How Can I Make a Control Transparent in .NET 3.5 WinForms?

Making a Control Transparent in .NET 3.5

In your .NET 3.5 Winforms application, you're encountering challenges in making a rectangle transparent when the select tool button is clicked. Despite your efforts using "ControlStyles.SupportsTransparentBackColor," the background remains opaque. To resolve this, consider leveraging custom controls that support transparency for compatibility with .NET 3.5.

Here's a specialized control that provides an "Opacity" property, allowing you to adjust the transparency level programmatically:

public class TranspCtrl : Control
{
    private int m_opacity = 100;

    // ... Other properties and methods

    public int Opacity
    {
        get
        {
            // Bounds check and adjust opacity value
        }
        set
        {
            m_opacity = value;
            // Invalidate the parent to request redrawing
        }
    }

    // ... Override CreateParams to enable transparent background
    // ... Override OnPaint to handle opacity blending and drawing
}

By utilizing this custom control, you can achieve transparent backgrounds for your controls, even in .NET 3.5, providing you with greater flexibility for your image editing tool.

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