Question

enter code hereWant to remove items from a Main list , but give an error ‘Unable to cast object of type <ExceptIterator>d__99'1[‘

    Public Class FieldCollectionItemCompare
    Implements System.Collections.Generic.IEqualityComparer(Of FieldCollectionItem)

    Public Shadows Function Equals(ByVal x As FieldCollectionItem, ByVal y As FieldCollectionItem) As Boolean Implements System.Collections.Generic.IEqualityComparer(Of FieldCollectionItem).Equals

        If x.UniqueID = y.UniqueID Then
            Equals = True
        Else
            Equals = False
        End If
    End Function

    Public Overloads Function GetHashCode(ByVal obj As FieldCollectionItem) As Integer Implements System.Collections.Generic.IEqualityComparer(Of FieldCollectionItem).GetHashCode
        GetHashCode = obj.UniqueID + obj.UniqueID
    End Function

    End Class

…here is what I am doing

FieldCollectionToProcessList = FieldCollectionToProcessList.Intersect(FieldCollectionRejected, New FieldCollectionItemCompare)

FieldCollectionToProcessList = FieldCollectionToProcessList.Intersect(FieldCollectionAccepted, New FieldCollectionItemCompare)

…all the list are As Generic.List(Of FieldCollectionItem)

Answer 1

You’re trying to assign the result of Intersect back to FieldCollectionToProcessList. The result isn’t a List(Of FieldCollectionItem) – it’s an IEnumerable(Of FieldCollectionItem). You’ll need to call ToList if you need to turn it back into a list. You could do it all in one step though:

// Variable names changed for sanity
list = list.Intersect(rejected, New FieldCollectionItemCompare).
            Intersect(accepted, New FieldCollectionItemCompare).
            ToList

Note that if you have Option Strict on, you should be able to see the error at compile time.

You might also want to consider creating a HashSet(Of FieldCollectionItem), then use IntersectWith.

None of this has anything to do with your custom equality comparer, although I note that you could just use:

Return x.UniqueID = y.UniqueID

for Equals, and

Return obj.UniqueID

for GetHashCode.