C#调用gdi32.dll函数绘制Rectangle填充颜色不起作用什么原因？

C#调用gdi32.dll函数绘制Rectangle填充颜色不起作用什么原因？
大家好，通过搜索代码在Winform C#下调用gdi32.dll做带填充色的矩形橡皮筋，已实现但填充色只能是默认的黑白色，经分析是Brush颜色设定无效，涉及代码如下：
[System.Runtime.InteropServices.DllImportAttribute("gdi32.dll")]
private static extern IntPtr CreateSolidBrush(BrushStyles enBrushStyle, int crColor);
private static extern bool Rectangle(IntPtr hdc, int X1, int Y1, int X2, int Y2);
void Draw(Graphiics g)
{
hdc = g.GetHdc();
gdiPen = CreatePen(penStyle, lineWidth, GetRGBFromColor(Color.FromArgb(255, 255, 255, 232)));// Pen颜色有效
gdiBrush = CreateSolidBrush(brushStyle, GetRGBFromColor(Color.FromArgb(255, 255, 255, 232))); // Brush颜色无论怎么改结果都不变
SelectObject(hdc, gdiPen);
SelectObject(hdc, gdiBrush);
Rectangle(hdc, p1.X, p1.Y, p2.X, p2.Y);
}
如何让设定的Rectangle填充色生效？暂只想用GDI32函数实现，基于效率考虑，不想用别的。


 [System.Runtime.InteropServices.DllImport("gdi32.dll")]
        private static extern IntPtr CreateSolidBrush(int crColor);

        [System.Runtime.InteropServices.DllImport("gdi32.dll")]
        private static extern bool Rectangle(IntPtr hdc, int X1, int Y1, int X2, int Y2);

        [System.Runtime.InteropServices.DllImport("gdi32.dll")]
        public static extern IntPtr CreatePen(int nPenStyle, int nWidth, int crColor);

        [System.Runtime.InteropServices.DllImport("gdi32.dll")]
        public static extern IntPtr SelectObject(IntPtr hdc, IntPtr pen);


        Point p1 = new Point(10, 10), p2 = new Point(200, 100);
        void Draw(Graphics g)
        {
            IntPtr hdc = g.GetHdc();
            IntPtr gdiPen = CreatePen(0, 5, GetRGBFromColor(Color.FromArgb(255, 255, 232)));// Pen颜色有效
            IntPtr gdiBrush = CreateSolidBrush(GetRGBFromColor(Color.FromArgb(255, 0, 0))); // Brush颜色无论怎么改结果都不变
            SelectObject(hdc, gdiPen);
            SelectObject(hdc, gdiBrush);
            Rectangle(hdc, p1.X, p1.Y, p2.X, p2.Y);
        }
        private int GetRGBFromColor(Color color)
        {
            byte r = color.R;
            byte g = color.G;
            byte b = color.B;
            //转化为32bit RGB值:
            int rgb = (r & 0xff) | ((g & 0xff) << 8) | ((b & 0xff) << 16);
            return rgb;
        }
        private void button1_Click(object sender, System.EventArgs e)
        {
            System.Drawing.Graphics gs = pictureBox1.CreateGraphics();
            Draw(gs);
        }

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