps the
TextBox control from drawing itself. Worse yet, when the
TextBox did paint, it still had a solid background.
After several weeks of frustration, I came up with this solution: subclass the
TextBox control and add a
PictureBox to the control that gets displayed, instead of the original
When the control needs to be painted, it does the following:
- temporarily sets the
UserPaint style to
- captures a bitmap of the original
TextBox by sending a
- switches back to
- uses a ColorMap to convert the solid background into a transparent/alpha background
- figures out where the caret should be and draws it if necessary
- then copies the new alpha image to the
PictureBox, which when drawn will be blended with the background
In addition, the
PictureBox had to be overridden so that any mouse event sent to it would be redirected to the
TextBox. Also, several public properties and methods of the
TextBox had to be overridden, such as
Using the code
This control can be used in the same manner as the standard
TextBox control. I have compiled the control as a control library (DLL) file. To use this control from the Visual Studio designer, simply right click on the tool bar and choose "Add/Remove Items" and then click "Browse" and browse to the AlphaBlendTextBox.dll file. After doing this, the AlphaBlendTextBox control should appear in your tool bar (under "My User Controls" if you have VS2003). Now you can drag the control to a form and use it just like the original
TextBox control. The control has a new property named
BackAlpha that will allow you to set the control's background
alphablend value. The range is 0 for transparent, 255 for opaque, or in between for translucent.
Another way to use the control is to add it to your project, using the "Project:Add Reference" menu. The control is under the
ZBobb namespace so you can then use code like below:
// Sample C# code
private ZBobb.AlphaBlendTextBox alphaBlendTextBox1;
private void Form1_Load(object sender, System.EventArgs e)
alphaBlendTextBox1 = new ZBobb.AlphaBlendTextBox();
alphaBlendTextBox1.Location = new System.Drawing.Point(32, 16);
this.alphaBlendTextBox1.Text = "Hello";
alphaBlendTextBox1.BackAlpha = 0; // Totally transparent
The control can also be used in Visual Basic .NET and I have included a sample VB project.
This is my first posting to the Code Project. The Code Project has been a great source of help to me in my programming and I am glad to finally be able to give something back. I look forward to hearing your comments and suggestions, and I hope that this posting can be useful to you.
- Version 1.0 released June 24, 2003.