.NET BackGroundWorker - InvalidOperationException : Cross-thread operation not valid

Question

I have a project coded in .NET Winforms. I need to implement a data-mining operation, print the text to TextBox and update the progress. I tried to use BackgroundWorker to do, but it throws a InvalidOperationException (Cross-thread operation not valid: Control ‘xxxxx’ accessed from a thread other than the thread it was created on) To narrow down the potential causes of the problem, I started a new project, included the following: Button - To start the BackgroundWorker Label - to print the text. And ProgressBar. However, the result is the same. I searched on SOF, and was told to use a delegate, but I am not familiar with it. This is the code sample that throws the error: ``` using System; using System.Collections.Generic; using System.ComponentModel; namespace TestProject { public partial class Form1 : Form { private readonly BackgroundWorker _bw = new BackgroundWorker(); public Form1() { InitializeComponent(); _bw.DoWork += RosterWork; _bw.ProgressChanged += BwProgressChanged; _bw.Run…

Accepted Answer

This is the problem:

```
label1.Text = i.ToString();
```

You're trying to change the label text within the `BackgroundWorker`, which is not running on a UI thread. The point of `BackgroundWorker` is to do all the non-UI work there, using `ReportProgress` to periodically "go back" to the UI thread and update the UI with the progress you're making.

So either you need to change `label1.Text` in `BwProgressChanged` as well, or you need to use `Control.Invoke`/`BeginInvoke` just as you would from any other background thread:

```
// Don't capture a loop variable in a lambda expression...
int copy = i;
Action updateLabel = () => label1.Text = copy.ToString();
label1.BeginInvoke(updateLabel);
```

For more about the copying part, see Eric Lippert's blog post, "Closing over the loop variable considered harmful". In this particular case it's only an issue because I'm using `BeginInvoke`. This could be changed to just:

```
Action updateLabel = () => label1.Text = i.ToString();
label1.Invoke(updateLabel);
```

... but now the background worker would always be waiting for the UI to catch up before it kept going, which in real life is usually not what you want. I generally prefer `BeginInvoke` over `Invoke`.

.NET BackGroundWorker - InvalidOperationException : Cross-thread operation not valid

Solution

Problem