Evaluating The DataBinder

I was trying to put together a quick piece of code as an example over the weekend and remembered how much I dislike using DataBinder in ASP.NET.

Let’s say a web service call gives back an array of simple objects (the sort of objects you’d see imported by a web reference):

class Parameter
{  
 public string Name;
 public string Value;
}

I wanted to dump the Parameter array to a web page as easily as possible. The following will not work.

<asp:Repeater id="Repeater1" runat="server">
   <ItemTemplate>
      <tr>
         <td>
            <%# DataBinder.Eval(Container.DataItem, "Name") %>
         </td>
         <td>
            <%# DataBinder.Eval(Container.DataItem, "Value") %>
         </td>
      </tr>
   </ItemTemplate>
</asp:Repeater>

The DataBinder, besides looking awkward, only finds public properties - it doesn’t find public fields. The quick hack to get around this is to replace the language agnostic DataBinder syntax with C# code:

<asp:Repeater id="Repeater1" runat="server">
    <ItemTemplate>
        <tr>
            <td>
                <%# ((Parameter)(Container.DataItem)).Name %>
            </td>
            <td>
                <%# ((Parameter)(Container.DataItem)).Value %>
            </td>
        </tr>
    </ItemTemplate>
</asp:Repeater>

Disadvantages:
There is a huge disadvantage in that if the Parameter class fields ever change, the code in the ASPX won’t generate an error until runtime. It’s also a problem if the ASPX moved from a C# project to a VB.NET project.

Advantages:
To me the second example looks cleaner – I can see an object and a property even through the parentheses of a cast. It’s also blazingly faster - two order of magnitude faster. In fact, in my tests the following:

<%# DataBindParameterName(Container.DataItem) %>

// in the code behind of the class:
protected string DataBindParameterName(object o)
{
    return ((Parameter)o).Name;
}

is still 300x faster than the reflection machinations DataBinder.Eval uses. Food for thought in perf critical scenarios.