Olivier's .NET Blog

Saving the state of a Graphics object

Today I discovered a new class : GraphicsContainer; it allows you to save the state of aGraphics object (transformation, clipping region, and rendering properties) and to restore it later.

This class is used in conjunction with 2 pairs of methods belonging to the Graphics class:

BeginContainer saves the state of the graphics object (and returns a GraphicsContainer object), EndContainer restores the state (when provided with a GraphicsContainer object). MSDN provides a sample here: http://msdn2.microsoft.com/en-us/library/azdschfw.aspx.

There exists overloads of BeginContainer that accept two rectangles and unit allowing the Graphics object to be applied a new transformation after its state is saved.

In the same maneer, but in a simpler way, the Graphics.Save and Graphics.Restore methods do the same job (Honestly, I don't understand why these two pairs of methods coexist...)

This led me to write a little helper class allowing to save/restore a Graphics with the using syntax.

Here is how to use this class (it is just a rewritten version of the MSDN sample):

protected override void OnPaint(System.Windows.Forms.PaintEventArgs e)
{
    Graphics g = e.Graphics;

    // Define transformation for container.
    RectangleF src = new RectangleF(0f, 0f, 200f, 200f);
    RectangleF dest = new RectangleF(100f, 100f, 150f, 150f);

    using (PaintHelper.SaveGraphics(g, dest, src, GraphicsUnit.Pixel))
    {
        // Fill red rectangle in container.
        g.FillRectangle(Brushes.Red, 0.0F, 0.0F, 200.0F, 200.0F);
    }

    // Fill untransformed rectangle with green.
    g.FillRectangle(Brushes.Green, 0.0F, 0.0F, 200.0F, 200.0F);
}

And the source code for the PaintHelper class:   

using System;
using System.Drawing;
using System.Drawing.Drawing2D;

namespace Delta.Internals
{
    public static class PaintHelper
    {
        private class GraphicsBackup : IDisposable
        {
            private Graphics graphics = null;
            private GraphicsContainer container = null;

            public GraphicsBackup(Graphics g)
            {
                Init(g);
                container = g.BeginContainer();
            }

            public GraphicsBackup(Graphics g, Rectangle dest, Rectangle src, GraphicsUnit unit)
            {
                container = g.BeginContainer(dest, src, unit);
            }

            public GraphicsBackup(Graphics g, RectangleF dest, RectangleF src, GraphicsUnit unit)
            {
                container = g.BeginContainer(dest, src, unit);
            }

            private void Init(Graphics g)
            {
                if (g == null) throw new ArgumentNullException("g");
                graphics = g;
            }

            public void Dispose()
            {
                graphics.EndContainer(container);
            }
        }

        public static IDisposable SaveGraphics(Graphics g)
        {
            return new GraphicsBackup(g);
        }

        public static IDisposable SaveGraphics(Graphics g,
            Rectangle dest, Rectangle src, GraphicsUnit unit)
        {
            return new GraphicsBackup(g, dest, src, unit);
        }

        public static IDisposable SaveGraphics(Graphics g,
            RectangleF dest, RectangleF src, GraphicsUnit unit)
        {
            return new GraphicsBackup(g, dest, src, unit);
        }
    }
}

Hope this helps.