Question

I’ve come across several methods of applying gradient styles to objects in a windows form application. All the methods involve overriding the OnPaint method. However, I am looking the change the style at runtime based on validation.

How can I apply the new gradient style to an already rendered button (like I can with BackColor)?

R,
C.

UPDATE: This is the code I am currently using. It appears to have no effect

private void Button_Paint(object sender, System.Windows.Forms.PaintEventArgs e)
    {
        Graphics g = e.Graphics;
        g.DrawString("This is a diagonal line drawn on the control",
            new Font("Arial", 10), System.Drawing.Brushes.Blue, new Point(30, 30));
        g.DrawLine(System.Drawing.Pens.Red, btn.Left, btn.Top,
            btn.Right, btn.Bottom);

        this.btn.Invalidate();
    }

Being called by

btn.Paint += new PaintEventHandler(this.Button_Paint);

FURTHER UPDATE WITH CURRENT CODE

private void Button_Paint(object sender, PaintEventArgs e)
{
Graphics g = e.Graphics;
g.DrawString("This is a diagonal line drawn on the control",
        new Font("Arial", 10), System.Drawing.Brushes.Blue, new Point(30, 30));
g.DrawLine(System.Drawing.Pens.Red, btn.Left, btn.Top,
        btn.Right, btn.Bottom);
}

private void btn_Click(object sender, EventArgs e)
{
btn.Paint += new PaintEventHandler(this.Button_Paint);();
btn.Invalidate();
}

Answer 1

There are two parts to this. One, as SLaks said, you need to draw the gradient in your Paint event handler. This would look something like this (my example here is a bit messy for the sake of brevity):

private void Button_Paint(object sender, PaintEventArgs e)
{
    Graphics g = e.Graphics;
    if (MyFormIsValid()) {
        g.DrawString("This is a diagonal line drawn on the control",
            new Font("Arial", 10), System.Drawing.Brushes.Blue, new Point(30, 30));
        g.DrawLine(System.Drawing.Pens.Red, btn.Left, btn.Top,
            btn.Right, btn.Bottom);
    }
    else {
        g.FillRectangle(
            new LinearGradientBrush(PointF.Empty, new PointF(0, btn.Height), Color.White, Color.Red),
            new RectangleF(PointF.Empty, btn.Size));
    }
}

Also, you need to do your validation and redraw the button when it is clicked:

btn.Click += Button_Click;

…

private void Button_Click(object sender, EventArgs e)
{
    DoValidations();
    btn.Invalidate();
}

Of course, you’ll have to implement the DoValidations() and MyFormIsValid() methods.

Here’s the whole thing as a runnable sample program: http://pastebin.com/cfXvtVwT