No se puede convertir un objeto de tipo ‘whereenumerableiterator`1[object]’ para escribir ‘system.linq.iqueryable`1[object]’

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I'm using some free source code to create an e-commerce site in WebForms. It is the Visual Basic version of Getting Started with ASP.NET 4.5 Web Forms and Visual Studio 2013. I have been successful until I coded the ProductDetails.aspx.vb page. I replaced my object names and properties accordingly, but when I run the project, I get this error:

Unable to cast object of type 'WhereEnumerableIterator`1[LethalLibrary.Book]' to type 'System.Linq.IQueryable`1[LethalLibrary.Book]'.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.InvalidCastException: Unable to cast object of type 'WhereEnumerableIterator`1[LethalLibrary.Book]' to type 'System.Linq.IQueryable`1[LethalLibrary.Book]'.

Does Linq from VS2017 differ from VS2013? How do I fix this?

Lo que he probado:

V.B.
Public Function GetBook(<QueryString("BookID")> BookId As Nullable(Of Integer)) As IQueryable(Of Book)
		Dim db = New BookContext()
		Dim query As IQueryable(Of Book) = db.Books
		If BookId.HasValue AndAlso BookId > 0 Then
			query = query.Where(CType(Function(p) p.BookID = BookId, Func(Of Book, Boolean)))
		ElseIf Not String.IsNullOrEmpty(BookTitle) Then
			query = query.Where(Function(p) String.Compare(p.BookTitle, BookTitle) = 0)
		Else
			query = Nothing
		End If
		Return query
	End Function

También he añadido

V.B.
.Select(Function(p) p)

hasta el final de mi declaración .Where, pero eso tampoco ha funcionado.

V.B.
<pre>Public Function GetBook(<QueryString("BookID")> BookId As Nullable(Of Integer)) As IQueryable(Of Book)
		Dim db = New BookContext()
		Dim query As IQueryable(Of Book) = db.Books
		If BookId.HasValue AndAlso BookId > 0 Then
			query = query.Where(CType(Function(p) p.BookID = BookId, Func(Of Book, Boolean))).Select(Function(p) p)
		Else
			query = Nothing
		End If
		Return query
	End Function

Solución 1

Intenta usar Enumerable.Cast(TResult) Método (IEnumerable) (System.Linq)[^] :

VB.NET
Dim query As IQueryable(Of Book) = db.Books.Cast(Of Book)() _
    .Where(Function(x) x.BookID = BookId) _
    .ToList()

o Enumerable.OfType(TResult) Método (IEnumerable) (System.Linq)[^]:

VB.NET
Dim query As IQueryable(Of Book) = db.Books.OfType(Of Book)() _
    .Where(Function(x) x.BookID = BookId) _
    .ToList()

Para obtener más detalles, consulte: Métodos LINQ admitidos y no admitidos (LINQ to Entities) | Documentos de Microsoft[^]

Solución 2

Encontré una solución en los foros de ASP.NET; yo añadí AsQueryable() hasta el final de mi query variable y funcionó!

V.B.
Public Function GetBook(BookId As Nullable(Of Integer)) As IQueryable(Of Book)
    Dim BookTitle As String = "aaaa"
    Dim db = New ApplicationDbContext()
    Dim query As IQueryable(Of Book) = db.Books
    If BookId.HasValue AndAlso BookId > 0 Then
        query = (query.Where(CType(Function(p) p.BookId = BookId, Func(Of Book, Boolean)))).AsQueryable()
    ElseIf Not String.IsNullOrEmpty(BookTitle) Then
        query = (query.Where(Function(p) String.Compare(p.BookTitle, BookTitle) = 0)).AsQueryable()
    Else
        query = Nothing
    End If
    Return query
End Function

Solución 3

AsQueryable también funcionó para mí (aunque en cSharp)
gracias

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