Transparent BackColor ignored when OnPaint is overridden in Button-derived controls

[Sekar47]

.NET version

.NET Framework 4.8

Did it work in .NET Framework?

No

Did it work in any of the earlier releases of .NET Core or .NET 5+?

No

Issue description

When creating a custom control that inherits from System.Windows.Forms.Button and overrides the OnPaint method, setting the BackColor property to Color.Transparent does not render a transparent background. Instead, the control renders a black background, even when placed on a colored or image-filled parent.

When the overridden OnPaint is commented out, the transparent background works as expected.

WindowsFormsApp1.zip

Steps to reproduce

1.Open the provided sample solution
2.Open the form (Form2) in the Designer.
3.In the Properties window of CustomButton:

• Set the BackColor property to Transparent.
• Set the ForeColor property to any visible color (e.g., Blue).
  4.Save and run the application.
  5.The CustomButton renders with a black background, even though the BackColor was set to Transparent.
  6.If set the BackColor to a non-transparent color (Eg: Color.Red).
  7.The CustomButton now displays the correct red background as expected.

[LeafShi1]

If your custom control is intended to extend the existing implementation of Button instead of completely replacing it, you should call base.OnPaint(e) in your overridden OnPaint method to retain the default drawing logic of Button.

protected override void OnPaint(PaintEventArgs e)
{
    Graphics g = e.Graphics;

    using (SolidBrush brush = new SolidBrush(this.BackColor))
    {
        g.FillRectangle(brush, this.ClientRectangle);
    }

    // Calculate rectangles for image and text
    CalculateTextAndImageBounds(g);

    // Draw border
    DrawBorder(g);

    // Draw text
    TextRenderer.DrawText(g, Text, Font, textRect, ForeColor, TextFormatFlags.HorizontalCenter | TextFormatFlags.VerticalCenter | TextFormatFlags.EndEllipsis);

    g.ResetClip();

    // Draw focus rectangle if focused
    if (Focused && ShowFocusCues)
    {
        ControlPaint.DrawFocusRectangle(g, ClientRectangle);
    }

    base.OnPaint(e);
}

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