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.vb)
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: Forms 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 call are needed. The following contains the code behind the Form1 class.
The following user32.DLL calls (SendMessage and ReleaseCapture) are required to support the dragging and dropping of the form in the absense of a caption bar on the form.
<summary>
The SendMessage function sends a message to a window or windows.
</summary>
<param name="hWnd"></param>
<param name="msg"></param>
<param name="wParam"></param>
<param name="lParam"></param>
<returns></returns>
<remarks></remarks>
Declare Auto Function SendMessage Lib "user32.dll" (ByVal hWnd As IntPtr, _
ByVal msg As Integer, ByVal wParam As IntPtr, ByVal lParam As IntPtr) As IntPtr
<summary>
ReleaseCapture releases a mouse capture
</summary>
<returns></returns>
<remarks></remarks>
Declare Function ReleaseCapture Lib "user32.dll" () As Boolean
/// <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>
Handles form drags
</summary>
<param name="sender"></param>
<param name="e"></param>
<remarks></remarks>
Private Sub pictureBox1_MouseDown(ByVal sender As System.Object, ByVal e
As System.Windows.Forms.MouseEventArgs) Handles pictureBox1.MouseDown
If e.Button = MouseButtons.Left Then
ReleaseCapture()
SendMessage(Me.Handle, &HA1, &H2, 2)
End If
End Sub
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>
Open new instance of form 2
</summary>
<param name="sender"></param>
<param name="e"></param>
<remarks></remarks>
Private Sub button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles button2.Click
Dim f As New Form2()
f.Show()
End Sub
<summary>
Open new instance of form 3
</summary>
<param name="sender"></param>
<param name="e"></param>
<remarks></remarks>
Private Sub button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles button3.Click
Dim f As New Form3()
f.Show()
End Sub
<summary>
Dispose main form and exit
</summary>
<param name="sender"></param>
<param name="e"></param>
<remarks></remarks>
Private Sub button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles button1.Click
Me.Dispose()
End Sub
End Class
Form 2 (Form2.vb)
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.