Error While Attempting to Use Custom Events

Question

I am trying to use the information from the Passing-Data-between-Windows-Forms white paper to write some custom events to pass information between forms.
Unfortunately, I keep getting a NullReferenceException when I attempt to raise the event. Below is a scaled down version of the relevant code that I have currently attempted.
Can someone please have a look and let me know if I am missing something?
BTW, I am using DevExpress forms if that makes a difference. I only included the class that is generating my custom event since that is where the code is failing. On the line where the NullReferenceException occurs, I have verified that the item is not null.

// Class that generates custom event
public partial class DiscountItemControl : DevExpress.XtraEditors.XtraUserControl
{
    // Add a delegate
    public delegate void ItemInsertedEventHandler(object sender, ItemInsertedEventArgs e);

    // AddCustomerForm an event of the delegate type
    public event ItemInsertedEventHandler ItemInsertedEvent;

    public void SaveAndClose()
    {
        // setup event args
        ItemInsertedEventArgs args = new ItemInsertedEventArgs(currentDiscountItem);

        // ********** THIS THROWS NullReferenceException *********
        // raise the event to notify listeners
        ItemInsertedEvent(this, args);

        this.Dispose();
    }
}

// Event arguments for event
public class ItemInsertedEventArgs : System.EventArgs
{
    private Item item;

    public ItemInsertedEventArgs(Item item)
    {
        this.item = item;
    }

    public Item InsertedItem
    {
        get
        {
            return this.item;
        }
    }
}

---

System.NullReferenceException was unhandled by user code   Message="Object reference not set to an instance of an object."   Source="PureService"   StackTrace:
at MarineService.Tests.DiscountItemControl.SaveAndClose(Boolean& result) in C:\Aaron\Dev\HIGH PRIORITY\ServiceModule\MarineService\ServiceModule\UtilityClasses\UserControls\Items\DiscountItemControl.cs:line 336
at MarineService.Tests.AddEditItemForm.btnSaveAndClose_Click(Object sender, EventArgs e) in C:\Aaron\Dev\HIGH PRIORITY\ServiceModule\MarineService\ServiceModule\AddEditItemForm.cs:line 326
at System.Windows.Forms.Control.OnClick(EventArgs e)
at DevExpress.XtraEditors.BaseButton.OnClick(EventArgs e)
at DevExpress.XtraEditors.BaseButton.OnMouseUp(MouseEventArgs e)
at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
at System.Windows.Forms.Control.WndProc(Message& m)
at DevExpress.Utils.Controls.ControlBase.WndProc(Message& m)
at DevExpress.XtraEditors.BaseControl.WndProc(Message& msg)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)   InnerException:

Answer 1

You don't have any event handlers attached. Usually custom event code has a check for that:

ItemInsertedEventHandler handler = this.ItemInsertedEvent;
if(handler != null) handler(this, args);

You need to have some code somewhere that adds a handler, i.e.

MyObject.ItemInsertedEvent += myHandler;

EDIT: Jon Skeet is right about race condition issues, you should use the local variable like he suggests. I've changed my example to match.

Answer 2

Chances are you don't have any subscribers for the event. If that's the case, the delegate field which backs your event will have a null value. You should check for that, like this:

ItemInsertedEventHandler handler = ItemInsertedEvent;
if (handler != null)
{
    handler(this, args);
}

The reason for using a local variable is to avoid a race condition, where the last handler is removed after the check but before the invocation.

Answer 3

This is because the event is null when you're calling it. You need to check if it's null beforehand:

if (ItemInsertedEvent != null) {
    ItemInsertedEvent(this, args);
}