c# newbie with null reference error

Nigel Alford

I apologize in advance if this has been answered in a previous forum or post. I'm new to c# and my common problem regards null reference, normally I solve it but I'm stuck on this one.

Backstory: this is a windows mobile application and this form needs to take input from combo box and four text boxes and create a name based on user input.

I'm guessing that I need to reference the name of the string to correct this, but I'm unsure of if that's true and how my brain has taken the rest of the day off. I've underlined the line that throws the null reference exception, Thanks in advance

namespace ElevationPoint

{

public partial class frmnewelevation : Form

{

public static frmsplitscreen staticsplitscreenForm = null;

private string name1;

public string Name1

{

get { return name1; }

set { name1 = value; }

}

public frmnewelevation()

{

InitializeComponent();

}

public frmnewelevation(FormInfo info)

{

InitializeComponent();

DIVBOX.SelectedItem = info.div;

PreBox.Text = info.pre;

SufBox.Text = info.post;

}

private void saveData()

{

throw new NotImplementedException();

}

private void MessageBox(string p)

{

throw new NotImplementedException();

}

private void mainmenu_Click(object sender, EventArgs e)

{

frmmainmenu.staticmainmenuForm.Show();

}

private void mbNext_Click(object sender, EventArgs e)

{

FormInfo info = new FormInfo();

info.div = DIVBOX.SelectedValue.ToString();

info.pre = PreBox.Text;

info.post = SufBox.Text;

this.name1 = DIVBOX.SelectedValue.ToString() + PreBox.Text + SufBox.Text + HIGHMPTEXT.Text + LOWMPTEXT.Text;

Boolean allOK = false;

{

if (((HIGHMPTEXT.Text.Length > 0) && (LOWMPTEXT.Text.Length > 0)) && ((PreBox.Text.Length > 0) || (SufBox.Text.Length > 0)))

allOK = true;

}

{

if (allOK)

{

saveData();

}

else

{

MessageBox("Missing Value, Check all text entries");

}

{

frmsplitscreen frm = new frmsplitscreen();

frm.info = info;

frm.Show();

}

6 Replies

Know the answer? Post it — somebody with the same question will find it here.

Nigel Alford15y ago
thanks for the help, it was simple solution (they often are in my case lol). I was using the wrong property; DIVBOX.SelectedValue.ToString();
DIVBOX.SelectedText.ToString();

Gotta love it
Vulpes15y ago
Doh, thanks for pointing that out, John :)

Must have been half-asleep when I made that post!
Guest User15y ago
You may need to move the null check for SelectedValue so that executes before the assignment to info.div:

FormInfo info = new FormInfo();
if (DIVBOX.SelectedValue != null)
{
info.div = DIVBOX.SelectedValue.ToString();
info.pre = PreBox.Text;
info.post = SufBox.Text;
        .
        .
        .
Vulpes15y ago
The only other possibility is that DIVBOX itself is null though that's impossible if you can actually see the combobox on the screen.

If you open up the InitializeComponent() method in the other partial class, can you see a line like this:

DIVBOX = new System.Windows.Forms.ComboBox();
Nigel Alford15y ago
Vulpes....I've added that exception and recieved the same error.

In the trouble shooting tips I'm recommended to "use the 'new' keyword to create an object instance. or Check to determine if the object is null before calling the method"

Thanks
Vulpes15y ago
Well, the only things that can be null in that line are:

1. DIVBOX itself - unlikely, unless you've added it manually.

2. DIVBOX.SelectedValue - possible if nothing is actually selected when the code executes.

I'd see if the exception still occurs if you do this:

private void mbNext_Click(object sender, EventArgs e)
{
FormInfo info = new FormInfo();
info.div = DIVBOX.SelectedValue.ToString();
info.pre = PreBox.Text;
info.post = SufBox.Text;

if (DIVBOX.SelectedValue != null)
{

this.name1 = DIVBOX.SelectedValue.ToString() + PreBox.Text + SufBox.Text + HIGHMPTEXT.Text + LOWMPTEXT.Text;
Boolean allOK = false;
{
if (((HIGHMPTEXT.Text.Length > 0) && (LOWMPTEXT.Text.Length > 0)) && ((PreBox.Text.Length > 0) || (SufBox.Text.Length > 0)))
allOK = true;
}
{
if (allOK)
{
saveData();
}
else
{
MessageBox("Missing Value, Check all text entries");
}
{
frmsplitscreen frm = new frmsplitscreen();
frm.info = info;
frm.Show();
}
}
} // end if
}