Transparent Borderless Forms in C#

Introduction

This article describes an approach to displaying transparent forms in a Windows application. Such may be useful to anyone wishing to display an odd shaped form for a splash screen or possibly a tool dialog with a transparent background.



Figure 1: Transparent Forms

Getting Started

The solution contains three Windows Form classes; each shows something different about a transparent form.

Figure 2:  Solution Explorer with the Project Visible

Code:  Form 1 (Form1.cs)

There is nothing much to creating the transparent form; setting the forms back color and transparency key property value to the same color is all that is required. In form 1, the form back color and transparency key are both set to lime green; this color does not occur within the image and so the image will not appear to have holes in it; do not use a transparency color that occurs in your images. That much takes care of making the form transparent. Setting the forms FormBorderStyle property to none removes the forms border and all that will remain when the form is displayed is an image.



Figure 3:  Form Transparency Key Property

Of course having a transparent, borderless form eliminates the ability to drag the form around the desktop; at least without doing a bit more work. In order to set up the form for dragging in this condition, a couple of DLL imports are needed. The following contains the code behind the Form1 class.

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Text;

using System.Windows.Forms;

using System.Runtime.InteropServices;

 

InteropServices was added to the list of defaults to support the use of DLL imports within the class.

 

namespace XparentForm

{

     public partial class Form1 : Form

     {

 

The following user32.dll imports (SendMessage and ReleaseCapture) are required to support the dragging and dropping of the form in the absense of a caption bar on the form.

        

#region Form Dragging API Support
//The SendMessage function sends a message to a window or windows.

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = false)]

static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, int wParam, int lParam);

//ReleaseCapture releases a mouse capture

[DllImportAttribute("user32.dll", CharSet = CharSet.Auto, SetLastError = false)]

public static extern bool ReleaseCapture();

#endregion

 

/// <summary>
/// default constructor

/// </summary>

public Form1()

{

     InitializeComponent();

}

 

The picturebox fills the entire draggable area of the form (those sections which are not visible cannot be used for a drag operation) and for that that reason the picture box controls mouse down event is used to support the borderless form dragging operations.

/// <summary>
/// Respond to the picture box mousedown event to
/// drag the borderless form around (left click and drag)/// </summary>

/// <param name="sender"></param>
/// <param name="e"></param>

      private void pictureBox1_MouseDown(object sender, MouseEventArgs e)

     {

           // drag the form without the caption bar

           // present on left mouse button

           if (e.Button == MouseButtons.Left)

           {

                ReleaseCapture();

                SendMessage(this.Handle, 0xa1, 0x2, 0);

            }

        }

 

The rest of the class code is pretty straightforward and is not particularly relevant to the topic. The code is annotation to explain the remaining event handlers.

/// <summary>

/// Close the Application

/// </summary>

/// <param name="sender"></param>

/// <param name="e"></param>

private void button1_Click(object sender, EventArgs e)

{

     // dispose

     this.Dispose();
}

/// <summary>

/// Creates a second transparent form

/// </summary>

/// <param name="sender"></param>

/// <param name="e"></param>

private void button2_Click(object sender, EventArgs e)

{

     // open up another transparent form, this one with a hole in it

     Form2 f = new Form2();

     f.Show();

}

/// <summary>

/// Creates a third transparent form

/// </summary>

/// <param name="sender"></param>

/// <param name="e"></param>

private void button3_Click(object sender, EventArgs e)

{

     Form3 f = new Form3();

     f.Show();

}

 

Form 2  (Form2.cs)

The code behind Form2 is the same as it used in Form1; the only differences between the two forms are the complexity of the image used as the borderless form and that the background color and transparency key property values are different than that used on Form1.

Form 3 (Form3.cs)

Form3 is another spin on the same thing used in forms 1 and 2. The difference here is that the picture box contains a white rectangle; the transparency key value is set also to white which will defeat any attempts to drag the form. This form does have a border around it to enable dragging the form If you set the border style to None then the form will no longer be draggable.

Summary

This article was intended to demonstrate an approach to creating transparent backed, borderless draggable forms. The approach may be used to generate some sort of custom UI or may be used to build a classier looking splash screen.