IMetadataAware.OnMetadataCreated is never called

Question

I created attribute class to attach metadata to properties, so I can display tooltips for the form's input fields.

HelpAttribute implements IMetadataAware:

Public Class HelpAttribute
    Inherits Attribute
    Implements System.Web.Mvc.IMetadataAware

    Public Sub New(text As String)
        _text = text
    End Sub

    Private _text As String
    Public ReadOnly Property Text As String
        Get
            Return _text
        End Get
    End Property

    Public Sub OnMetadataCreated(metadata As System.Web.Mvc.ModelMetadata) Implements System.Web.Mvc.IMetadataAware.OnMetadataCreated
        metadata.AdditionalValues.Add("HelpText", _text)
    End Sub
End Class

I utilize this metadata in my extension method:

<Extension()>
Public Function HelpFor(Of TModel, TProperty)(ByVal htmlHelper As HtmlHelper(Of TModel), expression As Expression(Of Func(Of TModel, TProperty))) As MvcHtmlString
    Dim metaData = ModelMetadata.FromLambdaExpression(Of TModel, TProperty)(expression, htmlHelper.ViewData)

    If metaData.AdditionalValues.ContainsKey("HelpText") Then
        Dim helpText = metaData.AdditionalValues("HelpText")
        Return MvcHtmlString.Create(String.Format("<span class=""help""></span><div class=""tooltip"" style=""display: none""><div class=""border-top""></div><div class=""close""><a href=""#"">close</a></div><br class=""clear""><div class=""content"">{1}</div></div>", htmlHelper.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldId(metaData.PropertyName), helpText, metaData.DisplayName))
    End If

    Return MvcHtmlString.Create(String.Format("<span class=""no_help""></span>", htmlHelper.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldId(metaData.PropertyName), metaData.DisplayName))
End Function

So I can call Html.HelpFor for any of my model's properties and if it has appropriate metadata I display a help icon which shows the tooltip on click (js).

That all works fine as long as HelpAttribute is defined in the same assembly as the classes that I decorate their properties with. Today I had to move HelpAttribute to a separate dll (different namespace as well), so I did that, I referenced the project and expected it to work. I do not get any compiler errors, the app works fine, but it does not display the help icons. I debuggedd the code and I see that the constructor of HelpAttribute is called for different properties with a proper text, but OnMetadataCreated is never called. Does anyone have an idea why that is and how to fix it?

Answer 1

Another reason this may not be called is if you have the wrong namespace referenced. So

using System.Web.ModelBinding;

will compile and not be hit, but you should be using

using System.Web.Mvc;

Answer 2

Once again I will answer my question myself. Apparently posting things on SO helps me structure the problem in my head. When I moved HelpAttribute to a seperate class library I had to reference System.Web.Mvc for IMetadataAware interface. I use .NET 4.0 and I automatically referenced MVC4 that I installed some time ago for testing purposes. It didn't get me any errors, it just did not work. When I changed System.web.Mvc to ver. 3.0 everything works smoothly.