【解決方法】タイプ ‘whereenumerableiterator`1 のオブジェクトをキャストできません[object]’ system.linq.iqueryable`1 と入力します[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?

私が試したこと:

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

私も追加しました

.Select(Function(p) p)

.Where ステートメントの最後まで続けましたが、それも機能しませんでした。

<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

解決策 1
解決策 2

解決策 1

使ってみる Enumerable.Cast(TResult) メソッド (IEnumerable) (System.Linq)[^] :

VB.NET

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

または Enumerable.OfType(TResult) メソッド (IEnumerable) (System.Linq)[^]:

VB.NET

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

詳細については、以下を参照してください。サポートされているおよびサポートされていない LINQ メソッド (LINQ to Entities) Microsoft ドキュメント[^]

解決策 2

ASP.NET フォーラムで解決策を見つけました。追加した AsQueryable() 私の終わりまで query 変数を変更してうまくいきました！

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

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