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Visual basic System.NullReferenceException, In a code i badly need x(.
Mr White
post Sep 2 2009, 12:43 AM
Post #1


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OS: Windows XP
CODE
Private Sub ComboBox1_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles ComboBox1.KeyPress
        e.KeyChar = vbCr
CType(TabControl1.SelectedTab.Controls.Item(0),  WebBrowser).Navigate(ComboBox1.Text)
        My.Settings.History.Add(ComboBox1.Text)
        ComboBox1.Items.Add(ToolStripComboBox2.Text)
    End Sub

This is the code that i have a System.NullReferenceException this line to be more specific CType(TabControl1.SelectedTab.Controls.Item(0), WebBrowser).Navigate(ComboBox1.Text).At Microsoft's site they said for this error to disable vb optimization which they don't tell you how laughing.gif .
So if someone know how to disable it or to resolve this NullReferenceException please tell me!
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stettybet0
post Sep 2 2009, 01:06 PM
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OS: Vista Ultimate SP2 64-bit, XP Pro SP3
A null reference exception means you are addressing something that doesn't exist... Are you sure that TabControl1.SelectedTab.Controls.Item(0) of tabcontrol1 and ComboBox1.Text both exist? A good way to see is to set a breakpoint, and then hover your mouse over those items... It will show you if they contain a value.
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Mr White
post Sep 2 2009, 02:06 PM
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Can you explain me how to do that because i am a noob in VB i am not a genius i am learning now laughing.gif it would be very good if you could tell me how to do it and yes combobox1 and tab control1 does exist this null reference come up when i added a history sistem in my browser. lookaround.gif
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W-Unit
post Sep 2 2009, 08:41 PM
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Full code would be helpful.
I don't know VB but I do know enough about OOP to understand your problem. In most OOPLs, a NullReferenceException is thrown in a few instances:
1. Attempt to reference a variable which you have declared, but to which no value has been assigned
2. Attempt to assign a value of null to a non-nullable type
3. Attempt to use a variable with a null value in an invalid context

Judging from your description, I'd rule out number 2. So, to answer your question, what are combobox1 and control1's types? Are these types nullable? If not, have you attempted to assign a value of null to either of them?
Also, make sure that these variables have been ASSIGNED, not just declared, before you try to use them. A null or unassigned value is not valid in most contexts.
Check your assignment statements and make sure they just use the "=" operator. Operators such as +=, -=, etc, are a bit deceptive in that they ARE NOT assignment operators. You also cannot use increment or decrement operators such as ++ on unassigned variables.

For debugging purposes, if you have a blank declaration statement for these variables in your code, i.e.
CODE
SomeType combobox1;

...try replacing them with a blank assignment, i.e.
CODE
SomeType combobox1 = new SomeType();

(not sure if that is how the construction operator works in VB but I think you get my point)

If that fixes the NullReferenceException, then your problem is that you are somehow winding up with a variable that has not been assigned to at the time when you try to reference its value (you'll probably wind up with a different exception in this case, but that's ok - you'll have found the problem. Assuming the exception isn't about the assignment itself, of course).

Otherwise please post complete code and the location of the exception in order for me to better assist.

This post has been edited by W-Unit: Sep 2 2009, 08:43 PM
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Mr White
post Sep 3 2009, 01:23 AM
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This is the full source code
CODE
Imports System.Windows.Forms.VisualStyles.VisualStyleElement.Menu

Public Class Form1


    Dim int As Integer = 0

    Private Sub Loading(ByVal sender As Object, ByVal e As Windows.Forms.WebBrowserProgressChangedEventArgs)
        ToolStripProgressBar1.Maximum = e.MaximumProgress
        ToolStripProgressBar1.Value = e.CurrentProgress
    End Sub

    Private Sub Done(ByVal sender As Object, ByVal e As Windows.Forms.WebBrowserDocumentCompletedEventArgs)

        TabControl1.SelectedTab.Text = CType(TabControl1.SelectedTab.Controls.Item(0), WebBrowser).DocumentTitle
        ComboBox1.Text = CType(TabControl1.SelectedTab.Controls.Item(0), WebBrowser).Url.ToString

    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        CType(TabControl1.SelectedTab.Controls.Item(0), WebBrowser).GoBack()
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        CType(TabControl1.SelectedTab.Controls.Item(0), WebBrowser).GoForward()
    End Sub

    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click

        CType(TabControl1.SelectedTab.Controls.Item(0), WebBrowser).Refresh()
    End Sub

    Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button4.Click
        CType(TabControl1.SelectedTab.Controls.Item(0), WebBrowser).Stop()
    End Sub

    Private Sub Button5_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button5.Click

        CType(TabControl1.SelectedTab.Controls.Item(0), WebBrowser).GoHome()

        ToolStripStatusLabel1.Text = CType(TabControl1.SelectedTab.Controls.Item(0), WebBrowser).StatusText
    End Sub


    Private Sub NewTabToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles NewTabToolStripMenuItem.Click
        Dim Browser As New WebBrowser
        TabControl1.TabPages.Add("New Page")
        TabControl1.SelectTab(int)
        Browser.Name = "FireBird"
        Browser.Dock = DockStyle.Fill
        TabControl1.SelectedTab.Controls.Add(Browser)
        AddHandler Browser.ProgressChanged, AddressOf Loading
        AddHandler Browser.DocumentCompleted, AddressOf Done
        int = int + 1
    End Sub

    Private Sub RemoveTabToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RemoveTabToolStripMenuItem.Click
        If Not TabControl1.TabPages.Count = 1 Then
        End If
        TabControl1.TabPages.RemoveAt(TabControl1.SelectedIndex)
        TabControl1.SelectTab(TabControl1.TabPages.Count - 1)
        int = int - 1


    End Sub
    Private Sub ComboBox1_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles ComboBox1.KeyPress
        e.KeyChar = vbCr
        CType(TabControl1.SelectedTab.Controls.Item(0), WebBrowser).Navigate(ComboBox1.Text)
        My.Settings.History.Add(ComboBox1.Text)
        ComboBox1.Items.Add(ToolStripComboBox2.Text)

    End Sub

    Private Sub RegularToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RegularToolStripMenuItem.Click
        CType(TabControl1.SelectedTab.Controls.Item(0), WebBrowser).GoBack()
    End Sub

    Private Sub BlackAndWhiteToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BlackAndWhiteToolStripMenuItem.Click
        CType(TabControl1.SelectedTab.Controls.Item(0), WebBrowser).GoForward()
    End Sub

    Private Sub ToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Toolstripmenuitem.Click
        CType(TabControl1.SelectedTab.Controls.Item(0), WebBrowser).GoHome()
    End Sub
    Private Sub Form1_load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        For Each ITEM As String In My.Settings.History
            ToolStripComboBox2.Items.Add(ITEM)
        Next

    End Sub

    Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
        For Each ITEM As String In My.Settings.History
            ToolStripComboBox2.Items.Add(ITEM)
        Next
    End Sub

    Private Sub ClearAllHistoryToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ClearAllHistoryToolStripMenuItem.Click
        If ToolStripComboBox2.Text Then
            My.Settings.History.Clear()
        End If
        If ToolStripComboBox2.Text Then
            ToolStripComboBox2.Items.Clear()

        End If
      


    End Sub
End Class


As for your options I chose
2.Attempt to reference a variable which you have declared, but to which no value has been assigned.

This is the full error NullReferenceException was unhandled bject reference not set to an instance of an object.
This appears every time when i debug my litle Browser and type a word in my combobox!
This is the line where the error shows up when i debug:
CType(TabControl1.SelectedTab.Controls.Item(0), WebBrowser).Navigate(ComboBox1.Text)

This post has been edited by Mr White: Sep 3 2009, 01:29 AM
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