Invert Text Color depending on BackColor

问题

I have a ProgressBar control like the following two:

The first is painted properly. As you can see, the second only has one 0, it's supposed to have two but the other cannot be seen because ProgressBar's ForeColor is the same as the TextColor. Is there a way I can paint the text in black when the ProgressBar below is painted in Lime and paint the text in Lime when the background is black?

回答1:

You can first draw the background and text, then draw the foreground lime rectangle using PatBlt method with PATINVERT parameter to combine foreground drawing with background drawing:

using System;
using System.Drawing;
using System.Runtime.InteropServices;
using System.Windows.Forms;

public class MyProgressBar : Control
{
    public MyProgressBar() 
    {
        DoubleBuffered = true;
        Minimum = 0; Maximum = 100; Value = 50;
    }
    public int Minimum { get; set; }
    public int Maximum { get; set; }
    public int Value { get; set; }
    protected override void OnPaint(PaintEventArgs e) 
    {
        base.OnPaint(e);
        Draw(e.Graphics);
    }
    private void Draw(Graphics g) 
    {
        var r = this.ClientRectangle;
        using (var b = new SolidBrush(this.BackColor))
            g.FillRectangle(b, r);
        TextRenderer.DrawText(g, this.Value.ToString(), this.Font, r, this.ForeColor);
        var hdc = g.GetHdc();
        var c = this.ForeColor;
        var hbrush = CreateSolidBrush(((c.R | (c.G << 8)) | (c.B << 16)));
        var phbrush = SelectObject(hdc, hbrush);
        PatBlt(hdc, r.Left, r.Y, (Value * r.Width / Maximum), r.Height, PATINVERT);
        SelectObject(hdc, phbrush);
        DeleteObject(hbrush);
        g.ReleaseHdc(hdc);
    }
    public const int PATINVERT = 0x005A0049;
    [DllImport("gdi32.dll")]
    public static extern bool PatBlt(IntPtr hdc, int nXLeft, int nYLeft,
        int nWidth, int nHeight, int dwRop);
    [DllImport("gdi32.dll")]
    public static extern IntPtr SelectObject(IntPtr hdc, IntPtr hgdiobj);
    [DllImport("gdi32.dll", EntryPoint = "DeleteObject")]
    public static extern bool DeleteObject(IntPtr hObject);
    [DllImport("gdi32.dll")]
    public static extern IntPtr CreateSolidBrush(int crColor);
}

Note: The controls is just for demonstrating the paint logic. For a real world application, you need to add some validation on Minimum, Maximum and Value properties.

来源：https://stackoverflow.com/questions/40953598/invert-text-color-depending-on-backcolor

标签

.net

winforms

gdi+

gdi