c# - update - vb net filesystemwatcher cross thread operation not valid




Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on (14)

Threading Model in UI

Please read the Threading Model in UI applications in order to understand basic concepts. The link navigates to page that describes the WPF threading model. However, Windows Forms utilizes the same idea.

The UI Thread

  • There is only one thread (UI thread), that is allowed to access System.Windows.Forms.Control and its subclasses members.
  • Attempt to access member of System.Windows.Forms.Control from different thread than UI thread will cause cross-thread exception.
  • Since there is only one thread, all UI operations are queued as work items into that thread:

BeginInvoke and Invoke methods

Invoke

BeginInvoke

Code solution

Read answers on question How to update the GUI from another thread in C#?. For C# 5.0 and .NET 4.5 the recommended solution is here.

I have a scenario. (Windows Forms, C#, .NET)

  1. There is a main form which hosts some user control.
  2. The user control does some heavy data operation, such that if I directly call the UserControl_Load method the UI become nonresponsive for the duration for load method execution.
  3. To overcome this I load data on different thread (trying to change existing code as little as I can)
  4. I used a background worker thread which will be loading the data and when done will notify the application that it has done its work.
  5. Now came a real problem. All the UI (main form and its child usercontrols) was created on the primary main thread. In the LOAD method of the usercontrol I'm fetching data based on the values of some control (like textbox) on userControl.

The pseudocode would look like this:

CODE 1

UserContrl1_LoadDataMethod()
{
    if (textbox1.text == "MyName") // This gives exception
    {
        //Load data corresponding to "MyName".
        //Populate a globale variable List<string> which will be binded to grid at some later stage.
    }
}

The Exception it gave was

Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on.

To know more about this I did some googling and a suggestion came up like using the following code

CODE 2

UserContrl1_LoadDataMethod()
{
    if (InvokeRequired) // Line #1
    {
        this.Invoke(new MethodInvoker(UserContrl1_LoadDataMethod));
        return;
    }

    if (textbox1.text == "MyName") // Now it wont give an exception
    {
    //Load data correspondin to "MyName"
        //Populate a globale variable List<string> which will be binded to grid at some later stage
    }
}

BUT BUT BUT... it seems I'm back to square one. The Application again become nonresponsive. It seems to be due to the execution of line #1 if condition. The loading task is again done by the parent thread and not the third that I spawned.

I don't know whether I perceived this right or wrong. I'm new to threading.

How do I resolve this and also what is the effect of execution of Line#1 if block?

The situation is this: I want to load data into a global variable based on the value of a control. I don't want to change the value of a control from the child thread. I'm not going to do it ever from a child thread.

So only accessing the value so that the corresponding data can be fetched from the database.


Same question : how-to-update-the-gui-from-another-thread-in-c

Two Ways:

  1. Return value in e.result and use it to set yout textbox value in backgroundWorker_RunWorkerCompleted event

  2. Declare some variable to hold these kind of values in a separate class (which will work as data holder) . Create static instance of this class adn you can access it over any thread.

Example:

public  class data_holder_for_controls
{
    //it will hold value for your label
    public  string status = string.Empty;
}

class Demo
{
    public static  data_holder_for_controls d1 = new data_holder_for_controls();
    static void Main(string[] args)
    {
        ThreadStart ts = new ThreadStart(perform_logic);
        Thread t1 = new Thread(ts);
        t1.Start();
        t1.Join();
        //your_label.Text=d1.status; --- can access it from any thread 
    }

    public static void perform_logic()
    {
        //put some code here in this function
        for (int i = 0; i < 10; i++)
        {
            //statements here
        }
        //set result in status variable
        d1.status = "Task done";
    }
}

Action y; //declared inside class

label1.Invoke(y=()=>label1.Text="text");


Along the same lines as previous answers, but a very short addition that Allows to use all Control properties without having cross thread invokation exception.

Helper Method

/// <summary>
/// Helper method to determin if invoke required, if so will rerun method on correct thread.
/// if not do nothing.
/// </summary>
/// <param name="c">Control that might require invoking</param>
/// <param name="a">action to preform on control thread if so.</param>
/// <returns>true if invoke required</returns>
public bool ControlInvokeRequired(Control c, Action a)
{
    if (c.InvokeRequired) c.Invoke(new MethodInvoker(delegate
    {
        a();
    }));
    else return false;

    return true;
}

Sample Usage

// usage on textbox
public void UpdateTextBox1(String text)
{
    //Check if invoke requied if so return - as i will be recalled in correct thread
    if (ControlInvokeRequired(textBox1, () => UpdateTextBox1(text))) return;
    textBox1.Text = ellapsed;
}

//Or any control
public void UpdateControl(Color c, String s)
{
    //Check if invoke requied if so return - as i will be recalled in correct thread
    if (ControlInvokeRequired(myControl, () => UpdateControl(c, s))) return;
    myControl.Text = s;
    myControl.BackColor = c;
}

Controls in .NET are not generally thread safe. That means you shouldn't access a control from a thread other than the one where it lives. To get around this, you need to invoke the control, which is what your 2nd sample is attempting.

However, in your case all you've done is pass the long-running method back to the main thread. Of course, that's not really what you want to do. You need to rethink this a little so that all you're doing on the main thread is setting a quick property here and there.


Follow the simplest (in my opinion) way to modify objects from another thread:

using System.Threading.Tasks;
using System.Threading;

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

        private void button1_Click(object sender, EventArgs e)
        {
            Action<string> DelegateTeste_ModifyText = THREAD_MOD;
            Invoke(DelegateTeste_ModifyText, "MODIFY BY THREAD");
        }

        private void THREAD_MOD(string teste)
        {
            textBox1.Text = teste;
        }
    }
}

Here is an alternative way if the object you are working with doesn't have

(InvokeRequired)

This is useful if you are working with the main form in a class other than the main form with an object that is in the main form, but doesn't have InvokeRequired

delegate void updateMainFormObject(FormObjectType objectWithoutInvoke, string text);

private void updateFormObjectType(FormObjectType objectWithoutInvoke, string text)
{
    MainForm.Invoke(new updateMainFormObject(UpdateObject), objectWithoutInvoke, text);
}

public void UpdateObject(ToolStripStatusLabel objectWithoutInvoke, string text)
{
    objectWithoutInvoke.Text = text;
}

It works the same as above, but it is a different approach if you don't have an object with invokerequired, but do have access to the MainForm


I find the check-and-invoke code which needs to be littered within all methods related to forms to be way too verbose and unneeded. Here's a simple extension method which lets you do away with it completely:

public static class Extensions
{
    public static void Invoke<TControlType>(this TControlType control, Action<TControlType> del) 
        where TControlType : Control
        {
            if (control.InvokeRequired)
                control.Invoke(new Action(() => del(control)));
            else
                del(control);
    }
}

And then you can simply do this:

textbox1.Invoke(t => t.Text = "A");

No more messing around - simple.


I have had this problem with the FileSystemWatcher and found that the following code solved the problem:

fsw.SynchronizingObject = this

The control then uses the current form object to deal with the events, and will therefore be on the same thread.


I know its too late now. However even today if you are having trouble accessing cross thread controls? This is the shortest answer till date :P

Invoke(new Action(() =>
                {
                    label1.Text = "WooHoo!!!";
                }));

This is how i access any form control from a thread.



This is not the recommended way to solve this error but you can suppress it quickly, it will do the job . I prefer this for prototypes or demos . add

CheckForIllegalCrossThreadCalls = false

in Form1() constructor .


You only want to use Invoke or BeginInvoke for the bare minimum piece of work required to change the UI. Your "heavy" method should execute on another thread (e.g. via BackgroundWorker) but then using Control.Invoke/Control.BeginInvoke just to update the UI. That way your UI thread will be free to handle UI events etc.

See my threading article for a WinForms example - although the article was written before BackgroundWorker arrived on the scene, and I'm afraid I haven't updated it in that respect. BackgroundWorker merely simplifies the callback a bit.


this.Invoke(new MethodInvoker(delegate
            {
                //your code here;
            }));




invoke