Object Reference Not Set

Question

There are similar questions around here but none that fit into my my particular case scenario.

I have a Windows Form with a Button. The button is attached to the event handler as follows:

private void mybutton_Click(object sender, EventArgs e)
{
    // do some processing here
}

In addition there is a combobox where a change in selection in supposed to trigger the button event handler as defined above.

private void mycombobox_SelectedIndexChanged(object sender, EventArgs e)
{
    mybutton_Click(sender, e); // this is the line which pops up the dialog
}

The code works exactly as intended at runtime but i get a dialog prompt at compile time which reads:

object reference not set to an instance of an object

There are no other errors or warning.

A google search tells me that this message is an error caused if the program is trying to access a member of a reference type variable which is set to null.

However when i run this code in debug mode, both the sender and event(e) variables are not null.

So why is this dialog popping up ?

And if this had been an error or warning - it should have shown as an error or warning but nothing of that sort happens.

Here's the screenshot:

Edits: Answering Questions Raised in Comments

There are no errors as you can see in the screenshot.

The program works great - just this pop up

The popup is caused by the line:mybutton_Click(sender, e); in the combobox selectedIndexChanged function.

The mybutton_Click(sender, e) does not use any of the arguments sender or e in the processing.

I have not installed any VS extensions either.

Answer 1

It is not a good design to call the Click-event of the button in the SelectedIndexChanged-event of the ComboBox and this might also be the reason for the error.

Better put your logic in a seperate method and call it in the Click- and the SelectedIndexChanged-event like this:

private void UpdateSomething()
{
    // Do whatever you want
}

private void mybutton_Click(object sender, EventArgs e)
{
    UpdateSomething();
}

private void mycombobox_SelectedIndexChanged(object sender, EventArgs e)
{
    UpdateSomething();
}

Answer 2

I think what is happening is that the events are firing in Design mode (which I seem to recall happening to me a few times when using WinForms back in the day).

To get around it what I did was handled the form load event, then in that I attached a listener to the SelectedIndexChanged. Then in Design mode the event isnt bound and wont fire, but at run time it is bound.

Something like:

public void form_OnLoaded(object sender, EventArgs e)
{
     myComboBox.SelectedIndexChanged += mycombobox_SelectedIndexChanged;
}