"Object reference not set to an instance of an object" Error while existing vb.net program.

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VS2010

Dear Coders,

I have given the code below which i was working. All goes well but when existing the program a null exception error happens with the message "Object reference not set to an instance of an object"

Please help

VB

Dim con As New SqlConnection
        Dim myconstring As String = "Data Source=ETA-TESTSRVR;Initial Catalog=Procurement;Integrated Security=True"
        con.ConnectionString = myconstring
        Dim sql As String = "SELECT mat_sub_no FROM e1 where project_id='" + ComboBox1.SelectedValue.ToString + "'"
        If sql.Length = 0 Then
            MessageBox.Show(sql.Length.ToString)
        End If
        Dim da As New SqlDataAdapter(sql, con)
        Dim dt As New DataTable
        da.Fill(dt)
        ListBox1.DataSource = dt
        ListBox1.DisplayMember = "Mat_Sub_no"
End Sub

Posted 7-Nov-12 0:48am

armarzook

Updated 7-Nov-12 0:57am

v2

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3 solutions

Solution 1

"Object reference not set to an instance of an object" error occurs when try to get/set null value.

In your code , this situation might be occurs many places

1.

C#

ComboBox1.SelectedValue.ToString

ComboBox1.SelectedValue does not fetch any value.

2.

C#

da.Fill(dt)

when you try to fill data adapter

..............

so, better to debug your code to find actual place of source of error

or use try, catch block with your code

1 more thing-

always use

C#

Convert.Tostring()

in the place of

C#

yourvalue.ToString()

Posted 7-Nov-12 1:16am

ashriv

Comments

armarzook 7-Nov-12 7:21am

Actually the error is showing in this line.

Dim sql As String = "SELECT mat_sub_no FROM e1 where project_id='" + ComboBox1.SelectedValue.ToString + "'"

Solution 3

If you have put this code inside New() method [Constructor]
then
make sure you have called InitializeComponents() before code

else

put code inside this condition

VB

If ComboBox1.SelectedValue Isnot Nothing then
 .... 'your code
End If

Happy Coding!
:)

Posted 7-Nov-12 20:00pm

Aarti Meswania

Comments

armarzook 10-Nov-12 8:38am

If ListBox1.SelectedValue Is Nothing Then
Me.E1TableAdapter.FillBy(Me.ProcurementDataSet.E1, projid:=CType(TextBox1.Text, Integer))
Else
MsgBox(ListBox1.SelectedValue)
Me.E1TableAdapter.FillBy1(Me.ProcurementDataSet.E1, ms_no:=ListBox1.SelectedValue)
End If

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ashriv · Accepted Answer · 2012-11-07T19:45:00

Solution 2

SQL

object obj =  ComboBox1.SelectedValue;
if(obj != null)
  Dim sql As String = "SELECT mat_sub_no FROM e1 where project_id='" + obj.ToString + "'"

use this code, it might be work with you.

Posted 7-Nov-12 19:45pm

ashriv