gistfile1.txt

void Func ()
{
    ThreadPool.QueueUserWorkItem (delegate {
        InvokeOnMainThread (() => {
            myLabel.Text = "Computation beginning...";
        });

        Thread.Sleep (10000); // Simulate downloading a large file or calculating lots of stuff

        InvokeOnMainThread (() => {
            myLabel.Text = "Computation complete!";
        });
    });
}