Generate multiple graphics objects to use in parallel

Question

I'm trying to create a graphics object that I can use to draw a metafile and I'm not sure what is the best way to do it. Ultimately the metafiles will be included in a Word document and we are looking to have them look good when printed.

At first I had something like this:

var ms = new MemoryStream();
using (var mfG = System.Drawing.Graphics.FromHwndInternal(IntPtr.Zero))
using (Metafile mf = new Metafile(ms, mfG.GetHdc(), EmfType.EmfPlusOnly, "Chart"))
using (var g = System.Drawing.Graphics.FromImage(mf))
{
    // do some drawing 
}

And this works, but I want to process multiple images in parallel and in that case it will generate errors because they are all trying to use the same graphics object.

So I tried (based on some suggestions on line to create a graphics object):

using (var mfG = new System.Drawing.Printing.PrinterSettings().CreateMeasurementGraphics())

But it still has the same problem. Apparently CreateMeasurementGraphics gives you the same graphics object every time.

I can wrap the code with a lock to make other threads wait. And maybe I should. But is there a better way to generate independent Graphics objects that will produce decent printable metafiles?

Answer 1

I don't know, if my answer will be of any help for you, but you can't draw from multiple threads to the same Graphics object.

What you can do: you can create multiple MetaFiles with a linked Graphics object and you can draw to the different files in Parallel. Obviously each thread must use it's own MetaFile<->Graphics duo.

In the very end you can combine the images by using a single Graphics object and the method "DrawImageUnscaled".

Example (Will try to write files to D:. You might want to change the destination):

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace MetaFileDrawTest
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);
            Pen[] p = new Pen[] { Pens.Black, Pens.Red, Pens.Green };
            // Draw to three MetaFiles in Parallel.
            Parallel.For(0, 3, i =>
            {
                using (var ms = new MemoryStream())
                {
                    using (var mfG = System.Drawing.Graphics.FromHwndInternal(IntPtr.Zero))
                    {
                        using (Metafile mf = new Metafile(ms, mfG.GetHdc(), EmfType.EmfPlusOnly, "Chart"))
                        {
                            using (var g = System.Drawing.Graphics.FromImage(mf))
                            {
                                // Draw a slightly different line each time to see a difference...
                                g.DrawLine(p[i], 10 * i, 10, 100, 100);
                            }

                            // Save the Metafile to the disc. (You might keep it in memory as well)
                            mf.Save("D:\\test" + i + ".wmf");
                        }
                    }
                }
            });

            // Draw the three metafiles to the form (just to see how they look like)
            for (int i = 0; i <= 2; i++)
            {
                e.Graphics.TranslateTransform(100 * (i + 1), 0);
                e.Graphics.DrawImageUnscaled(Image.FromFile("D:\\test" + i + ".wmf"), 0, 0);
                e.Graphics.ResetTransform();
            }

            // Create a file metafile to draw all single images to (one by one)
            using (var ms2 = new MemoryStream())
            {
                using (var mfG = System.Drawing.Graphics.FromHwndInternal(IntPtr.Zero))
                {
                    using (Metafile mf = new Metafile(ms2, mfG.GetHdc(), EmfType.EmfPlusOnly, "Chart"))
                    {
                        using (var g = System.Drawing.Graphics.FromImage(mf))
                        {
                            g.DrawImageUnscaled(Image.FromFile("D:\\test0.wmf"), 0, 0);  
                            g.DrawImageUnscaled(Image.FromFile("D:\\test1.wmf"), 0, 0);
                            g.DrawImageUnscaled(Image.FromFile("D:\\test2.wmf"), 0, 0);
                        }
                        // Save the combined file to disc
                        mf.Save("D:\\complete.wmf");
                        // draw the combined file to the form.
                        e.Graphics.DrawImageUnscaled(mf, 0, 0);
                    }
                }
            }
        }
    }