Non-blocking drag and drop in a Windows Forms app

Question

Non-blocking drag and drop in a Windows Forms app

FleetCommand 21

Hi. I'm writing a Windows Forms app that supports accepting files via drag-and-drop. Unfortunately, my code seems to disrupt the app from which the dragging started.

Here is my code:

public Form1()
{
    InitializeComponent();
    AllowDrop = true; //In the actual app, I set this in the form designer.
}

private void Form1_DragEnter(object sender, DragEventArgs e)
{
    if ((e.Data == null) || e.Data.GetDataPresent(DataFormats.FileDrop)) e.Effect = DragDropEffects.Copy;
}

private void Form1_DragDrop(object sender, DragEventArgs e)
{
    if ((e.Data == null) || !e.Data.GetDataPresent(DataFormats.FileDrop)) return;
    string[] FilesToGet = (string[])e.Data.GetData(DataFormats.FileDrop);
    if (!PreliminaryValidation()) { return; }
    StartProcessing(FilesToGet);
}

private bool PreliminaryValidation()
{
    // Replace this with the actual validation code
    return MessageBox.Show(
        "You have not read the manual! Would you like to proceed at your own risk?",
        "Saftey alert",
        MessageBoxButtons.YesNo
    ) switch { DialogResult.Yes => true, DialogResult.No => false };
}

private void StartProcessing(string[] FilesToGet)
{
    this.SuspendLayout();
    listView1.Items.Clear();
    foreach (var item in FilesToGet)
    {
        _ = listView1.Items.Add(item);
    }
    this.ResumeLayout(false);

    // Processing code goes here.
    throw new NotImplementedException();
}

When I run my app and drag a group of files onto it, the following happens:

The originating app (probably File Explorer) becomes unresponsive.
My app saves a list of files in the FilesToGet array.
After a brief check, my app may displays a message box to confirm a potentially risky operation. (For the sake of this question's applicability, the message box always appears.)
My app starts processing the files. It could take up hours, depending on the number, size, and complexity of the files.
The originating app becomes responsive again.

Now the question: How can I make step 5 happen right after step 2?

Jack J Jun 25,296 Reputation points

2022-03-14T07:16:10.66+00:00

@FleetCommand , is any update? Have you tried the solution provided by Castorix31? Does it work for you?
FleetCommand 21 Reputation points

2022-03-15T00:31:22.037+00:00

With all due respect, Castorix31's reply is not even an answer to my question. I asked, "How can I make step 5 happen right after step 2?"

While I am grateful for community volunteers trying to help, I regret that so far, the participants have been posting random things about multi-threading. My question IS NOT about multi-threading or making my app responsive. It concerns preventing drag-and-drop from rendering other apps unresponsive.
RLWA32 49,636 Reputation points

2022-03-15T02:02:11.327+00:00

Using a BackgroundWorker would allow you to return immediately from the dragdrop method which would prevent the originating application from freezing while your process performed its file operations. The fact that the file operations are performed on a background thread also prevents your process user interface from being blocked while that is going on. So two objectives would be achieved.
Castorix31 90,686 Reputation points

2022-03-15T02:06:30.637+00:00

It concerns preventing drag-and-drop from rendering other apps unresponsive.

What renders the other app unresponsive is your MessageBox
The code I posted by putting the MessageBox in a thread make it responsive.
FleetCommand 21 Reputation points

2022-03-15T05:24:22.803+00:00

Our esteemed @RLWA32 already explained that you are wrong. Please re-read his comment as many times as you need.
FleetCommand 21 Reputation points

2022-03-15T05:30:16.277+00:00

And I'm still gripping with BackgroundWorker. Microsoft examples don't compile and none of my books have anything on this topic. My problem is passing filesToGet to the worker thread and then receiving the process results.

My next problem will be how to not run the process in a separate worker thread because that's what I do not want.
Castorix31 90,686 Reputation points

2022-03-15T09:51:27.27+00:00

Did you test it ???

The MessageBox in ANOTHER THREAD makes Explorer or other app RESPONSIVE
The MessageBox in the UI THREAD makes Explorer or other app UNRESPONSIVE
RLWA32 49,636 Reputation points

2022-03-15T09:54:39.633+00:00

Refer to the new answer I posted that contains a link to BackgroundWorker sample and suggestion to avoid separate worker thread.
soren60 81 Reputation points

2022-03-15T09:58:25.607+00:00
A test with

new System.Threading.Thread(new System.Threading.ThreadStart(delegate { MessageBox.Show("Yo!"); })).Start();

shows he's obviously right : the explorer is not blocked anymore

Accepted answer

5 additional answers

Your answer

Jack J Jun 25,296 Reputation points

2022-03-14T07:16:10.66+00:00

@FleetCommand , is any update? Have you tried the solution provided by Castorix31? Does it work for you?
FleetCommand 21 Reputation points

2022-03-15T00:31:22.037+00:00

With all due respect, Castorix31's reply is not even an answer to my question. I asked, "How can I make step 5 happen right after step 2?"

While I am grateful for community volunteers trying to help, I regret that so far, the participants have been posting random things about multi-threading. My question IS NOT about multi-threading or making my app responsive. It concerns preventing drag-and-drop from rendering other apps unresponsive.
RLWA32 49,636 Reputation points

2022-03-15T02:02:11.327+00:00

Using a BackgroundWorker would allow you to return immediately from the dragdrop method which would prevent the originating application from freezing while your process performed its file operations. The fact that the file operations are performed on a background thread also prevents your process user interface from being blocked while that is going on. So two objectives would be achieved.
Castorix31 90,686 Reputation points

2022-03-15T02:06:30.637+00:00

It concerns preventing drag-and-drop from rendering other apps unresponsive.

What renders the other app unresponsive is your MessageBox
The code I posted by putting the MessageBox in a thread make it responsive.
FleetCommand 21 Reputation points

2022-03-15T05:24:22.803+00:00

Our esteemed @RLWA32 already explained that you are wrong. Please re-read his comment as many times as you need.
FleetCommand 21 Reputation points

2022-03-15T05:30:16.277+00:00

And I'm still gripping with BackgroundWorker. Microsoft examples don't compile and none of my books have anything on this topic. My problem is passing filesToGet to the worker thread and then receiving the process results.

My next problem will be how to not run the process in a separate worker thread because that's what I do not want.
Castorix31 90,686 Reputation points

2022-03-15T09:51:27.27+00:00

Did you test it ???

The MessageBox in ANOTHER THREAD makes Explorer or other app RESPONSIVE
The MessageBox in the UI THREAD makes Explorer or other app UNRESPONSIVE
RLWA32 49,636 Reputation points

2022-03-15T09:54:39.633+00:00

Refer to the new answer I posted that contains a link to BackgroundWorker sample and suggestion to avoid separate worker thread.
soren60 81 Reputation points

2022-03-15T09:58:25.607+00:00

A test with

new System.Threading.Thread(new System.Threading.ThreadStart(delegate { MessageBox.Show("Yo!"); })).Start();

shows he's obviously right : the explorer is not blocked anymore

Answer 1

As long as you insist on using a modal message box in the dragdrop method then the originating application will freeze until you dismiss the message box and return from the method.

See my earlier comment about using a timer.

For example,

    public partial class Form1 : Form
    {
        string[] FilesToGet;
        bool ready = true;
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_DragDrop(object sender, DragEventArgs e)
        {
            if ((e.Data == null) || !e.Data.GetDataPresent(DataFormats.FileDrop))return;
            FilesToGet = (string[])e.Data.GetData(DataFormats.FileDrop);
            ready = false;
            timer1.Start();
            //if (!PreliminaryValidation()) { return; }
            //StartProcessing(FilesToGet);
        }

        private void Form1_DragEnter(object sender, DragEventArgs e)
        {
            if (ready && ((e.Data == null) || e.Data.GetDataPresent(DataFormats.FileDrop)))
                e.Effect = DragDropEffects.Copy;
            else
                e.Effect = DragDropEffects.None;
        }
        private bool PreliminaryValidation()
        {
            // Replace this with the actual validation code
            DialogResult result = MessageBox.Show(
                "You have not read the manual! Would you like to proceed at your own risk?",
                "Saftey alert",
                MessageBoxButtons.YesNo
            );

            return (result == DialogResult.Yes);
        }
        private void StartProcessing()
        {
            this.SuspendLayout();
            listView1.Items.Clear();
            foreach (var item in FilesToGet)
            {
                _ = listView1.Items.Add(item);
            }
            this.ResumeLayout(false);
            // Processing code goes here.
            //throw new NotImplementedException();
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            timer1.Stop();

            if (!ready)
            {

                if(PreliminaryValidation())
                {
                    StartProcessing();
                }

                ready = true;
            }
        }
    }

FleetCommand 21 Reputation points

2022-03-16T12:56:19.39+00:00

Woo-hoo! I'd post many emojis here if this website allowed it. (It doesn't. [sad emoji])

Now, this is amazing. I didn't know a Timer could do that.

On the other hand, I had absolute confidence that parallelism and multi-threading aren't the solutions. Anyway, right now, I feel I've seen enough System.NullReferenceException to last me ten lifetimes. I hope I don't have to use threads again until I completely forget this unsavory case.

And as for you, RLWA32-6355, thanks a lot for your persistence.
RLWA32 49,636 Reputation points

2022-03-16T15:28:44.217+00:00

You're welcome. You should reconsider performing long-running file operations on the UI thread. It's not a good idea.
FleetCommand 21 Reputation points

2022-03-17T02:53:09.56+00:00

Oh, don't worry about my app's performance. I can't disclose much about it, but I can tell you about a past case: I had inherited the development of a half-complete app that processed a compressed archive and displayed the results in a grid. At the time, it needed 1m 28s to process our sample archive. It had everything your multi-threaded fancy can dream of: A progress bar, a test label to announce the name of the sub-item it was processing, and a worker thread. I removed them all. After that, the app would take only 0.1s to open the sample archive. You see, context-switching isn't cheap.

Answer 2

RLWA32 49,636

You need to return from the dragdrop method before processing the files. Also, don't use a modal message box inside the method.

FleetCommand 21 Reputation points

2022-03-08T00:39:39.46+00:00

You need to return from the dragdrop method before processing the files.

How?
RLWA32 49,636 Reputation points

2022-03-08T09:22:48.807+00:00

You could kick off a BackgroundWorker before returning from the method. It would run on a separate thread to process the files.
FleetCommand 21 Reputation points

2022-03-08T17:49:33.887+00:00

Hm... I have 20 books about C#, but none of them mention the BackgroundWorker class. Looks like I have to borrow or buy a new book, learn about BackgroundWorker, and hope you pointed me in the right direction.

And I still need to figure out where to put the modal message box.

Answer 3

Castorix31 90,686

The main problem is your MessageBox

You can replace

MessageBox.Show("Yo!");

by :

new System.Threading.Thread(new System.Threading.ThreadStart(delegate { MessageBox.Show("Yo!"); })).Start();

RLWA32 49,636 Reputation points

2022-03-08T19:36:27.437+00:00

My app starts processing the files. It could take up hours, depending on the number, size, and complexity of the files

I commented about the MessageBox also. In any event, I doubt this type of lengthy file handling should be performed on the UI thread.

Answer 4

Karen Payne MVP 35,586 Volunteer Moderator

High level look at using TAP (Task-based Asynchronous Pattern) which supports cancellation and optionally IProgress. When done right your application will be completely responsive along with providing feedback to the user. Many will settle for a BackGroundWorker component which in many cases is fine but using TAP pattern provides more options than a BackGroundWorker and is available in not just desktop solutions but any device e.g. mobile and web.

To get an idea about TAP, check out the code in this class project which has examples for TAP and events which keeps the caller responsive. Also check out the following GitHub repository for other code samples.

Take time to learn asynchronous programming and your app will be responsive and not mess with other windows apps.

FleetCommand 21 Reputation points

2022-03-15T00:17:34.14+00:00

When done right your application will be completely responsive along with providing feedback to the user.

This has nothing to do with the question I asked. All I wish to do is for my app to signal the completion of the drop before proceeding with its job.

Answer 5

@FleetCommand , Have you considered putting step3 and step4 into one thread? Then you will not face the blocking problem.

I make a code example and based on my test, there is no blocking indeed.

 private void Form1_DragEnter(object sender, DragEventArgs e)  
        {  
            if ((e.Data == null) || e.Data.GetDataPresent(DataFormats.FileDrop)) e.Effect = DragDropEffects.Copy;  
        }  
        private void Form1_DragDrop(object sender, DragEventArgs e)  
        {  
             
            if ((e.Data == null) || !e.Data.GetDataPresent(DataFormats.FileDrop)) return;  
            string[] FilesToGet = (string[])e.Data.GetData(DataFormats.FileDrop);  
  
            Task.Run(() =>  
            {  
                MessageBox.Show("Yo!");  
                Form1.StartProcessing(FilesToGet);  
  
            });  
             
  
  
        }  
        private static void StartProcessing(string[] filesToGet)  
        {  
  
            for (int i = 0; i < 100000000; i++)  
            {  
                Console.WriteLine(i);  
            }  
        }  
  
        private void button1_Click(object sender, EventArgs e)  
        {  
            MessageBox.Show("Test");  
        }

However, If you insist on using Messagebox in the method, Inevitably, you need to manually click on the messagebox so that the program will not freeze.

I used 100000000 loop and button to check if the app is responsive.

As the following result shown, it will not block the app when we output the number in another thread.

Best Regards,
Jack

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FleetCommand 21 Reputation points

2022-03-16T00:48:14.407+00:00

Thank you for posting the first answer that attempts to consider all criteria. It means a lot to me.

Sadly, it still falls short of being practical.

Upon implementing it, I encounter a System.NullReferenceException at line 20 of Form1.Designer.cs, at this instruction: base.Dispose(disposing);

Step 4 (refer to my original post) fails. Unlike your example, at step 4, my app doesn't sleep! It processes data. Now, it cannot return them.

Putting the message box in the worker thread is something that Castorix31 also tried. Such a message box does not block the UI, causing race conditions. Perhaps I must update my example to better impress upon the reader why the message box is so vital.

I posted my example code with the assumption that the reader knows whatever I do in it has a purpose.

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Non-blocking drag and drop in a Windows Forms app

5 additional answers

Your answer