title: System.FormatException in MVVMCross
body: <p>The following code updates properties on-the-fly via mvvmcross messaging protocol. </p>
  
  <p>The problem that I am facing now, when user clears either <code>age</code> or <code>category</code> textviews, then I am getting systemformat exception, I guess when <code>textview</code> gets blank, it does not parse it to a number?</p>
  
  <blockquote>
    <p>System.FormatException: Input string was not in a correct format.</p>
  </blockquote>
  
  <p><strong>MainViewModel</strong></p>
  
  <pre><code>private readonly IMvxMessenger _messenger;
  private readonly MvxSubscriptionToken _token;
  
  public MainViewModel(IMvxMessenger messenger) {
  
      _messenger = messenger;
      _token = messenger.Subscribe&lt;SelectedItemMessage&gt;(OnMessageReceived);;
  }
  
  private void OnMessageReceived(SelectedItemMessage obj)
  {
      Age = obj.Age;
      Category= obj.Category;
  }
  </code></pre>
  
  <p><strong>DetailViewModel</strong></p>
  
  <pre><code>private readonly IMvxMessenger _messenger;
  
  public DetailViewModel(IMvxMessenger messenger) {
      _messenger = messenger;
  }
  
  public void UpdateMethod() {
      var message = new SelectedItemMessage(this,  age, category); 
      _messenger.Publish(message, typeof(SelectedItemMessage));
  }
  </code></pre>
  
  <p><strong>SelectedItemMessage</strong></p>
  
  <pre><code>public SelectedItemMessage(object sender, double age, int category) : base(sender)
      {
          Age = age;
          Category = category;
      }
  
      public double Age { get; set; }
      public int Category{ get; set; }
  }
  </code></pre>