Question

我并不习惯使用VB.NET，但是我发现了谷歌搜索不足的错误。我已经创建了这个数据访问类，它有一个方法可以使用Yield和vb.net的对象初始化快捷方式

Public Class FMACMarketingRepository
    Private cvConnection As SqlConnection
    Private cvCommand As SqlCommand
    Private cvReader As SqlDataReader

    Public Sub New(Role As InfotelLibrary.Data.DatabaseRole)
        cvConnection = New SqlConnection(InfotelLibrary.Data.ConnectionString(Role, InfotelLibrary.Data.DatabaseName.Mongoose))
    End Sub

    Public Iterator Function GetAllFrontPageBanners() As IEnumerable(Of FrontPageBanner)
        Using dbConnection As IDbConnection = cvConnection
            Using cmd As IDbCommand = dbConnection.CreateCommand()
                cmd.CommandType = CommandType.StoredProcedure
                cmd.CommandText = "sel_AllFmacFrontPageBanners"
                Using reader As IDataReader = cmd.ExecuteReader()
                    If reader Is Nothing Then
                        Yield Nothing
                    End If
                    While reader.Read()
                        Yield New FrontPageBanner() With
                        {
                            .Banner_Id = CType(reader("Banner_ID"), Integer),
                            .Geo_Id = CType(reader("Geo_ID"), Long),
                            .Title = CType(reader("Title"), String),
                            .Description = CType(reader("Description"), String),
                            .Link = CType(reader("Link"), String),
                            .Image = CType(reader("Image"), Byte()),
                            .SpecialOffer = CType(reader("Special_Offer"), Boolean)
                        }
                    End While
                End Using
            End Using
        End Using
    End Function
End Class

Intellisense中有0个错误，它会构建，但是当我运行网页时会出现错误

System.InvalidCastException：无法转换类型＆＃39; VB $ StateMachine_4_GetAllFrontPageBanners＆＃39;输入＆＃39; System.Collections.Generic.List`1 [InfotelData.Mongoose.Data.FrontPageBanner]＆＃39;。

Line 7:  
Line 8:      Protected Sub Page_Load(ByVal sender As Object, e As EventArgs) Handles Me.Load
Line 9:          Dim banners As List(Of FrontPageBanner) = cvRepo.GetAllFrontPageBanners()
Line 10:         If banners.Count() > 0 Then
Line 11:             rptUploadedBanners.DataSource = banners

调试只会在遇到page_load时出现相同的错误。

我认为应该责备用户错误。

Answer 1

GetAllFrontPageBanners会返回IEnumerable(Of FrontPageBanner) （*） ，您正试图将其存储在List(Of FrontPageBanner)中。我很惊讶它没有给出编译时错误（你可能处于Option Strict Off模式）

您需要使用.ToList从可枚举中创建一个列表，例如：

Dim banners As List(Of FrontBanner) = cvRepo.GetAllFrontPageBanners.ToList

_{（*） Iterator块内部将函数转换为生成的“class-state-machine”（实现IEnumerable（Of FrontBanner））在你的情况下）
这是你得到的奇怪的名字，但你可以（并且应该）将它视为源代码中给出的返回类型
有关here}

的更多信息

无法转换类型＆＃39; VB $ StateMachine_4_GetAllFrontPageBanners＆＃39;键入IEnumerable（Of FrontPageBanner）

1 个答案: