Description

 I have created a small class in a single ASP.NET 4.5 web forms page that is instantiated and then stored in a List, then saved/loaded from ViewState:

<Serializable>

Private Class _PageLevelToolTip

    Public ClientId As String

    Public TipText As String

End Class

Private Property _pageLevelToolTipCollection As List(Of _PageLevelToolTip)

    Get

        Return ViewState("_toolTipCollection")

    End Get

    Set(value As List(Of _PageLevelToolTip))

        ViewState("_toolTipCollection") = value

    End Set

End Property

This class only appears on one page in the whole application.

Users are reporting an intermittent error on subsequent page loads:

    [A]System.Collections.Generic.List1[selfassessment+_PageLevelToolTip] cannot be cast to [B]System.Collections.Generic.List1[selfassessment+_PageLevelToolTip].

    Type A originates from 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' in the context 'LoadNeither' at location 'C:\Windows\Microsoft.Net\assembly\GAC_64\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll'. Type B originates from 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' in the context 'LoadNeither' at location 'C:\Windows\Microsoft.Net\assembly\GAC_64\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll'.

After searching for days I found a tip that seems to solve the issue, by casting to the type explicitly:

Private Property _pageLevelToolTipCollection As List(Of _PageLevelToolTip)

    Get

        Return CType(ViewState("_toolTipCollection"), List(Of _PageLevelToolTip)) ' Revision here '

    End Get

    ' Setter removed for brevity '

End Property

Is this a bug?

We cannot test this accurately because of the wildly intermittent nature. This is a page-level private object and list that only exist in a single page.

Although the error does not trigger in the above manner, I can recreate it by doing the following process:

    . Load page (ensuring class is saved within viewstate)

    . Edit page in VS and save

    . Load page (to load class from Viewstate)

This would imply that the object changes in some way part-way through postbacks, although it's structure does not, or the compiled page changes and won't recognise the original. 

I hope this helps in the diagnosis.  Another person is reporting the same fault from a different approach, but the answer given does not help here: https://connect.microsoft.com/VisualStudio/feedback/details/779108/xaml-designer-in-vs2012-throws-invalidcastexception