// worker
class Worker
{
    public Action<string> OnComplete;

    public Worker( Action<string> _aOnComplete )
    {
        OnComplete = _aOnComplete;
    }

    public Start( int _iNumber )
    {
        Thread.Sleep( 2000 );
        OnComplete?.Invoke( "Worker #{_iNumber.ToString()} finished." );
    }
}

// ui
class MainForm : System.Windows.Forms.Forms
{
    // ui zeug

    public void Initialize()
    {
        for( int i = 0; i < 100; i++ )
        {
            Worker w = new Worker( ( string _pszMsg ) => { TextBox1.Invoke( new MethodInvoker( () => { TextBox1.Text += "{_pszMsg}\n" } ) ) } );
            w.Start( i );
        }
    }
}