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Transparent drop shadow in C# (GDI+ and Windows Forms)

, 18 Jun 2007 CPOL
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How to create a panel with a transparent drop shadow in C# using GDI+.

Screenshot - dropshadow.png

Introduction

I had to create a transparent fuzzy drop shadow similar to those found in various Windows applications for a project I'm working on. Want to have them too? Then read on! (or just download the sources).

Background

Unfortunately, there's no support at all for these drop shadows in Visual Studio 2005/WinForms (there is in WPF!). Fortunately, I only needed panels with shadows. And shadows for rectangular shapes are easy. I just took the method I use for web pages and applied it to WinForms.

The trick is as follows. Using strategically placed and tiled pictures of a shadow, we can simulate one. When transparent .PNG's are used, it looks just like the real thing.

The image parts we need are outlined in red below:

Sample Image

Normally, I'd start with creating these images. Luckily for you, I actually did that and I have provided them with the sources. Time to start coding!

The project

File > New > Project, and select Class Library. Call it ShadowPanel and click OK. Now rename class1.cs to ShadowPanel.cs. That's the file we'll work with.

It's always handy to create a test project. So right click the solution in Solution Explorer and select Add > New Project. Pick Windows Application this time and call it ShadowPanelTest.

Properties

First, we need some properties. It would be nice to provide at least a border and a background color. Time to open ShadowPanel.cs and add the following:

using System;
using System.Text;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;         

namespace ShadowPanel
{
    public class ShadowPanel : Panel
    {
        private Color _panelColor;         

        public Color PanelColor
        {
            get { return _panelColor; }
            set { _panelColor = value; }
        }         

        private Color _borderColor;         

        public Color BorderColor
        {
            get { return _borderColor; }
            set { _borderColor = value; }
        }

We can't use the original panel BackgroundColor property, because that's for the entire panel, including the shadow area! So I created a PanelColor property that defines the color for the area inside the shadow.

And about those references to System.Drawing and System.Drawing.Drawing2D. You might have to add them manually by right-clicking on References in Solution Explorer and selecting them in the list.

Images

Now for the images we can use to actually draw the shadow. You can download them with the sources if you haven't done that already. Create a folder in the project called 'Images' and add the tshadowXXXX.png files. Select them in Solution Explorer and change the Build Action to Embedded resource. That way the files will be compiled into the .dll and you won't need to distribute them with your application.

I added the images as static in the code (kudos to Remco Schrijvers for proposing this). I also added two private members to describe the size of the images (they are 5X5 pixel images) and the offset (margin) of the shadow from the left and top. Please add the following code to the ShadowPanel class:

private int shadowSize = 5;
private int shadowMargin = 2;

// static for good performance
static Image shadowDownRight = new Bitmap(typeof(ShadowPanel), "Images.tshadowdownright.png");
static Image shadowDownLeft = new Bitmap(typeof(ShadowPanel), "Images.tshadowdownleft.png");
static Image shadowDown = new Bitmap(typeof(ShadowPanel), "Images.tshadowdown.png");
static Image shadowRight = new Bitmap(typeof(ShadowPanel), "Images.tshadowright.png");
static Image shadowTopRight = new Bitmap(typeof(ShadowPanel), "Images.tshadowtopright.png");

OnPaint()

Everything is prepared for us to do the cool stuff. Creating GDI+ drawing code! Let's override the OnPaint method of the Panel base class:

protected override void OnPaint(PaintEventArgs e)
{
    base.OnPaint(e);

    // Get the graphics object. We need something to draw with  
    Graphics g = e.Graphics;

First we'll do the tiled images on the bottom and the right:

Sample Image

I think the code says it all. Just add after Graphics.g = e.Graphics;

// Create tiled brushes for the shadow on the right and at the bottom.
TextureBrush shadowRightBrush = new TextureBrush(shadowRight, WrapMode.Tile);
TextureBrush shadowDownBrush = new TextureBrush(shadowDown, WrapMode.Tile);   

// Translate (move) the brushes so the top or left of the image matches the top or left of the
// area where it's drawed. If you don't understand why this is necessary, comment it out. 
// Hint: The tiling would start at 0,0 of the control, so the shadows will be offset a little.
shadowDownBrush.TranslateTransform(0, Height - shadowSize);
shadowRightBrush.TranslateTransform(Width - shadowSize, 0);   

// Define the rectangles that will be filled with the brush.
// (where the shadow is drawn)
Rectangle shadowDownRectangle = new Rectangle(
    shadowSize + shadowMargin,                      // X
    Height - shadowSize,                            // Y
    Width - (shadowSize * 2 + shadowMargin),        // width (stretches)
    shadowSize                                      // height
    );

Rectangle shadowRightRectangle = new Rectangle(
    Width - shadowSize,                             // X
    shadowSize + shadowMargin,                      // Y
    shadowSize,                                     // width
    Height - (shadowSize * 2 + shadowMargin)        // height (stretches)
    );

// And draw the shadow on the right and at the bottom.
g.FillRectangle(shadowDownBrush, shadowDownRectangle);
g.FillRectangle(shadowRightBrush, shadowRightRectangle);

You can add the ShadowPanel to the test project by opening the Form1.cs designer. If you build the project once, the ShadowPanel should appear in the toolbox. Just drag it to the form and run it. You'll (hopefully) get the following result:

Sample Image

By the way, I set my form's background color to white, so it might look different for you.

Bring in the corners

To draw the small bitmaps in the corners, just add the following:

// Now for the corners, draw the 3 5X5 pixel images.
g.DrawImage(shadowTopRight, new Rectangle(Width - shadowSize, 
            shadowMargin, shadowSize, shadowSize));
g.DrawImage(shadowDownRight, new Rectangle(Width - shadowSize, 
            Height - shadowSize, shadowSize, shadowSize));
g.DrawImage(shadowDownLeft, new Rectangle(shadowMargin, 
            Height - shadowSize, shadowSize, shadowSize));

Almost too easy.

Filling and bordering

We already have the PanelColor and the BorderColor properties. Now it's time to put them to use.

// Fill the area inside with the color in the PanelColor property.
// 1 pixel is added to everything to make the rectangle smaller. 
// This is because the 1 pixel border is actually drawn outside the rectangle.
 Rectangle fullRectangle = new Rectangle(
    1,                                              // X
    1,                                              // Y
    Width - (shadowSize + 2),                       // Width
    Height - (shadowSize + 2)                       // Height
    );

if (PanelColor != null)
{
    SolidBrush bgBrush = new SolidBrush(_panelColor);
    g.FillRectangle(bgBrush, fullRectangle);
} 

// Draw a nice 1 pixel border it a BorderColor is specified
if (_borderColor != null)
{
    Pen borderPen = new Pen(BorderColor);
    g.DrawRectangle(borderPen, fullRectangle);
}

Time to check our results so far:

Sample Image

Well, that's about it. We're almost done.

Finishing up

Just a formality, end the OnPaint method with:

            // Memory efficiency
            shadowDownBrush.Dispose();
            shadowRightBrush.Dispose(); 

            shadowDownBrush = null;
            shadowRightBrush = null;
        }
    }
}

Done!

Points of interest

Of course, I won't stop you from adding more functionality. Please do! And let me know if you appreciated this tutorial!

History

  • June 17, 2007 - First version.

License

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

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About the Author

Mark de Haan
Web Developer
Netherlands Netherlands
I'm a UI designer living in The Netherlands. Currently I'm using C# and Visual Studio 2005 with ASP.NET and Windows Forms.
 
I'm looking forward to working with WPF professionally (expression blend is just fantastic!)
 


Comments and Discussions

 
Generalanother cool gradient panel PinmemberOwfAdmin5-Aug-07 10:28 

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