I started messing around with multithreading for a CPU intensive batch process I'm running. Essentially I'm trying to condense multiple single page tiffs into single PDF documents. This works fine with a foreach loop or standard iteration but can be very slow for several 100 page documents. I tried the following based on a some examples I found to use multithreading and it has significant performance improvements however it obliterates the page order instead of 1,2,3,4 it will be 1,3,4,2,6,5 on what thread completes first.

My question is how would I utilize this technique while maintaining the page order and if I can will it negate the performance benefit of the multithreading? Thank you in advance.

PdfDocument doc = new PdfDocument();
string mail = textBox1.Text;
string[] split = mail.Split(new string[] { Environment.NewLine }, StringSplitOptions.None);

int counter = split.Count();

// Source must be array or IList.
var source = Enumerable.Range(0, 100000).ToArray();
// Partition the entire source array.
var rangePartitioner = Partitioner.Create(0, counter);
double[] results = new double[counter];
// Loop over the partitions in parallel.
Parallel.ForEach(rangePartitioner, (range, loopState) =>
{
    // Loop over each range element without a delegate invocation.
    for (int i = range.Item1; i < range.Item2; i++)
    {
        f_prime = split[i].Replace(" " , "");
        PdfPage page = doc.AddPage();
        XGraphics gfx = XGraphics.FromPdfPage(page);
        XImage image = XImage.FromFile(f_prime);
        double x = 0;
        gfx.DrawImage(image, x, 0);

    }
});