title: Get the Source value in ConvertBack() method for IValueConverter implementation in WPF binding
body: <p>I am binding a dependency property to textboxex in WPF. The property is a string that has some values separated by '/' (example: "1/2/3/4" ). I need to bind individual values to separate textboxes which is fine with following implementation of <code>Convert()</code> method:</p>
  
  <pre class="lang-js prettyprint-override"><code>public object Convert(object value, Type targetType, object parameter,System.Globalization.CultureInfo culture)
  {
    if (!string.IsNullOrEmpty(value as string))
    {
      String[] data = (value as string).Split('/');
      return data[Int16.Parse(parameter as string)];
    }
    return String.Empty;
  }
  </code></pre>
  
  <p>And I am using the <code>ConverterParameter</code> in <code>xaml</code> to specify the position of wanted value. 
  However, the problem is with <code>ConvertBack()</code> method. I do not know, how to get the source value so I could just add or change just one value in the string (on the specified position).</p>
  
  <p>Thanks for any help.</p>