OnPaint Paint问题

Question

I have created a custom checkBox to increase the height of the rectangle of the checkbox . I have override the OnPaint method as given below. When I make the control transparent, background becomes white, instead of transparent. What could be the issue?

    protected override void OnPaint(PaintEventArgs pevent)
    {        
        base.OnPaint(pevent);        
        int h = this.Height;

        if (BackColor == Color.Transparent)
        {
            pevent.Graphics.Clear(this.Parent.BackColor);            
        }
        else
        {
            pevent.Graphics.Clear(BackColor);
        }
        Rectangle rc = new Rectangle(new Point(0, 0), new Size(h, h));
        ControlPaint.DrawCheckBox(pevent.Graphics, rc,
            this.Checked ? ButtonState.Checked : ButtonState.Normal);

        SizeF stringMeasure = pevent.Graphics.MeasureString(Text, Font);

        int topOffset = (int)(ClientRectangle.Height - stringMeasure.Height) / 2;

        pevent.Graphics.DrawString(Text, Font, new SolidBrush(ForeColor), this.Height, topOffset);
     }

Answer 1

I am not sure if you derived your checkbox from the given class CheckBox or you derived it from Control. If you derived from control, you should add in the constructor the following line:

public CustomChecBox()
        : base()
    {
        SetStyle(ControlStyles.SupportsTransparentBackColor, true);
    }

Note, that there is also a function called OnPaintBackground(...) which you can override, but you should not need to to this. In your OnPaint()-function, you should replace

if (BackColor == Color.Transparent)
    {
        pevent.Graphics.Clear(this.Parent.BackColor);            
    }
    else
    {
        pevent.Graphics.Clear(BackColor);
    }

with

pevent.Graphics.Clear(BackColor);

or

pevent.Graphics.FillRectangle(new SolidBrush(this.BackColor), this.ClientRectangle);

But this is, what happens in OnPaintBackground(...). If you set the BackColor to transparent and you added the SetStyle-Methid in the contructor, then your control should be really transparent. I hope this helps.