Cross-thread operation not valid: Control accessed

Question

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.

Answer 1

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

Answer 2

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;
        }
    }
}

Answer 3

As per Prerak K's update comment (since deleted):

I guess I have not presented the question properly.

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 corresponding data can be fetched from the database.

The solution you want then should look like:

UserContrl1_LOadDataMethod()
{
    string name = "";
    if(textbox1.InvokeRequired)
    {
        textbox1.Invoke(new MethodInvoker(delegate { name = textbox1.text; }));
    }
    if(name == "MyName")
    {
        // do whatever
    }
}

Do your serious processing in the separate thread before you attempt to switch back to the control's thread. For example:

UserContrl1_LOadDataMethod()
{
    if(textbox1.text=="MyName") //<<======Now it wont give exception**
    {
        //Load data correspondin to "MyName"
        //Populate a globale variable List<string> which will be
        //bound to grid at some later stage
        if(InvokeRequired)
        {
            // after we've done all the processing, 
            this.Invoke(new MethodInvoker(delegate {
                // load the control with the appropriate data
            }));
            return;
        }
    }
}

Answer 4

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 .

Answer 5

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:

• If there is no work for UI thread, then there are idle gaps that can be used by a not-UI related computing.
• In order to use mentioned gaps use System.Windows.Forms.Control.Invoke or System.Windows.Forms.Control.BeginInvoke methods:

BeginInvoke and Invoke methods

• The computing overhead of method being invoked should be small as well as computing overhead of event handler methods because the UI thread is used there - the same that is responsible for handling user input. Regardless if this is System.Windows.Forms.Control.Invoke or System.Windows.Forms.Control.BeginInvoke.
• To perform computing expensive operation always use separate thread. Since .NET 2.0 BackgroundWorker is dedicated to performing computing expensive operations in Windows Forms. However in new solutions you should use the async-await pattern as described here.
• Use System.Windows.Forms.Control.Invoke or System.Windows.Forms.Control.BeginInvoke methods only to update a user interface. If you use them for heavy computations, your application will block:

Invoke

• System.Windows.Forms.Control.Invoke causes separate thread to wait till invoked method is completed:

BeginInvoke

• System.Windows.Forms.Control.BeginInvoke doesn't cause the separate thread to wait till invoked method is completed:

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.

Answer 6

For example to get the text from a Control of the UI thread:

Private Delegate Function GetControlTextInvoker(ByVal ctl As Control) As String

Private Function GetControlText(ByVal ctl As Control) As String
    Dim text As String

    If ctl.InvokeRequired Then
        text = CStr(ctl.Invoke(
            New GetControlTextInvoker(AddressOf GetControlText), ctl))
    Else
        text = ctl.Text
    End If

    Return text
End Function