The vast majority of answers use Control.Invoke which is a race condition waiting to happen. For example, consider the accepted answer:
string newText = "abc"; // running on worker thread
this.Invoke((MethodInvoker)delegate {
someLabel.Text = newText; // runs on UI thread
});
If the user closes the form just before this.Invoke is called (remember, this is the Form object), an ObjectDisposedException will be likely fired.
The solution is to use SynchronizationContext, specifically SynchronizationContext.Current as hamilton.danielb suggests (other answers rely on specific SynchronizationContext implementations which is completely unnecessary). I would slightly modify his code to use SynchronizationContext.Post rather than SynchronizationContext.Send though (as there's typically no need for the worker thread to wait):
public partial class MyForm : Form
{
private readonly SynchronizationContext _context;
public MyForm()
{
_context = SynchronizationContext.Current
...
}
private MethodOnOtherThread()
{
...
_context.Post(status => someLabel.Text = newText,null);
}
}
Note that on .NET 4.0 and up you should really be using tasks for async operations. See n-san's answer for the equivalent task-based approach (using TaskScheduler.FromCurrentSynchronizationContext).
Finally, on .NET 4.5 and up you can also use Progress<T> (which basically captures SynchronizationContext.Current upon its creation) as demonstrated by Ryszard Dzegan's for cases where the long-running operation needs to run UI code while still working.