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Hi sir I am getting null reference exception. kindly help

What I have tried:

Protected Sub ddlstate_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ddlstate.SelectedIndexChanged
        If ddlfin_year.SelectedIndex = 0 Then
            ClientScript.RegisterStartupScript(Me.GetType, "msgg", "alert('Please select Financial Year');", True)
            ddlstate.SelectedIndex = 0
            Return
        End If
        If ddlmnth.SelectedIndex = 0 Then
            ClientScript.RegisterStartupScript(Me.GetType, "msgg", "alert('Please select Month');", True)
            ddlstate.SelectedIndex = 0
            Return
        End If
        con.Open()
        Dim da As New SqlDataAdapter
        Dim dt As New DataTable
        Dim cmd As New SqlCommand
        Dim read As SqlDataReader



        cmd = New SqlCommand("Select Fin_Year,state_code,allocation from StateBrief_NSAP where Fin_Year='" & ddlfin_year.SelectedItem.Text & "' And   state_code='" & ddlstate.SelectedItem.Value & "'", con)
        da = New SqlDataAdapter(cmd)

        dt = New DataTable()

        da.Fill(dt)

        read = cmd.ExecuteReader()
        read.Read()
        If dt.Rows.Count > 0 Then
            While read.Read

                Dim allocation As String = CStr(read("allocation"))



                Session("alloc") = allocation






            End While
            txtAllocation.Text = Session("alloc").ToString()


            txtAllocation.Enabled = False
        Else


            txtAllocation.Enabled = True
            txtAllocation.Text = ""




        End If



        read.Close()



    End Sub
Posted
Updated 26-Feb-17 20:57pm
Comments
Member 7870345 27-Feb-17 2:26am    
Hello:
In wich line the exception ocurrs?
Member 12988273 27-Feb-17 2:41am    
txtAlloc.Text = Session("allocation").ToString()
Michael_Davies 27-Feb-17 2:34am    
First use the debugger, when the exception occurs hover the mouse over the variables on the offending line to see which of them is Null then work out why and deal with the condition.

Second why are you filling a DataTable then using a Reader?

Third in the defining statement of your variables da, dt and cmd you instantiate them with New, you then instantiate them again in code, why, "Dim da As New SqlDataAdapter" will create the variable and allocate a new instance, having never used that instance you then create a new one before you use actually use it; "da = New SqlDataAdapter(cmd)". Same goes for dt and cmd.
Member 12988273 27-Feb-17 2:56am    
i did all as you told but still getting error txtAlloc.Text = Session("allocation").ToString()
Michael_Davies 27-Feb-17 2:59am    
in your code it is "alloc" not "allocation" on the Session() call. Have you made a spelling mistake that will then return Null as alloc cannot be found but allocation might be?

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!
 
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Hello:
If the error is in the txtAlloc.Text=Session("allocation").ToString() line, it seems that Session("allocation") is null, so when is called the ToString function the exception raises.
 
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