title: Binding to TimeSpan field with IMultiValueConverter
body: <p>I am getting an exception for this binding</p>
  
  <pre><code>&lt;TextBlock.Text&gt;
    &lt;MultiBinding Converter="{StaticResource converter}" Mode="OneWay"&gt;
      &lt;Binding Path="TimeSpanProperty" /&gt;
      &lt;Binding Path="FormsttingOption" /&gt;
    &lt;/MultiBinding&gt;
  &lt;/TextBlock.Text&gt;
  
  
  // converter.Convert code
  if (formsttingOption == DurationFormat.Minutes)
    return Math.Round(timeSpan.TotalMinutes);
  else
    return duration;
  </code></pre>
  
  <p>Value produced by BindingExpression is not valid for target property.; Value='164' MultiBindingExpression:target element is 'TextBlock' (Name=''); target property is 'Text' (type 'String')</p>
  
  <p>The converter will, based on formatting option, convert TimeSpan value. In above example, FormattingOption is minutes, which means that TimeSpan value will be converted to minutes (164).</p>
  
  <p>For some reason Text property does not accept this value. Anyone knows the reason, and solution?</p>