Actual coordinate after ScaleTransfrom()

I am drawing a rectangle in a WinForms application in C# and I want to get the actual coordinates of the rectangle after applying ScaleTransform() method.

Graphics g = e.Graphics;
g.ScaleTransform(2.0F,2.0F,System.Drawing.Drawing2D.MatrixOrder.Append);
g.DrawRectangle(pen, 20, 40, 100,100)

Solution

Once you have set a ScaleTransform in your Graphics object (or any transform for that matter), you can use it to transform the points of your rectangle (or any other points).

For example:

// your existing code
Graphics g = e.Graphics;
g.ScaleTransform(2.0F,2.0F,System.Drawing.Drawing2D.MatrixOrder.Append);

// say we have some rectangle ...
Rectangle rcRect = new Rectangle(20, 40, 100, 100);

// make an array of points
Point[] pPoints =
{
    new Point(rcRect.Left, rcRect.Top),      // top left
    new Point(rcRect.Right, rcRect.Top),     // top right
    new Point(rcRect.Left, rcRect.Bottom),   // bottom left
    new Point(rcRect.Right, rcRect.Bottom),  // bottom right
};

// get a copy of the transformation matrix
using (Matrix mat = g.Transform)
{
    // use it to transform the points
    mat.TransformPoints(pPoints);
}

Note the using syntax above - this is because, as MSDN says:

Because the matrix returned and by the Transform property is a copy of the geometric transform, you should dispose of the matrix when you no longer need it.

As a slightly less wordy alternative, you can do the same thing using the TransformPoints method of the Graphics class (MSDN here) - so construct your array of points as above, then just do this:

g.TransformPoints(CoordinateSpace.Page, CoordinateSpace.World, pPoints);

MSDN describes the relevant coordinate spaces used in the above function:

GDI+ uses three coordinate spaces: world, page, and device. World coordinates are the coordinates used to model a particular graphic world and are the coordinates you pass to methods in the .NET Framework. Page coordinates refer to the coordinate system used by a drawing surface, such as a form or a control. Device coordinates are the coordinates used by the physical device being drawn on, such as a screen or a printer. The Transform property represents the world transformation, which maps world coordinates to page coordinates.