在插入查询对象引用未处理的异常VB.Net

Question

这是我在vb.net的代码，当我通过从VB.net插入查询到SQL服务器我在查询异常该对象引用未设置为实例对象， 在sql文件中允许为null，ID为自动增加。我的同伴的 一个说，使用记录集，并告诉条件的财产以后这样的，但我已经完全不知道有关的新的VB.net， 请帮我订的记录，我应该怎么办，

崩溃|复制代码

Imports System.Data 
Imports System.Data.SqlClient 
Public Class MaintenanceTask 
    Dim cn As New System.Data.SqlClient.SqlConnection 
    Sub connect() 
     cn = New System.Data.SqlClient.SqlConnection("Data Source=localhost;Initial Catalog=Fleet Maintainance;Integrated Security=True") 
    End Sub 
    Sub lockall() 
     cBx1.Enabled = False 
     cBx2.Enabled = False 
     tBx1.Enabled = False 
     tBx2.Enabled = False 
     tBx3.Enabled = False 
    End Sub 
    Sub unlockall() 
     cBx1.Enabled = True 
     cBx2.Enabled = True 
     tBx1.Enabled = True 
     tBx2.Enabled = True 
     tBx3.Enabled = True 
    End Sub 
    Sub setall() 
     cBx1.Text = "" 
     cBx2.Text = "" 
     tBx1.Text = "" 
     tBx2.Text = "" 
     tBx3.Text = "" 
    End Sub 
    Sub updatecombo1() 
     Call connect() 
     Dim cd As New System.Data.SqlClient.SqlCommand("SELECT [Name],[NameID] FROM [RepairName] order by [Name]", cn) 
     Dim adp As New System.Data.SqlClient.SqlDataAdapter(cd) 
     Dim ds As New DataSet 
     adp.Fill(ds) 
     'ComboBox2.Items.Clear() 
     cBx1.DisplayMember = "Name" 
     cBx1.ValueMember = "NameID" 
     cBx1.DataSource = ds.Tables(0) 
    End Sub 
    Sub updatecombo2() 
     Call connect() 
     Dim cd As New System.Data.SqlClient.SqlCommand("SELECT [RepairType],[RepairTypeID] FROM [RepairType] order by [RepairType]", cn) 
     Dim adp As New System.Data.SqlClient.SqlDataAdapter(cd) 
     Dim ds As New DataSet 
     adp.Fill(ds) 
     'ComboBox2.Items.Clear() 
     cBx2.DisplayMember = "RepairType" 
     cBx2.ValueMember = "RepairTypeID" 
     cBx2.DataSource = ds.Tables(0) 
    End Sub 
    Sub updatecombo3() 
     Call connect() 
     Dim cd As New System.Data.SqlClient.SqlCommand("SELECT [Service],[ServiceID] FROM [Service] order by [Service]", cn) 
     Dim adp As New System.Data.SqlClient.SqlDataAdapter(cd) 
     Dim ds As New DataSet 
     adp.Fill(ds) 
     'ComboBox2.Items.Clear() 
     cBx1.DisplayMember = "Service" 
     cBx1.ValueMember = "ServiceID" 
     cBx1.DataSource = ds.Tables(0) 
    End Sub 

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click 

     If cBx1.Enabled = True Then 
      Dim str As String = "insert into MaintenanceTask (MainID, TypeID, PartCost, LaborCost, Total) values (" & (cBx1.SelectedValue.ToString()) & ", " & (cBx2.SelectedValue.ToString()) & ", " & CInt(tBx1.Text) & "," & CInt(tBx2.Text) & "," & CInt(tBx3.Text) & ")" 
      Call connect() 
      Dim cd As New System.Data.SqlClient.SqlCommand(str, cn) 
      cd.Connection.Open() 
      cd.ExecuteNonQuery() 
      cd.Connection.Close() 
      MsgBox(" New Task is added successfully ") 
      Call lockall() 
      Me.Close() 
      Call IssueWorkOrder.listView2load() 

     Else 
      MsgBox(" Task is not added Try again ") 

     End If 

    End Sub 

    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click 
     AddService.Show() 
     AddService.Visible = True 
    End Sub 

    Private Sub RadioButton1_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RadioButton1.CheckedChanged 
     RadioButton1.Text = "Preventive" 
    End Sub 

    Private Sub RadioButton2_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RadioButton2.CheckedChanged 
     RadioButton2.Text = "Repair" 
    End Sub 

    Private Sub RadioButton1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RadioButton1.Click 
     Call updatecombo3() 
     Label3.Text = "Service" 
     cBx2.Enabled = False 
    End Sub 

    Private Sub RadioButton2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RadioButton2.Click 
     Call updatecombo1() 
     Call updatecombo2() 
     Label3.Text = "Repair" 
     cBx2.Enabled = True 
    End Sub 

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click 
     Me.Close() 
    End Sub 
End Class 

Answer 1

您的主要问题是您的插入字符串。 你不检查组合的值是有效还是无效，并调用toString在VB项目NULL（没有）

Dim str As String = "insert into MaintenanceTask (MainID, TypeID, PartCost, " & _ 
         "LaborCost, Total) values (" & (cBx1.SelectedValue.ToString()) & _ 
         ", " & (cBx2.SelectedValue.ToString()) & ", " & CInt(tBx1.Text) & _ 
         "," & CInt(tBx2.Text) & "," & CInt(tBx3.Text) & ")" 

如果cBx1.SelectedValue为NULL你不能调用的方法（的ToString ）在一个NULL对象上，这是收到的NullReferenceException。 但是你有一个名为Sql Injection一个更严重的问题（请阅读文章）

Dim str As String = "insert into MaintenanceTask (MainID, TypeID, PartCost, " & _ 
         "LaborCost, Total) values (@main, @tp, @cost, @labcost, @tot)" 

    Dim cd As New SqlCommand(str, cn) 
    cd.Parameters.AddWithValue("@main", if(cbx1.SelectedValue == null, DBNull.Value, cbx1.SelectedValue.ToString) 
    cd.Parameters.AddWithValue("@tp", if(cbx2.SelectedValue == null, DBNull.Value, cbx2.SelectedValue.ToString) 
    cd.Parameters.AddWithValue("@cost", Convert.ToInt32(tBx1.Text)) 
    cd.Parameters.AddWithValue("@labcost", Convert.ToInt32(tBx2.Text)) 
    cd.Parameters.AddWithValue("@totale", Convert.ToInt32(tBx3.Text)) 
    cd.Connection.Open() 
    cd.ExecuteNonQuery() 
    cd.Connection.Close() 
    MsgBox(" New Task is added successfully ") 
    Call lockall() 
    Me.Close() 
    Call IssueWorkOrder.listView2load() 

其他问题转化为一个文本框的整数。
如果输入不是有效的数值，你会得到一个错误，你需要一点错误处理之前 连接对象是一个稀缺资源，你应该确保完成后关闭。你的代码没有错误处理，万一错误，你的连接永远不会关闭。