从 SqlDataReader 检索数据时出现 InvalidCastException

Question

我怎样才能摆脱：

System.InvalidCastException: 'Conversion from string "Type" to type 'Integer' is not valid.'"

在线：

Dim usertype = Reader.GetString("Type")

这是我的完整代码：

Private Sub OKbt1_Click(sender As Object, e As EventArgs) Handles OKbt1.Click
    Call Connect() ' [ connection to module ]'
    Dim Reader As SqlDataReader

    Try
        Dim command As New SqlCommand("select * from uinfo where password = '" & PASStb2.Text & "'", sqlConn)
        Reader = command.ExecuteReader
        Reader.Read()

        Dim count As Integer = 0

        While Reader.Read
            count = count + 1
        End While

        If count = 1 Then

            ' ** MY ERROR **
            Dim usertype = Reader.GetString("Type")

            If usertype = "admin" Then
                'MsgBox("username and password are correct")
                MAIN_MENU.Show()

                For a = 0 To 500

                Next
                Me.Hide()
                sqlConn.Close()
                sqlConn.Dispose()

            ElseIf usertype = "user" Then

                For a = 0 To 500

                Next
                Me.Hide()
                'MsgBox("username and password are correct")
                USERMENU.Show()

            End If

        ElseIf count > 1 Then
            MsgBox("username and password are duplicate")

        Else
            MsgBox("username and password are not correct")
        End If

        sqlConn.Close()

    Catch ex As SqlException
        MsgBox(ex.Message)

    Finally
        sqlConn.Dispose()

    End Try
End Sub

Answer 1

'Reader.GetString' 接受整数参数，而不是字符串。或者，做

Reader.Item("Type").ToString()

Answer 2

将用户类型声明为 Dim 用户类型为字符串，然后像 usertype = Reader.Item("Type").ToString() 一样为其赋值。您还需要通过 Reader.GetString("Type") 检查值 return 它可能为空。打开“Option Strict” 这将有助于长期运行。

Answer 3

 **Dim usertype = Reader.GetString("Type") // MY ERROR**

您传递的字符串值显然是错误的

GetString 函数接受整数。您可能想要实例化一个整数数据并通过 getstring 传递它。

Dim usertype = Reader.GetString(data)

Answer 4

这是因为您可能无法从 "Reader" 对象中正确获取数据

试试这个：

Dim command As SqlCommand = New SqlCommand("SELECT  * FROM uinfo WHERE  password = '" & PASStb2.Text & "'", connection)
    connection.Open()
    Dim READER As SqlDataReader = command.ExecuteReader()
    If READER.HasRows Then
        While READER.Read()
            Console.WriteLine("{0}" & vbTab & "{1}", READER.GetInt32(0), READER.GetString(1))
        End While
    Else
        Console.WriteLine("No rows found.")
    End If

    READER.Close()

Answer 5

使用参数。我让服务器进行计数。为您节省几行代码。

Private Sub OKbt1_Click(sender As Object, e As EventArgs) Handles OKbt1.Click
        Call Connect() ' [ connection to module ]'
        Dim Reader As SqlDataReader
        Try
            Using cmd As New SqlCommand("select usertype, Count(*) from uinfo Group By usertype where [password] = @password;", sqlConn)
                cmd.Parameters.Add("@password", SqlDbType.VarChar).Value = PASStb2.Text
                Reader = cmd.ExecuteReader()
                If Reader.HasRows Then
                    Reader.Read()
                    Dim usertype As String = Reader.GetString(0)
                    Dim count As Integer = Reader.GetInt32(1)
                    If count = 1 Then
                        If usertype = "admin" Then
                            MAIN_MENU.Show()
                            Hide()
                        ElseIf usertype = "user" Then
                            USERMENU.Show()
                            Hide()
                        End If
                    ElseIf count > 1 Then
                        MsgBox("username and password are duplicate")
                    End If
                Else
                    MsgBox("username and password are not correct")
                End If
            End Using
        Catch ex As Exception
            MsgBox(ex.Message)
        Finally
            sqlConn.Close()
            sqlConn.Dispose()
        End Try
    End Sub

Answer 6

SqlDataReader.GetString Method (Int32) 需要一个整数（列索引）作为参数。所以你需要

Dim usertype as String = Cstr(Reader("Type"))

或

Dim usertype = Reader.GetString(Reader.GetOrdinal("Type"))

或

Dim usertype = Reader.GetFieldValue(Of String)("Type")

请注意，这些可能性中没有一个可以处理 DBnull。

从 SqlDataReader 检索数据时出现 InvalidCastException

InvalidCastException when retrieving data from SqlDataReader

sql

vb.net

runtime