NullReference異常是未處理

Question

Imports System 
Imports SnmpSharpNet 

Public Class Form1 
    Private Sub DataGridView1_CellContentClick(ByVal sender As System.Object, ByVal e As System.Windows.Forms.DataGridViewCellEventArgs) Handles DataGridView1.CellContentClick 

    End Sub 
Private Sub Button1_Click_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click 
    DataGridView1.Columns.Add("NameOne", "Column One") 
    DataGridView1.Columns.Add("NameTwo", "Column Two") 

    Dim dataGridRow As New DataGridViewRow() 
    Dim cells As DataGridViewCell() = New DataGridViewCell(1) {} 
    Dim txt1A As New DataGridViewTextBoxCell() 
    Dim txt1B As New DataGridViewTextBoxCell() 
    txt1A.Value = "Host" 
    dataGridRow.Cells.Add(txt1A) 
    txt1A.[ReadOnly] = False 
    dataGridRow.Cells.Add(txt1B) 
    dataGridRow.Height = 20 
    DataGridView1.Rows.Add(dataGridRow) 



    dataGridRow = New DataGridViewRow() 
    cells = New DataGridViewCell(1) {} 

    Dim txt2A As New DataGridViewTextBoxCell() 
    Dim cbo1 As New DataGridViewComboBoxCell() 

    cbo1.Items.Add("1.3.6.1.2.1.1.1.0") 
    cbo1.Items.Add("1.3.6.1.2.1.1.2.0") 
    cbo1.Items.Add("1.3.6.1.2.1.1.3.0") 
    cbo1.Items.Add("1.3.6.1.2.1.1.4.0") 
    cbo1.Items.Add("1.3.6.1.2.1.1.5.0") 

    cbo1.Value = cbo1.Items(0) 

    cbo1.[ReadOnly] = False 
    txt2A.Value = "OIDs" 
    dataGridRow.Cells.Add(txt2A) 
    txt2A.[ReadOnly] = True 
    dataGridRow.Cells.Add(cbo1) 
    dataGridRow.Height = 20 
    DataGridView1.Rows.Add(dataGridRow) 

    Dim requestOid() As String 

    requestOid = New String() {cbo1.Selected} 

    dataGridRow = New DataGridViewRow() 
    cells = New DataGridViewCell(1) {} 
    Dim txt3A As New DataGridViewTextBoxCell() 
    Dim cbo2 As New DataGridViewComboBoxCell() 

    cbo2.Items.Add("Get") 
    cbo2.Items.Add("GetNext") 


    cbo2.Value = cbo2.Items(0) 

    cbo2.[ReadOnly] = False 
    txt3A.Value = "SNMP Operation" 
    dataGridRow.Cells.Add(txt3A) 
    txt3A.[ReadOnly] = True 
    dataGridRow.Cells.Add(cbo2) 
    dataGridRow.Height = 20 
    DataGridView1.Rows.Add(dataGridRow) 

    dataGridRow = New DataGridViewRow() 
    cells = New DataGridViewCell(1) {} 
    Dim txt4A As New DataGridViewTextBoxCell() 
    Dim txt4B As New DataGridViewTextBoxCell() 
    txt4A.Value = "Community String" 
    txt4B.Value = "public" 
    dataGridRow.Cells.Add(txt4A) 
    dataGridRow.Cells.Add(txt4B) 
    dataGridRow.Height = 20 
    DataGridView1.Rows.Add(dataGridRow) 



End Sub 
Private Sub dataGridView1_EditingControlShowing(ByVal sender As Object, ByVal e As DataGridViewEditingControlShowingEventArgs) 

    Dim comboControl As DataGridViewComboBoxEditingControl = TryCast(e.Control, DataGridViewComboBoxEditingControl) 
    If comboControl IsNot Nothing Then 
     ' Set the DropDown style to get an editable ComboBox 
     If comboControl.DropDownStyle <> ComboBoxStyle.DropDown Then 
      comboControl.DropDownStyle = ComboBoxStyle.DropDown 
     End If 
    End If 
End Sub 


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

    Dim result As Dictionary(Of Oid, AsnType) 
    Dim requestoid() As String 
    Dim cbo1 As New DataGridViewComboBoxCell() 
    Dim txt1B As New DataGridViewTextBoxCell() 
    Dim txt4B As New DataGridViewTextBoxCell() 
    Dim host As String = txt1B.Value 

    Dim community As String 
    community = txt4B.Value 
    requestoid = New String() {cbo1.Selected} 

    Dim snmp As New SimpleSnmp 
    snmp = New SimpleSnmp(host, community) 

    'abc.Text = txtsnmpaction.SelectedItem 
    result = snmp.Get(SnmpVersion.Ver1, requestoid) 

    'result = snmp.GetNext(SnmpVersion.Ver1, requestoid) 

    'If (txtsnmpaction = "GetBulk") 
    'result = snmp.GetBulk(New String() {".1.3.6.1.2", ".1.3.6.1.3"}) 
    ' End If 
    If Not snmp.Valid Then 

     MessageBox.Show("Invalid hostname/community") 

    End If 
    If result IsNot Nothing Then 
     Dim kvp As KeyValuePair(Of Oid, AsnType) 
     For Each kvp In result 
      MessageBox.Show("kvp.Key.ToString") 
      MessageBox.Show(SnmpConstants.GetTypeName(kvp.Value.Type)) 
      MessageBox.Show(kvp.Value.ToString()) 

     Next kvp 
    Else 
     MessageBox.Show("No results received") 
    End If 



    End Sub 
End Class 

我米得到一個錯誤NullReference異常在這行代碼

Dim snmp As New SimpleSnmp 
     snmp = New SimpleSnmp(host, community) 

處理請幫助我..！

Answer 1

nullreference是因爲主機和社區字符串爲空。你將它們設置爲DataGridViewTextBoxCell的一個新實例的值，這個值是無關緊要的。

您是否試圖將它們設置爲datagridview中的特定行？

如果是這樣，你應該usging類似：什麼是一個NullReferenceException，如何解決呢]

host = DataGridView1.Rows(1).Cells(1).Value.ToString 
community = DataGridView1.Rows(1).Cells(2).Value.ToString