How can I set the opacity or transparency of a Panel in WinForms?

I was wondering how to change or modify the transparency of a Panel in C#, not the whole form, but the panel only.. I've seen many C# tutorials on Opacity, but its for the Form. im looking for how it could be possible with the Panel only. Thank You!

For whoever is still looking for a totally transparent panel, I found a nice solution in this blog by William Smash who in turn has taken it from Tobias Hertkorn on his T# blog. I thought its worth posting it as an answer here.

C# code:

public class TransparentPanel : Panel
{
    protected override CreateParams CreateParams 
    {            
        get {
            CreateParams cp =  base.CreateParams;
            cp.ExStyle |= 0x00000020; // WS_EX_TRANSPARENT
            return cp;
            }
    }
    protected override void OnPaintBackground(PaintEventArgs e) 
    {
        //base.OnPaintBackground(e);
    }
}

VB.Net code:

Public Class TransparentPanel
Inherits Panel
    Protected Overrides ReadOnly Property CreateParams() As System.Windows.Forms.CreateParams
        Get
            Dim cp As CreateParams = MyBase.CreateParams
            cp.ExStyle = cp.ExStyle Or &H20 ''#WS_EX_TRANSPARENT
            Return cp
        End Get
    End Property
    Protected Overrides Sub OnPaintBackground(ByVal e As System.Windows.Forms.PaintEventArgs)
    ''#MyBase.OnPaintBackground(e)
    End Sub
End Class

Yes, opacity can only work on top-level windows. It uses a hardware feature of the video adapter, that doesn't support child windows, like Panel. The only top-level Control derived class in Winforms is Form.

Several of the 'pure' Winform controls, the ones that do their own painting instead of letting a native Windows control do the job, do however support a transparent BackColor. Panel is one of them. It uses a trick, it asks the Parent to draw itself to produce the background pixels. One side-effect of this trick is that overlapping controls doesn't work, you only see the parent pixels, not the overlapped controls.

This sample form shows it at work:

public partial class Form1 : Form {
    public Form1() {
        InitializeComponent();
        this.BackColor = Color.White;
        panel1.BackColor = Color.FromArgb(25, Color.Black);
    }
    protected override void OnPaint(PaintEventArgs e) {
        e.Graphics.DrawLine(Pens.Yellow, 0, 0, 100, 100);
    }
}

If that's not good enough then you need to consider stacking forms on top of each other. Like this.

Notable perhaps is that this restriction is lifted in Windows 8. It no longer uses the video adapter overlay feature and DWM (aka Aero) cannot be turned off anymore. Which makes opacity/transparency on child windows easy to implement. Relying on this is of course future-music for a while to come. Windows 7 will be the next XP :)

Based on information found at http://www.windows-tech.info/3/53ee08e46d9cb138.php, I was able to achieve a translucent panel control using the following code.

public class TransparentPanel : Panel
{
    protected override CreateParams CreateParams
    {
        get
        {
            var cp = base.CreateParams;
            cp.ExStyle |= 0x00000020; // WS_EX_TRANSPARENT

            return cp;
        }
    }

    protected override void OnPaint(PaintEventArgs e) =>
        e.Graphics.FillRectangle(new SolidBrush(this.BackColor), this.ClientRectangle);
}

The caveat is that any controls that are added to the panel have an opaque background. Nonetheless, the translucent panel was useful for me to block off parts of my WinForms application so that users focus was shifted to the appropriate area of the application.

Panel with opacity:

public class GlassyPanel : Panel
{
    const int WS_EX_TRANSPARENT = 0x20;  

    int opacity = 50;

    public int Opacity
    {
        get
        {
            return opacity;
        }
        set
        {
            if (value < 0 || value > 100) throw new ArgumentException("Value must be between 0 and 100");
            opacity = value;
        }
    }

    protected override CreateParams CreateParams
    {
        get
        {
            var cp = base.CreateParams;
            cp.ExStyle = cp.ExStyle | WS_EX_TRANSPARENT;

            return cp;
        }
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        using (var b = new SolidBrush(Color.FromArgb(opacity * 255 / 100, BackColor)))
        {
            e.Graphics.FillRectangle(b, ClientRectangle);
        }

        base.OnPaint(e);
    }
}

Try this:

panel1.BackColor = Color.FromArgb(100, 88, 44, 55);

change alpha(A) to get desired opacity.

Don't forget to bring your Panel to the Front when dynamically creating it in the form constructor. Example of transparent panel overlay of tab control.

panel1 = new TransparentPanel();
panel1.BackColor = System.Drawing.Color.Transparent;
panel1.Location = new System.Drawing.Point(0, 0);
panel1.Name = "panel1";
panel1.Size = new System.Drawing.Size(717, 92);
panel1.TabIndex = 0;
tab2.Controls.Add(panel1);
panel1.BringToFront(); 

// <== otherwise the other controls paint over top of the transparent panel

As far as I know a Panel can have a transparent color only, you can not control the opacity of the panel. So, you can have some parts of a panel completely transparent but not a 50% to say something.

To use transparency you must define the transparent color property.

I just wanted to add to the William Smash solution as I couldn't get to his blog so answers which may have been in there to my simple questions could not be found.

Took me a while to realise, but maybe I was just having a moment...

If you haven't had to do so already you'll need to add a reference to System.Windows.Forms in the project properties.

Also you'll need to add

Imports System.Windows.Forms 

to the file where you're adding the override class.

For OnPaintBackground you'll need to add a reference for System.Drawing then

Imports System.Drawing.Printing.PrintEventArgs

some comments says that it works and some say it doesn't It works only for your form background not any other controls behind

This does work for me. In below example, Alpha range can be a value between 0 to 255. Previously, I made a mistake by thinking that it must be a value of percentage.

Dim x as integer = 230 Panel1.BackColor = Color.FromArgb(x, Color.Blue)