Why Date Format in ASP NET MVC Differs when used inside Html Helper?

Question

I just came across a very interesting issue. If I use ViewData to pass a DateTime value to the view and then display it inside a textbox, even though I'm using String.Format in the exact same manner, I get different formatting results when using the Html.TextBox helper.

<%= Html.TextBox("datefilter", String.Format("{0:d}", ViewData["datefilter"]))%>
<input id="test" name="test" type="text" value="<%: String.Format("{0:d}", ViewData["datefilter"]) %>" />

The above code renders the following html:

<input id="datefilter" name="datefilter" type="text" value="2010-06-18" />
<input id="test" name="test" type="text" value="18/06/2010" />

Notice how the fist line that uses the Html helper produces the date format in one way while the second one produces a very different output. Any ideas why?

Note: I'm currently in Brazil, so the standard short date format here is dd/MM/yyyy.

Answer 1

The reason this happens is because the TextBox helper uses the value stored inside ViewData["datefilter"] because its name is datefilter and completely ignores the second argument you are passing which is supposed to format the date. Try changing the name of the textbox.

A better solution is to use editor templates and strongly typed views instead of ViewData. Here's an example.

Model:

public class MyModel
{
    [DisplayFormat(DataFormatString = "{0:dd/MM/yyyy}", 
                   ApplyFormatInEditMode = true)]
    public DateTime Date { get; set; }
}

Controller:

public class HomeController : Controller
{
    public ActionResult Index()
    {
        var model = new MyModel
        {
            Date = DateTime.Now
        };
        return View(model);
    }
}

View:

<%@ Page Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage<SomeNs.Models.MyModel>" %>

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
    <%: Html.EditorFor(x => x.Date) %>
</asp:Content>