Common causes of: Object reference not set to an instance of an object error

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January 18th, 2005, 08:34 AM

Phoenix

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Common causes of: Object reference not set to an instance of an object error

(mods, you might want to make this a sticky)

Considering the large number of "Object reference not set to an instance of an object" errors abound, I figured its my "public duty" to give the main reasons, and thus... reduce the number of superflous threads created on the matter:

Cause one:

Not declaring variables!

Yes, I know it sounds obvious, but MAKE SURE that you've explicity declared the variable, and don't forget to use the appropriate scope!

One common mistayke is when working with Codebehind Classes, consider:

XHTML / Tags:

Code:

 
<body>
<asp:Literal id="litOne" runat="server" />
</body>

ASP.Net Codebehind Class

Code:

Public Class PageOne
Inherits System.Web.UI.Page
 
Dim strHello As String
 
Sub Page_Load(ByVal e As System.EventArgs) Handles MyBase.Load
	strHello = "Hello!"
	litOne.Text = strHello
End Sub
 
End Class

Remember! Codebehinds arn't handled the same way as inline/<script> pages!

You need to declare EVERY control you want to interact with in the codebehind class

...Do this just after the Inherits statement:

Code:

Public Class PageOne
Inherits System.Web.UI.Page
 
Protected WithEvents litOne As System.Web.UI.WebControls.Literal
 
...

Personally, I use "Protected WithEvents" as its the most flexible and "protects" the object, feel free to use other scopes and attributes as you see fit.

Cause 2: Bad scoping!

Indeed...

One common cause is this:

Code:

Sub Page_Load(ByVal e As System.EventArgs) Handles MyBase.Load
 
	Private strHello As String
 
End Sub
 
Sub Button1_Click(ByVal e As System.EventArgs) Handles Button1.Click
 
	strHello = "Hello!"
 
End Sub

This won't work because strHello is accessible only by Sub Page_Load

Bad inits/constructs
Remember, some classes have constuctors

Consider DirectoryInfo:

Code:

Sub Page_Load() ....
 
Dim dirInfo As System.IO.DirectoryInfo
 
dirInfo.GetDirectory("C:\")
 
End Sub

In the above example, we've only defined dirInfo *AS* a DirectoryInfo class, we haven't actually created it

So we need to do this instead:

Code:

Sub Page_Load() ....
 
Dim dirInfo As System.IO.DirectoryInfo
 
dirInfo = New System.IO.DirectoryInfo("C:\")
 
End Sub

That should do it....

Remember, ALWAYS refer to the MSDN Library when enquiring about classes and their initilisation (sp?) methods.... why trust some "1337 script kiddie!" site when you've got the official reference at http://www.msdn.com ? Or better yet, get the compiled version on CD, don't need to worry about long download times or the constant "was this article of any use to you?" screen.

HTH
-1337_d00d

(and yes, I'm aware of the "mistayke" thing, it was supposed to be ironic, but clearly the humour is lost on you)

Comments on this post

BigGwyn agrees!

medialint agrees!

January 18th, 2005, 04:19 PM

Shadow Wizard

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very nice job, dude... I see you really got into it, eh?

we already have about 5 stickies in this forum, I'll consult with the others moderators and we'll see if there is room for another one... this might as well be attached to existing sticky tutorial.
anyways, thanks - this would probably going to be very useful information.

January 18th, 2005, 06:08 PM

Phoenix

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"Just doing my part"

...I figured I needed to get back into the mod's good books after flaming all the newbs for a while last week

January 19th, 2005, 03:31 AM

Shadow Wizard

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lol
you didn't flame anyone, and even if you did it was justified as they had non standard code! *shudder*

well, as you see this thread has been stuck already, now let's hope ppl will read it before posting problems of that kind.

January 19th, 2005, 08:17 AM

Phoenix

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patience David... patience, climb the ladder...

today... sticky forum topics

tomorrow... THE WORLD!! muwahahahahahhhaha!!!

January 31st, 2005, 04:27 AM

Damnrad24

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Your a pro

Yo dude

You'll probably want to shoot me, but I have this exact error you so passionately talk
about. I have read through your posting but it still hasn't helped me and wonder
if you would mind having a look see. I'm quite a novice in .net but have the coding backround just
need a slight hand.

thanks in advance.

January 31st, 2005, 05:17 AM

Shadow Wizard

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Quote:

Originally Posted by Damnrad24

post your problem as new thread and we will see if we can help. in your post, give full details aboout what exactly is wrong, along with relevant code causing errors.

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January 31st, 2005, 05:17 AM

Phoenix

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I've been having a few different causes of the error recently too, so here's an ammendum:

Not "IDing" user controls!

Consider:

XHTML

Code:

<body>
<MyPrefix:MyControl runat="server" />
</body>

ASP.Net

Code:

 
Protected MyControl As MyNamespace.MySite.MyControl
 
Sub Page_Load() Handles MyBase.Load
MyControl.MyProperty = MyValue
End Sub

...won't work

You need to declare the ID="" attribute:

XHTML

Code:

<body>
<MyPrefix:MyControl id="MyControl" runat="server" />
</body>

HTH

Last edited by 1337_d00d : January 31st, 2005 at 05:29 AM.

January 31st, 2005, 05:19 AM

Shadow Wizard

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Quote:

Show us your code and I'll take a look (paste it here, I'm sure the mods won't mind... and even then, you can delete it when you're done)

nope, not good idea. this thread is tutorial, it's pointless to make it something else. new problem deserve to be in its own thread.

#10

January 31st, 2005, 05:37 AM

Damnrad24

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Thanks

Quote:

Originally Posted by Shadow Wizard

post your problem as new thread and we will see if we can help. in your post, give full details aboout what exactly is wrong, along with relevant code causing errors.

Will post as a new thread, Look forward to the response! Thank again

#11

July 1st, 2005, 08:23 AM

dotnetdev_03

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wow good Job!!!

wonderful Job

#12

January 6th, 2006, 06:40 AM

Timo

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Phoenix,

I am truly gratefull for this sticky. I suffered from a cause 3(bad init) for two days before you and your post rescued me. I should have known better. Thanks for getting me out of that mess!

Cheers,
Timo

#13

March 7th, 2007, 02:15 PM

pebkac

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There is another possible solution. I knew there had to be because I had my application working on other computers, but one of them gave me this error (Windows XP).

If you had previously gotten this error:

"Failed to access IIS metabase"

and fixed it with (at the command prompt)

aspnet_regiis -ga IUSR_<computername>
aspnet_regiis -ga IWAM_<computername>

.. and subsequently you get the "Object reference ..." error

then the fix is to run (at the command prompt)

aspnet_regiis -i

I don't know why this problem only occurred on one of my machines. Anyway it seems to be a common problem so I thought I'd post my solution as an alternative. I hope it helps someone.

Comments on this post

Shadow Wizard agrees: thanks for sharing and welcome to the forum.

#14

March 12th, 2007, 09:10 PM

morris_osu

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Personally, I get this error most often when I'm trying to reference a "buried" control within my codebehind.

For example...

.aspx:

Code:

    <asp:Repeater ID="Repeater1" runat="server">
        <ItemTemplate>
            <asp:Label ID="testID" runat="server" />
        </ItemTemplate>
    </asp:Repeater>

.aspx.cs

Code:

    protected void Page_Load(object sender, EventArgs e)
    {
        testID.Text = "this won't work";
    }

Pretty sure this is relevant.

#15

March 30th, 2007, 01:21 PM

jmurrayhead

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Quote:

Originally Posted by morris_osu

Personally, I get this error most often when I'm trying to reference a "buried" control within my codebehind.

For example...

.aspx:

Code:

    <asp:Repeater ID="Repeater1" runat="server">
        <ItemTemplate>
            <asp:Label ID="testID" runat="server" />
        </ItemTemplate>
    </asp:Repeater>

.aspx.cs

Code:

    protected void Page_Load(object sender, EventArgs e)
    {
        testID.Text = "this won't work";
    }

Pretty sure this is relevant.

Just to add to the above, to reference the "buried" control do such:

vb.net Code:

Original - vb.net Code
Private Sub Page_Load(sender As Object, e As EventArgs) Dim testIDtxt As TextBox testIDtxt = Repeater1.Controls.(0).FindControl("testID") testIDtxt.Text = "this WILL work" End Sub

 
    Private Sub Page_Load(sender As Object, e As EventArgs)
        Dim testIDtxt As TextBox
        testIDtxt = Repeater1.Controls.(0).FindControl("testID")
        testIDtxt.Text = "this WILL work"
    End Sub

Written in VB, but same concept.

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