Why is ComputeHash() still blocking UI thread in an async task?

Question

So let me explain : I'm making a file signature scanner in C# using MD5 to compute the hashes. A problem I encountered is that ComputeHash() would block the UI thread. So then I thought of Task.Run() , which would solve my problems, at least I hoped so .

Even when putting the hash computing (and the whole) in an async task, it would still block, or at least, slow down the UI thread . And if I remove that hash computing, the UI thread isn't blocked anymore.

Here is my little snippet of code :

Task.Run(() =>
        {
            if (int.Parse(label5.Text) != int.Parse(label7.Text))
            {
                listBox1.SelectedIndex++;
                label11.Text = listBox1.SelectedItem.ToString();

                /*progressBar1.Increment(1);
                label5.Text = progressBar1.Value.ToString();
                int percentage = Convert.ToInt32(progressBar1.Value / (double)progressBar1.Maximum * 100);
                label2.Text = "Scanning files (" + percentage + "%)";*/
                label2.Text = "Scanning";

                label9.Text = currentThreats.ToString();
                try
                {
                    StringBuilder buff = new StringBuilder();
                    using (MD5 md5 = MD5.Create())
                    {
                        using (FileStream stream = File.OpenRead(label11.Text))
                        {
                            byte[] hash = md5.ComputeHash(stream);
                            buff.Append(BitConverter.ToString(hash).Replace("-", string.Empty));
                        }
                    }

                    if (Reference.VirusList.Contains(buff.ToString())) currentThreats++;
                }
                catch { }

                scanned++;
                label5.Text = scanned.ToString();
            }
            else
            {
                StopCurrentScan(false);
            }
        });

NOTE : This is being run in a Timer with an interval of 1 millisecond . Just precising it in case it might help solve the problem.

Answer 1

Well, now my UI thread isn't lagging out as hell anymore since I followed the steps below that people suggested me to do :

1. I used await Task.Run() instead of Task.Run() to log exceptions, and I added Control.CheckForIllegalCrossThreads = true; just after InitializeComponent so I could better understand my mistakes.

2. I removed ALL UI thread calls (like updating UI elements) from the async task, I instead (in my case) use a timer that updates the UI elements I need from local variables.

3. Similar to 2. but reversed, that means only "resource-intensive" tasks (like ComputeHash() ) are in my async task.