NullReference Exception was unhandled - vb.net

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
I m getting an error NullReference Exception was handled at this line of code
Dim snmp As New SimpleSnmp
snmp = New SimpleSnmp(host, community)
Kindly help me..!

Answer 1

The nullreference is because the host and community strings are blank. You are setting them to the value of a new instance of a DataGridViewTextBoxCell which is nothing.
Are you trying to set them to a particular row in the datagridview?
If so you should be usging something like:
host = DataGridView1.Rows(1).Cells(1).Value.ToString
community = DataGridView1.Rows(1).Cells(2).Value.ToString