Question

我正在使用vb.net

为sql数据库编写一个非常基本的数据库输入表单

执行代码

dsnewrow = ds.Tables("Car Details").NewRow()

我在此之前在同一个子句中声明了dsnewrow：

 Dim dsnewrow As DataRow

和ds在这里声明：

Dim ds As DataSet

在表单的公共类中。

我知道我的ds变量存在问题，但我不明白它是什么，有人可以帮忙吗？

完整的计划：

Imports System.Data.SqlClient
Imports System.Windows.Forms

Public Class Form1
    Dim inc As Integer
    Dim ds As New DataSet
    Dim da As SqlDataAdapter
    Dim maxrows As Integer

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles btnDelete.Click
        Dim cb As SqlCommandBuilder = New SqlCommandBuilder(da)

        If MessageBox.Show("Are you sure that you want to delete this record?", _
                           "Delete", MessageBoxButtons.YesNo, _
                           MessageBoxIcon.Warning) = Windows.Forms.DialogResult.No Then
            MsgBox("Operation Cancelled")
        Else
            ds.Tables("Car Details").Rows(inc).Delete()
            maxrows = maxrows - 1
            inc = 0
            NavigateRecords()
            da.Update(ds, "authors2")
        End If

    End Sub

    Public Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Me.Car_DetailsTableAdapter.Fill(Me.CourseworkDataSet.Car_Details)
        Dim sqlstring As String
        Dim conn As SqlConnection
        Dim connstring As String


        connstring = "Data Source=(LocalDB)\v11.0;AttachDbFilename=C:\Users\Daniel\Documents\Coursework.mdf;Integrated Security=True;Connect Timeout=30"

        conn = New SqlConnection(connstring)
        conn.Open()
        MsgBox("successful")

        sqlstring = "SELECT * FROM Car Details"
        da = New SqlDataAdapter(sqlstring, conn)
        ds = New DataSet
        da.Fill(ds, "Car Details")

        conn.Close()
        conn = Nothing

        maxrows = ds.Tables("Car Details").Rows.Count
        inc = 0
        navigaterecords()
    End Sub

    Private Sub Car_DetailsBindingNavigatorSaveItem_Click(sender As Object, e As EventArgs) Handles Car_DetailsBindingNavigatorSaveItem.Click
        Me.Validate()
        Me.Car_DetailsBindingSource.EndEdit()
        Me.TableAdapterManager.UpdateAll(Me.CourseworkDataSet)

    End Sub

    Private Sub NavigateRecords()
        MakeTextBox.Text = ds.Tables("Car Details").Rows(inc).Item(0)
        ModelTextBox.Text = ds.Tables("Car Details").Rows(inc).Item(1)
        Engine_SizeTextBox.Text = ds.Tables("Car Details").Rows(inc).Item(2)
        ColourTextBox.Text = ds.Tables("Car Details").Rows(inc).Item(3)
        DoorsTextBox.Text = ds.Tables("Car Details").Rows(inc).Item(4)
        Buy_PriceTextBox.Text = ds.Tables("Car Details").Rows(inc).Item(5)
        Sell_PriceTextBox = ds.Tables("Car Details").Rows(inc).Item(6)

    End Sub

    Private Sub btnFwd_Click(sender As Object, e As EventArgs) Handles btnFwd.Click
        If inc <> maxrows - 1 Then
            inc = inc + 1
            NavigateRecords()
        Else
            MsgBox("No More Records")

        End If
    End Sub

    Private Sub btnBack_Click(sender As Object, e As EventArgs) Handles btnBack.Click
        If inc > 0 Then
            inc = inc - 1
            NavigateRecords()
        Else
            MsgBox("First Record")
        End If
    End Sub

    Private Sub btnFirst_Click(sender As Object, e As EventArgs) Handles btnFirst.Click
        If inc <> 0 Then
            inc = 0
            NavigateRecords()
        End If

    End Sub

    Private Sub btnLast_Click(sender As Object, e As EventArgs) Handles btnLast.Click
        If inc <> maxrows - 1 Then
            inc = maxrows - 1
            NavigateRecords()

        End If
    End Sub

    Private Sub btnUpdate_Click(sender As Object, e As EventArgs) Handles btnUpdate.Click
        Dim cb As SqlCommandBuilder = New SqlCommandBuilder(da)
        ds.Tables("Car Details").Rows(inc).Item(0) = MakeTextBox.Text
        ds.Tables("Car Details").Rows(inc).Item(1) = ModelTextBox.Text
        ds.Tables("Car Details").Rows(inc).Item(2) = Engine_SizeTextBox.Text
        ds.Tables("Car Details").Rows(inc).Item(3) = ColourTextBox.Text
        ds.Tables("Car Details").Rows(inc).Item(4) = DoorsTextBox.Text
        ds.Tables("Car Details").Rows(inc).Item(5) = Buy_PriceTextBox.Text
        ds.Tables("Car Details").Rows(inc).Item(6) = Sell_PriceTextBox.Text

    End Sub

    Private Sub btnAdd_Click(sender As Object, e As EventArgs) Handles btnAdd.Click
        btnUpdate.Enabled = False
        btnAdd.Enabled = False
        btnConfirm.Enabled = True
        btnDelete.Enabled = False

        ModelTextBox.Clear()
        MakeTextBox.Clear()
        Engine_SizeTextBox.Clear()
        ColourTextBox.Clear()
        DoorsTextBox.Clear()
        Buy_PriceTextBox.Clear()
        Sell_PriceTextBox.Clear()
    End Sub

    Private Sub btnConfirm_Click(sender As Object, e As EventArgs) Handles btnConfirm.Click
        If inc <> -1 Then
            Dim cb As SqlCommandBuilder = New SqlCommandBuilder(da)
            Dim dsnewrow As DataRow

            dsnewrow = ds.Tables("Car Details").NewRow()

            dsnewrow.Item("Make") = MakeTextBox.Text
            dsnewrow.Item("Model") = ModelTextBox.Text
            dsnewrow.Item("Engine Size") = Engine_SizeTextBox.Text
            dsnewrow.Item("Colour") = ColourTextBox.Text
            dsnewrow.Item("Doors") = DoorsTextBox.Text
            dsnewrow.Item("Buy Price") = Buy_PriceTextBox.Text
            dsnewrow.Item("Sell Price") = Sell_PriceTextBox.Text

            btnConfirm.Enabled = False
            btnAdd.Enabled = True
            btnUpdate.Enabled = True
            btnDelete.Enabled = True

        End If
    End Sub

    Private Sub btnCancel_Click(sender As Object, e As EventArgs) Handles btnCancel.Click
        btnUpdate.Enabled = True
        btnAdd.Enabled = True
        btnConfirm.Enabled = True
        btnDelete.Enabled = True
        inc = 0
        NavigateRecords()
    End Sub
    End Class

Answer 1

您只是声明变量ds。您应该先创建一个实例，然后才能使用这些方法。

Dim ds As DataSet

应该是

Dim ds As New DataSet

或

Dim ds As DataSet
ds = New DataSet

Answer 2

Form_Load中的这一行是错误的

sqlstring = "SELECT * FROM Car Details"

包含空格时的表名应括在方括号

中

sqlstring = "SELECT * FROM [Car Details]"

执行查询时，它不起作用，您的数据集永远不会使用new进行初始化。然后，当您尝试使用它时，您将获得NULL引用异常。我想你在the exceptions in a Form_Load event are ignored

的64位系统上运行这段代码

WindowsApplication1.exe中发生System.NullReferenceException附加信息：未将对象引用设置为对象的实例

2 个答案: