ASP.NET MVC Display Template for strings is used for integers
I recently hit an issue with ASP.NET MVC display templates. Say this is my model:
public class Model
{
public int ID { get; set; }
public string Name { get; set; }
}
this is the controller:
public class HomeController : Controller
{
public ActionResult Index()
{
return View(new Model());
}
}
and this is my view:
<%@ Page Language="C#" Inherits="System.Web.Mvc.ViewPage<DisplayTemplateWoes.Models.Model>" %>
<!DOCTYPE html>
<html>
<head runat="server">
<title>Index</title>
</head>
<body>
<div>
<%: Html.DisplayForModel() %>
</div>
</body>
</html>
If I need for some reason a display template for all strings I will create a String.ascx partial view like this:
<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<string>" %>
<%: Model %> (<%: Model.Length %>)
And here is the problem - at runtime the following exception is thrown: "The model item passed into the dictionary is of type 'System.Int32', but this dictionary requires a model item of type 'System.String'"
It seems that String.ascx is used for both the integer and string property of the Model class. I expected it to be used only for the string property - after all it is named String.ascx not Object.ascx or Int32.ascx.
Is this by design? If yes - is it documented somewhere? If not - can it be considered a bug?
This seem to be by design. You will have to make string template more general. String template works as default template for every non-complex model that doesn't have it's own template.
Default template for string (FormattedModelValue is object):
internal static string StringTemplate(HtmlHelper html) {
return html.Encode(html.ViewContext.ViewData.TemplateInfo.FormattedModelValue);
}
Template selection looks like this:
foreach (string templateHint in templateHints.Where(s => !String.IsNullOrEmpty(s))) {
yield return templateHint;
}
// We don't want to search for Nullable<T>, we want to search for T (which should handle both T and Nullable<T>)
Type fieldType = Nullable.GetUnderlyingType(metadata.RealModelType) ?? metadata.RealModelType;
// TODO: Make better string names for generic types
yield return fieldType.Name;
if (!metadata.IsComplexType) {
yield return "String";
}
else if (fieldType.IsInterface) {
if (typeof(IEnumerable).IsAssignableFrom(fieldType)) {
yield return "Collection";
}
yield return "Object";
}
else {
bool isEnumerable = typeof(IEnumerable).IsAssignableFrom(fieldType);
while (true) {
fieldType = fieldType.BaseType;
if (fieldType == null)
break;
if (isEnumerable && fieldType == typeof(Object)) {
yield return "Collection";
}
yield return fieldType.Name;
}
}
So if you want to create template only for string, you should do it like this (String.ascx):
<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<object>" %>
<% var model = Model as string; %>
<% if (model != null) { %>
<%: model %> (<%: model.Length %>)
<% } else { %>
<%: Model %>
<% } %>