C# Object reference not set to an instance of an object

Question

I have the following code:

panel4.Controls.Clear();
        panel4.Controls.Add(dgv.addDatagrid(animal, experiment, pluginIdentifier));
        dgv.ConfigureDatagrid();

This gets called when a user clicks a node on a treeview. When I click on it the first time, it works as expected and obtains the datagrid and adds it to the panel. However, any subsequent calls, I get the 'Object reference not set to an instance of an object' error on the panel4.Controls.Clear() line. Anybody know why?

Thanks.

Something in your code is killing panel4, as Controls will never be null afaik.. start by posting code of ConfigureDatagrid method. Another option is that you got the wrong line and maybe dgv is null. — user447356
– user447356, Commented Jan 31, 2011 at 10:55
Could you add some more code? Obviously panel4 is reset somewhere. — Vlad
– Vlad, Commented Jan 31, 2011 at 10:56
Wow, amazing how many related questions share the same title... Nulls suck. — R. Martinho Fernandes
– R. Martinho Fernandes, Commented Jan 31, 2011 at 11:10

WraithNath · Accepted Answer · 2011-01-31 10:53:42Z

2

If you put a breakpoint on panel4.Controls.Clear(); and check for null on all controls the second time the breakpoint is hit you should be able to see which one is throwing the error.

its likely to be panel4 or dgv

answered Jan 31, 2011 at 10:53

WraithNath

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3 Comments

Darren Young Over a year ago

How can I check for nulls on all controls? Sorry if this is a silly question....I'm fairly new on debugging. Thanks.

WraithNath Over a year ago

When the breakpoint is hit and goes yellow you should be able to hover over the variable / control names to see the value

R. Martinho Fernandes Over a year ago

@Darren: When the debugger breaks on that line, just hover with the mouse over the text 'panel4'. You will see a small box popup showing the value of that variable. Alternatively, you can type 'panel4' into the Watch window, or type '?panel4' in the Immediate window.

Jack Allan · Accepted Answer · 2011-01-31 10:54:45Z

0

Just guessing based on your code... It could either be that panel4 is null or panel4.Controls is null. Take a look at where and how panel4 is used and make sure it's correctly initialized

answered Jan 31, 2011 at 10:54

Jack Allan

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Comments

Sony Mathew · Accepted Answer · 2011-01-31 11:00:15Z

0

The error happens becoze of any field's value becomes null. Debug it where the Controlls.Add function is calling and see any parameter has null value

answered Jan 31, 2011 at 11:00

Sony Mathew

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