Question

我一直在尝试创建一个返回类型对象的函数。下面的这个函数目前在我的数据类中，可以从任何继承数据类的对象调用。我的问题是当我尝试返回我的对象时。当我在函数内部时它会很好地填充。当我回到调用对象时，它全部为空，就像它是一个新对象。

这是函数....

    Public Overloads Function GetClassFromDB(ByVal ID As Integer) As Object
        Try
            Dim BaseObject As New Object
            Dim objDerived As Type = MyBase.GetType()
            Dim TableName As String = String.Empty
            Dim SQL As New LottoPayload.SQLiDataClass
            Dim SQLString As String = String.Empty
            Dim SQLCommand As SQLiteCommand = Nothing
            Dim SQLConnection As SQLiteConnection = Nothing
            Dim SQLiteDRObj As SQLiteDataReader = Nothing
            Dim SQLResultsTable As New DataTable

            'Create an instance of the base object
            BaseObject = Activator.CreateInstance(objDerived)
            'Get the tablename from the object
            TableName = objDerived.GetProperty("TableName").GetValue(BaseObject, Nothing).ToString

            SQLString = "SELECT * FROM " & TableName & " WHERE ID = '" & ID.ToString & "' LIMIT 1"
            SQLConnection = SQL.GetSqlConnection()

            SQLConnection.Open()
            SQLCommand = New SQLiteCommand(SQLConnection)
            SQLCommand.CommandText = SQLString

            SQLiteDRObj = SQLCommand.ExecuteReader()

            SQLResultsTable.Load(SQLiteDRObj)
            If SQLResultsTable.Rows.Count > 0 Then
                For Each Row As DataRow In SQLResultsTable.Rows

                    For Each Column As DataColumn In Row.Table.Columns

                        Dim ColumnName As String = Column.ColumnName.ToString
                        Console.WriteLine(Row.Item(ColumnName))
                        Dim ColumnValue As Object = Row.Item(ColumnName)
                        Console.WriteLine(Column.DataType.Name & "    ")
                        Select Case Column.DataType.Name
                            Case "Int64"
                                Dim ConvertedValue As Integer
                                Integer.TryParse(ColumnValue.ToString, ConvertedValue)
                                objDerived.GetProperty(ColumnName).SetValue(BaseObject, ConvertedValue, Nothing)
                            Case "Double"
                                Dim ConvertedValue As Double
                                Double.TryParse(ColumnValue.ToString, ConvertedValue)
                                objDerived.GetProperty(ColumnName).SetValue(BaseObject, ConvertedValue, Nothing)
                            Case "String"
                                Dim ConvertedValue As String
                                ConvertedValue = ColumnValue.ToString
                                objDerived.GetProperty(ColumnName).SetValue(BaseObject, ConvertedValue, Nothing)
                            Case Else
                                Dim ConvertedValue As String
                                ConvertedValue = "NotSet"
                                objDerived.GetProperty(ColumnName).SetValue(BaseObject, ConvertedValue, Nothing)
                        End Select

                    Next
                Next
            End If

            Return objDerived

            SQLiteDRObj.Close()
            SQLConnection.Close()
            SQLConnection.Dispose()
            'Return False
        Catch ex As Exception
            Return False
        End Try
    End Function

这就是它的调用方式。

        Dim objResults As New myapp.objResult
        Dim ID As Integer = 3
        objResults.GetClassFromDB(ID)

问题＃1 - 当我从GetClassFromDB返回对象时，objResults是空的，尽管它有数据直到Return objDerived。为什么会这样？

问题＃2 - 有没有办法可以消除Select Case区域并使用反射合并代码？

提前感谢您的帮助。

Object Is Filled Here Object is Empty Here

Answer 1

鉴于BaseObject实际上是一个objDerived对象，表示创建的对象的类型而不是对象，您不应该返回objDerived而不是Type本身？如果你没有首先使用像objDerived这样令人震惊的名字，那就更清楚了。

Answer 2

问题是你的方法返回一个对象，但你没有在任何地方将它赋值给一个变量。您需要将方法的返回值分配给变量

Dim someVariable As Object = objResults.GetClassFromDB(ID)

另外，你的方法很奇怪，如果代码成功运行它会返回一个对象，但是如果有一个异常，你会默认吞下一个布尔值（这也是一个不好的做法）。

Answer 3

您正在返回错误的对象。 objDerived是一个Type，而不是您正在设置的对象。您需要返回BaseObject。另外，要意识到关闭和处理SqlCommand和Connection的代码不会被命中，因为它在返回运算符之后。这里最好将连接和命令包装在Using子句中，这将确保它们在超出范围时将被处理掉。看看以下是否有效：

Public Overloads Function GetClassFromDB(ByVal ID As Integer) As Object
        Try
            Dim BaseObject As New Object
            Dim objDerived As Type = MyBase.GetType()
            Dim TableName As String = String.Empty
            Dim SQL As New LottoPayload.SQLiDataClass
            Dim SQLString As String = String.Empty
            Using SQLConnection As SQLiteConnection = Nothing
            Using SQLCommand As SQLiteCommand = Nothing
            Dim SQLiteDRObj As SQLiteDataReader = Nothing
            Dim SQLResultsTable As New DataTable

            'Create an instance of the base object
            BaseObject = Activator.CreateInstance(objDerived)
            'Get the tablename from the object
            TableName = objDerived.GetProperty("TableName").GetValue(BaseObject, Nothing).ToString

            SQLString = "SELECT * FROM " & TableName & " WHERE ID = '" & ID.ToString & "' LIMIT 1"
            SQLConnection = SQL.GetSqlConnection()

            SQLConnection.Open()
            SQLCommand = New SQLiteCommand(SQLConnection)
            SQLCommand.CommandText = SQLString

            SQLiteDRObj = SQLCommand.ExecuteReader()

            SQLResultsTable.Load(SQLiteDRObj)
            If SQLResultsTable.Rows.Count > 0 Then
                For Each Row As DataRow In SQLResultsTable.Rows

                    For Each Column As DataColumn In Row.Table.Columns

                        Dim ColumnName As String = Column.ColumnName.ToString
                        Console.WriteLine(Row.Item(ColumnName))
                        Dim ColumnValue As Object = Row.Item(ColumnName)
                        Console.WriteLine(Column.DataType.Name & "    ")
                        Select Case Column.DataType.Name
                            Case "Int64"
                                Dim ConvertedValue As Integer
                                Integer.TryParse(ColumnValue.ToString, ConvertedValue)
                                objDerived.GetProperty(ColumnName).SetValue(BaseObject, ConvertedValue, Nothing)
                            Case "Double"
                                Dim ConvertedValue As Double
                                Double.TryParse(ColumnValue.ToString, ConvertedValue)
                                objDerived.GetProperty(ColumnName).SetValue(BaseObject, ConvertedValue, Nothing)
                            Case "String"
                                Dim ConvertedValue As String
                                ConvertedValue = ColumnValue.ToString
                                objDerived.GetProperty(ColumnName).SetValue(BaseObject, ConvertedValue, Nothing)
                            Case Else
                                Dim ConvertedValue As String
                                ConvertedValue = "NotSet"
                                objDerived.GetProperty(ColumnName).SetValue(BaseObject, ConvertedValue, Nothing)
                        End Select

                    Next
                Next
            End If


            End Using ' Sql Command
            End Using ' Sql Connection
            Return BaseObject

            'Return False
        Catch ex As Exception
            Return False
        End Try
    End Function

结束班

从第二个屏幕截图中，您似乎没有为GetClassFromDB（ID）方法的返回设置任何内容。它可能应该是这样的：

Dim result = objResults.GetClassFromDB(ID)
' Do something with result

VB.net返回填充对象不起作用的函数

3 个答案: