Почему данный код блокирует UI?

private async void MenuOpen_Click(object sender, RoutedEventArgs e) {
            Microsoft.Win32.OpenFileDialog fileDialog = new Microsoft.Win32.OpenFileDialog();
            fileDialog.DefaultExt = ".txt";
            fileDialog.Filter = "Text file|*.txt|All files|*.*";
            fileDialog.Multiselect = false;

            if (!(bool)fileDialog.ShowDialog())
                return;

            string content = String.Empty;

            using (var file = new StreamReader(fileDialog.FileName)) {
                content = await file.ReadToEndAsync();
            }

            tbLeft.Text = content;
        }