title: How to calculate a value in WPF Binding
body: <p>I have an app which uses two sliders to generate a product used elsewhere in the code.  What I would like is to have the product value bound to a textblock or tooltip, for example, to look something like "10 x 15 = 150".</p>
  
  <p>The first part is easy, and looks like this:</p>
  
  <pre><code>&lt;TextBlock.Text&gt;
      &lt;MultiBinding StringFormat="{}{0} x {1}"&gt;
          &lt;Binding ElementName="amount_slider" Path="Value" /&gt;
          &lt;Binding ElementName="frequency_slider" Path="Value"/&gt;
      &lt;/MultiBinding&gt;
  &lt;/TextBlock.Text&gt;
  </code></pre>
  
  <p>But what's a nice easy way to get the product in there as well?</p>
  
  <p>Using Pavlo Glazkov's solution, I modified it to look like this:</p>
  
  <pre><code>public class MultiplyFormulaStringConverter : IMultiValueConverter
  {
      public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
      {
          var doubleValues = values.Cast&lt;double&gt;().ToArray();
          double x = doubleValues[0];
          double y = doubleValues[1];
          var leftPart = x.ToString() + " x " + y.ToString();
          var rightPart = (x * y).ToString();
          var result = string.Format("{0} = {1}", leftPart, rightPart);
          return result;
      }
  
      public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
      {
          throw new NotImplementedException();
      }
  }
  </code></pre>
  
  <p>And the all-important </p>
  
  <pre><code>&lt;Window.Resources&gt;
      &lt;local:MultiplyFormulaStringConverter x:Key="MultiplyFormulaStringConverter"/&gt;
  &lt;/Window.Resources&gt;
  </code></pre>
  
  <p>Thanks!</p>