ASP.NET MVC ForEach-Loop Sugar

by paul on 8/26/2010 3:13:52 PM

When you make the move from classic ASP.NET MVC webforms, one of the funny questions you hear is, “Where is the repeater control?”. The MVC veteran responds, “It’s called a For-Loop”. However, if you are used to using the ASP.NET DataList control for doing things like providing repeating columns, then a standard ForEach-Loop won’t do without some help. While I’m sure there are a number of ways to handle this, I want to demonstrate a simple re-usable way to extend what information is available in the foreach loop.

To provide additional information to the loop, I’ve created an extension method that takes an IEnumerable<T> and returns a list of IEnumerable<ForEachItem<T>>. The extra data for the loop is provided in the ForEachItem<T> class.

public class ForEachItem<T> { public T Item { get; set; } public bool First { get; set; } public bool Last { get; set; } public int Index { get; set; } public bool NewGroup { get; set; } }

Here is a list of what each Property represents:

  • Item: This provides access to the original data item
  • First: This boolean is true if it is the first item in the list
  • Last: This boolean is true if it is the last item in the list
  • Index: This is the index of the item in the list
  • NewGroup: This bool represents if this item is the first in a new group (explained below)

The extension method simply iterates through the source list and creates a ForEachItem instance for each item.

public static IEnumerable<ForEachItem<T>> ToFor<T>(this IEnumerable<T> items) { return ToFor(items, 0); } public static IEnumerable<ForEachItem<T>> ToFor<T>(this IEnumerable<T> items, int group) { var list = new List<ForEachItem<T>>(); foreach (var item in items) { var fei = new ForEachItem<T> { First = (list.Count == 0), Item = item, Index = list.Count }; fei.NewGroup = (group > 0 && (list.Count % group == group - 1)); list.Add(fei); } if (list.Count > 0) list[list.Count - 1].Last = true; return list; }

The first extension method ToFor(items) can be used when you don’t require grouping. The second method allows you to provide an integer which will set NewGroup to true for every X items. This can be used to produce things like the RepeatColumns function of the DataList. Here is some example usage to create a table from a loop that has 4 columns across:

 

<table> <tr> <% foreach (var m in users.ToFor(4)) { %> <td><%= m.Item.Name %></td> <% if (m.NewGroup) app.Write("</tr><tr>"); %> <% } %> </tr> </table>

Another use could be to format a menu of links for a website that need to have a pipe character between the links:

<% foreach (var p in pages.ToFor()) { app.Write("<a href='{0}'>{1}</a>", p.Item.Href, p.Item.Title); if (!p.Last) app.Write("&nbsp;&nbsp;|&nbsp;&nbsp;"); } %>

If you find this useful, feel free to leave a comment and let me know. I’d also love to hear if you have other techniques that you use to solve the same sorts of problems.

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