AudioSink problem

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;

public sealed class App : Application {
    public App () {
        this.Startup += this.Application_Startup;
    }
    TextBox log;
    private void Application_Startup (object sender, StartupEventArgs e) {
        StackPanel spMain = new StackPanel();
        Button btnStart = new Button() {
            Content = "Start"
        };
        btnStart.Click += new RoutedEventHandler((object s, RoutedEventArgs ev) => {
            if (CaptureDeviceConfiguration.AllowedDeviceAccess || CaptureDeviceConfiguration.RequestDeviceAccess()) {
                CaptureSource captureSource = new CaptureSource() {
                    VideoCaptureDevice = null,
                    AudioCaptureDevice = CaptureDeviceConfiguration.GetDefaultAudioCaptureDevice()
                };
                captureSource.CaptureFailed += new EventHandler<ExceptionRoutedEventArgs>((object se, ExceptionRoutedEventArgs eve) => {
                    log.Dispatcher.BeginInvoke(new Action(() => {
                        log.Text += "CaptureFailed\n";
                    }));
                });
                SampleAudioSink audioSink = new SampleAudioSink(log) {
                    CaptureSource = captureSource
                };
                captureSource.Start();
            }
        });
        spMain.Children.Add(btnStart);
        log = new TextBox() {
            AcceptsReturn = true,
            Height = 600,
            IsReadOnly = true,
            VerticalScrollBarVisibility = ScrollBarVisibility.Auto
        };
        spMain.Children.Add(log);
        this.RootVisual = spMain;
    }
}
internal sealed class SampleAudioSink : AudioSink {
    TextBox log;
    int i = 0;
    internal SampleAudioSink (TextBox log) {
        this.log = log;
    }
    protected override void OnCaptureStarted () {
        log.Dispatcher.BeginInvoke(new Action(() => {
            log.Text += "OnCaptureStarted\n";
        }));
    }
    protected override void OnCaptureStopped () {
        log.Dispatcher.BeginInvoke(new Action(() => {
            log.Text += "OnCaptureStopped\n";
        }));
    }
    protected override void OnFormatChange (AudioFormat audioFormat) {
        log.Dispatcher.BeginInvoke(new Action(() => {
            log.Text += "OnFormatChange\n";
        }));
    }
    protected override void OnSamples (long sampleTimeInHundredNanoseconds, long sampleDurationInHundredNanoseconds, byte[] sampleData) {
        log.Dispatcher.BeginInvoke(new Action(() => {
            log.Text += string.Format("OnSamples ({0})\n", ++i);
        }));
    }
}