Question

我需要知道如何将以下内容转换为VB.net以进行Async / Await项目：

public static class Extensions {

    public static IEnumerable<T> Select<T>(
        this SqlDataReader reader, Func<SqlDataReader, T> projection) {

        while (reader.Read()) {
            yield return projection(reader);
        }
    }
}

用法：

'call the database asynchronously... and 'await' the results
Using reader = Await Db.GetReaderAsync("spGetDashboard", ParamList)
    Return reader.Select(Function(r)
                             Return New DashboardInfo With {.RowNum = CType(r.Item("RowNum"), Long?)}
                         End Function)

这是我到目前为止所做的：

<Extension()>
Public Function [Select](Of T)(reader As SqlDataReader, projection As Func(Of SqlDataReader, T)) As IEnumerable(Of T)
    While reader.Read()
        Return CType(projection(reader), IEnumerable(Of T))
    End While
End Function

以下是例外：

System.InvalidCastException：无法转换类型的对象键入'WindowsApplication1.DashboardInfo' 'System.Collections.Generic.IEnumerable`1 [WindowsApplication1.DashboardInfo]'。在WindowsApplication1.Extensions.Select [T]（SqlDataReader阅读器， Func`2投影）in C：\ Dev \ AsyncDbTesting \ AsyncDbTesting \ Library \ Extensions.vb：第523行在 WindowsApplication1.Process.VB $ StateMachine_1_GetDashboardAsync.MoveNext（）在C：\ Dev \ AsyncDbTesting \ AsyncDbTesting \ Process.vb：第30行 ---从抛出异常的先前位置开始的堆栈跟踪结束--- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess（任务任务） System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification（任务任务）在System.Runtime.CompilerServices.TaskAwaiter`1.GetResult（）在 WindowsApplication1.Form1.VB $ StateMachine_0_Button1_Click.MoveNext（）在C：\ Dev \ AsyncDbTesting \ AsyncDbTesting \ Form1.vb：第17行

Answer 1

因为您尝试将类型为T的项目转换为IEnumerable(Of T)类型而引发异常。

假设您正在使用.Net framework 4.5，您想要的代码是

<Extension()>
Public Iterator Function [Select](Of T)(reader As SqlDataReader, projection As Func(Of SqlDataReader, T)) As IEnumerable(Of T)
    While reader.Read()
        Yield projection(reader)
    End While
End Function

请注意，.Net framework 4.5之前的VB.NET中不存在Yield和迭代器的概念。

如何将Datareader投影到预定义的对象中

1 个答案: