The program closes when Parallel.For?

Question

C++ / C#

Hello, there is a program that calculates the trajectory of an object. Because of the long calculation, I used Parallel.For, but the program simply closed (most often without an error). Who knows what's wrong?

private void выполнитьРасчетToolStripMenuItem1_Click(object sender, EventArgs e)
        {
            //System.Windows.Forms.Control.CheckForIllegalCrossThreadCalls = false;
            Random rand = new Random();
            var ob = new object();
            var before = DateTime.Now;
            List = new List<double[]>();
            Parallel.For(0, Prec, val =>
            {
                double b;
                double c;
                double d = 0;//theta1
                double m = 0;//phi1
                double f = 0;
                double g = 0;//theta2
                double h = 0;//phi2
                double i1 = 0;
                double k = 0;
                int l = 0;
                for (b = rand.Next() % 900 / 10.0, c = rand.Next() % 3600 / 10.0; (Math.Abs(c - 180.0) / 180.0 > Math.Sin(b * Math.PI / 180.0)); b = rand.Next() % 900 / 10.0, c = rand.Next() % 3600 / 10.0) ;
                c = 180.0 + (c - 180.0) / Math.Sin(b * Math.PI / 180.0);
                dllsub(ref A, ref b, ref c, out d, out m, out f, out g, out h, out i1, out k, out l);
                if (g > ThetaO - Aamp && g < ThetaO + Aamp)
                {
                    lock (List)
                    {
                        List.Add(project(d, m, g, h, A, f, h));
                    }
                }
            });
            double mit = MinT(List);
            for (int i = 0; i < List.Count; i++) List[i][2] = Time(List[i][3], List[i][2] - mit, A);
            List.Sort(new QuantComparer());
            this.Refresh();
            foreach (double[] a1 in DrawL) richTextBox1.Text = richTextBox1.Text + (int)Math.Round(a1[0]) + "\t" + (int)Math.Round(a1[1]) + "\t" + Math.Round(a1[2]) + "\n";
        }

Sometimes this error pops up: Attempt to read or write to protected memory. This is often an indication that other memory is corrupted.
In the output window, everything is written incomprehensible, except for this: The program '[7132] WindowsFormsApplication2.vshost.exe: Managed (v4.0.30319)' has exited with code 127 (0x7f).
dllsub - procedure on fortran'e, with the help of which we get the coordinates of the body. We put them on the list.

Answer 1

Most likely trap in unmanaged code, which probably does not support multithreading. You can try to put a global lock on the dllsub() call to make sure.
Code style sucks.

Answer 2

first: never use a random class from multiple threads, it is non-thread-safe and sometimes (in practice at high frequency of generation) the instance "breaks" and starts always returning 0
second: the dllsub, project method remains a mystery, since they use external variables - this is important