System.NullReferenceException: Object reference not set to an instance of an object

Question

I am using asp.net LoginView to show different data to authenticated and anonymous users.

<asp:LoginView ID="LoginView1" Runat="server">
    <LoggedInTemplate>
        <asp:Label ID="Foo" runat="server" />
    </LoggedInTemplate>
    <AnonymousTemplate>
        <asp:Label ID="Bar" runat="server" />
    </AnonymousTemplate>
</asp:LoginView>

I then access these labels in my c# file like this:

Label Foo = (Label)LoginView1.FindControl("Foo");
Foo.Text = "whatever";

The error I am getting reads:

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

It looks like it did not find your Label, try testing that Foo is not null before trying to use it. — Mark Hall
– Mark Hall, Commented Jun 27, 2013 at 20:01
Welcome to Stack Overflow! Almost all cases of NullReferenceException are the same. Please see "What is a NullReferenceException in .NET?" for some hints. — John Saunders
– John Saunders, Commented Jun 27, 2013 at 20:06

Jon Skeet · Accepted Answer · 2013-06-27 20:00:27Z

1

Well presumably at execution time, the user isn't logged in - so there's no control with an ID of Foo, so FindControl is returning null. You should either detect whether the user is logged in or not separately and ask for the right control, or check whether Foo is null before you use it. (You might want to consider renaming your local variable to foo to be more in tune with C# conventions, too.)

answered Jun 27, 2013 at 20:00

Jon Skeet

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1 Comment

Karl Anderson Over a year ago

how are you not a C# MVP for life? Really, Microsoft needs to bestow the lifetime achievement award in C# for you. :-)

Karl Anderson · Accepted Answer · 2013-06-27 20:00:57Z

0

Try this:

Label Foo = (Label)LoginView1.FindControl("Foo");
if(Foo != null)
{
    Foo.Text = "whatever";
}

Now you will not get the error, but if Foo is null, then your label's text will not update. You need to determine why it cannot find the Label named "Foo".

answered Jun 27, 2013 at 20:00

Karl Anderson

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2 Comments

Anders Lindén Over a year ago

Isnt checking for null is strange here? How will there not be a control named Foo if you created one?

Karl Anderson Over a year ago

@AndersLindén - because there are two different templates; LoggedInTemplate and AnonymousTemplate, so there is a possibility that the Foo label is not there, because if the AnonymousTemplate is active, then the Bar label would exist and the Foo label would not.

Bazinga · Accepted Answer · 2013-06-27 20:23:21Z

0

You need to check for nulls:

object labelObj = LoginView1.FindControl("Foo")
if(labelObj != null)
{
    Label Foo = (Label)labelObj;
    if(Foo.Text!=null)
        Foo.Text="whatever";
}

edited Jun 27, 2013 at 20:23

answered Jun 27, 2013 at 20:08

Bazinga

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1 Comment

Po-ta-toe Over a year ago

Not very efficient. FindControl() result should be cached so you don't need to call it twice.

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