Strange rotation in C# graphics

I am using OnPaint method in my class Class1 : Panel.

protected override void OnPaint(PaintEventArgs e)
{
    base.OnPaint(e);
    Graphics g = e.Graphics; 
}

to rotate and draw rectangle I am using

开发者_Python百科

Matrix m = new Matrix();
m.RotateAt(90, rotationPoint);
g.Transform = m;
g.FillRectangle(Brushes.Black, rectangle)

the problem is, that rotation isn't working as I want it to.

Red square is rotation point and it's located in the middle-top of rectangle. How to set x, y and rotation point so rotation would work properly?

After rotating at 90 degress it should look like this

red pixel is still at the same location.

---

Rotation point is not the point, which you want to rotate. It is point, around which graphics is rotated. So if you draw a rectangle on the top of the graphics and want to rotate it (rectangle) - then you should set rotation point as center of graphics and rotate image to 90 degrees.
Here is example, that does almost what you want:

base.OnPaint(e);

var g = e.Graphics;
var width = g.VisibleClipBounds.Width;
var height = g.VisibleClipBounds.Height;
var rotationPoint = new PointF(width / 2, height / 2); ;

// draw center point
g.FillRectangle(Brushes.Red, new RectangleF(rotationPoint.X - 5, rotationPoint.Y - 5, 10, 10));

using (var path = new GraphicsPath())
{
    var rectangle = new RectangleF((width - 10) / 2, 0, 10, 10);
    var m = new Matrix();
    m.RotateAt(90, rotationPoint);
    path.AddRectangle(rectangle);
    path.Transform(m);

    // draw rotated point
    g.FillPath(Brushes.Black, path);
}