Using extension methods as binding expressions

I recently ran into an issue with an asp.net ListView where I wanted to “bind” to an extension method on the entity I was working with.

So, my object looked something like this:

namespace ExtensionMethodBinding
{
 public class MyCustomObject
 {
   public string Property1 { get; set; }
   public string Property2 { get; set; }
   public string Property3 { get; set; }
 }

}

namespace ExtensionMethodBinding.Extensions
{
 public static class MyCustomPropertyExtensions
 {
   public static string SomeExtensionMethod(this MyCustomObject instance)
   {
     return String.Format("{0} - {1} - {2}",
       instance.Property1,
       instance.Property2,
       instance.Property3);
   }
 }

}

I deliberately put the extension method in a different namespace (since that is usually the case).
And the list view item template looks something like this:

<ItemTemplate>
 <tr>
 <td>
   <%# Eval("Property1") %>
 </td>
 <td>
   <%# Eval("Property2") %>
 </td>
 <td>
   <%# Eval("Property3") %>
 </td>
 <td>
   <%# ((MyCustomObject)Container.DataItem).SomeExtensionMethod() %>' />
 </td>
 </tr>
</ItemTemplate>

And when running it, this little error message turns up:

“CS0246: The type or namespace name ‘MyCustomObject’ could not be found (are you missing a using directive or an assembly reference?)”

I finally figured it out, you have to add a couple of things at the top of the page for it to understand the extension method syntax, like this: