Question

I have a generic list of custom objects and would like to reduce that list to objects where a specific property value is not in a list of exclusions.

I have tried the following:

Private Sub LoadAddIns()   // Get add-in templates   Dim addIns = GetTemplates(TemplateTypes.AddIn)   // Get the current document   Dim sectionId As String = CStr(Request.QueryString('sectionId'))   Dim docId As Integer = CInt(Split(sectionId, ':')(0))   Dim manual = GetTempManual(docId)   Dim content As XElement = manual.ManualContent   // Find which templates have been used to create this document.   Dim usedTemplates = (From t In content.<header>.<templates>.<template> _                        Select CInt(t.<id>.Value)).ToList   // Exclude add-ins that have already been used.   If usedTemplates IsNot Nothing Then     addIns = addIns.Where(Function(a) usedTemplates.Contains(a.TemplateID) = False)   End If   // Bind available add-ins to dropdown   With ddlAddIns     .DataSource = addIns     .DataTextField = 'Title'     .DataValueField = 'TemplateID'     .DataBind()     .Items.Insert(0, New ListItem('[select an add-in]', 0))   End With End Sub 

but get the error:

System.InvalidCastException: Unable to cast object of type ‘WhereListIterator1[MyApp.Classes.Data.Entities.Template]' to type 'System.Collections.Generic.List1[MyApp.Classes.Data.Entities.Template]’.

How can I select only the templates where the template id is not in the list of exclusions?

Answer 1

Tack a ToList() extension to the end of the Where extension to convert it back to a List of the appropriate type.

If usedTemplates IsNot Nothing Then     addIns = addIns.Where(Function(a) usedTemplates.Contains(a.TemplateID) = False) _                    .ToList() End If