Databinder.Eval using Lamda Expressions

Introduction

We all have done it, and we all know how it can be frustrating when dealing with magic strings. While not terribly hard to fix or work around, the Databinder.Eval("") method needs to use strings in order to work. However, using some extension methods (or a helper class) can remove the dependency on these magic strings and allow your code to use lambda expressions instead.

Using the Code

public static class ExtensionMethods
{
    public static string Evaluate<T>(this ListViewItem item, 
	Func<T, object> expression)
    {
        var result = expression((T)item.DataItem);
        if (result != null)
            return result.ToString();
        return null;
    }

    public static TOutput Evaluate<T, TOutput>
	(this ListViewItem item, Func<T, TOutput> expression)
    {
        return (TOutput)expression((T)item.DataItem);
    }
}		 

So, with these, instead of using code that looks like this:

<asp:GridView ID="gvName" runat="server" AutoGenerateColumns="False"
       HorizontalAlign="Center">
     <Columns>
         <asp:TemplateField HeaderText="County Name">
             <ItemTemplate>
                 <%# Eval("County.Name") %>
             </ItemTemplate>
         </asp:TemplateField>
     </Columns>
</asp:GridView>

You can use the slightly more verbose, but more stable lambda version:

 <asp:GridView ID="gvName" runat="server" AutoGenerateColumns="False" 
	HorizontalAlign="Center">
   <Columns> 
     <asp:TemplateField HeaderText="County Name">               
       <ItemTemplate>
         <%# Container.Evaluate<AddressEntity>(c=>c.County.Name) %> 
       </ItemTemplate>
     </asp:TemplateField>            
   </Columns>
</asp:GridView>  

Points of Interest

Just remember to include the namespace of wherever you placed your code either at the top of the page or in the web.config.

The second version of the Evaluate method allows you to return the data typed as it actually is instead of returning it as the string. This is useful if you need to call page methods and don't want to cast it once to a string with Eval(), then recast it back.

<%# DoSomething(Container.Evaluate<AddressEntity, int?>(c=>c.County.CountyID)) %>   

History

• 04/07/2010 - Original draft