title: How to use a MultiBinding on DataGridTextColumn?
body: <p>I have 2 properties as <code>Height</code> and <code>Width</code> on my <code>ImageDimension</code> object and I want to bind them together so it displays something like 50x60 (a x character in between)? How can I achieve this? The Code below gives me an error saying </p>
  
  <blockquote>
    <p>"Object reference not set to an object instance."</p>
  </blockquote>
  
  <pre><code>&lt;cst:CustomDataGrid x:Name="grdImageDimension"  
                      ItemsSource="{Binding ImageDimensions, IsAsync=True}"  
     &lt;DataGridTextColumn Header="ImageDimension" Width="50"&gt;
        &lt;DataGridTextColumn.Binding&gt;
           &lt;MultiBinding StringFormat="{}{0} + {1}"&gt;
              &lt;Binding Path="ImageDimensions.Height" /&gt;
              &lt;Binding Path="ImageDimensions.Width" /&gt;                              
           &lt;/MultiBinding&gt;
        &lt;/DataGridTextColumn.Binding&gt;
     &lt;/DataGridTextColumn&gt;
  &lt;/cst:CustomDataGrid&gt;
  </code></pre>
  
  <p>ViewModel:</p>
  
  <pre><code>Public Class ImageDimensionsVM
  
      Private m_ImageDimensions As ObservableCollection(Of ImageDimension)
      Public Property ImageDimensions() As ObservableCollection(Of ImageDimension)
          Get
              Return m_ImageDimensions
          End Get
          Set(value As ObservableCollection(Of ImageDimension))
              m_ImageDimensions = value
          End Set
      End Property
  </code></pre>