Question

I have this event in a webservice:

public event FindProductsByCharacteristicsCompletedEventHandler FindProductsByCharacteristicsCompleted
    {
        [MethodImpl(MethodImplOptions.Synchronized)]
        add
        {
            _findProductsByCharacteristicsCompleted += value;
        }
        [MethodImpl(MethodImplOptions.Synchronized)]
        remove
        {
            _findProductsByCharacteristicsCompleted -= value;
        }
    }

And im then checking if the event value is null with this later in the class:

private void OnFindProductsByCharacteristicsOperationCompleted(object arg)
    {
        var handler = _findProductsByCharacteristicsCompleted;
        if (handler == null)
            return;
        handler(this, new FindProductsByCharacteristicsCompletedEventArgs(completedEventArgs.Results, completedEventArgs.Error, completedEventArgs.Cancelled, completedEventArgs.UserState));
    }

Answer 1

Your event implementation looks like it is an endless recursion. You are using the property itself in its implementation.
Change it to this:

private FindProductsByCharacteristicsCompletedEventHandler
            _findProductsByCharacteristicsCompleted;

public event FindProductsByCharacteristicsCompletedEventHandler
                 FindProductsByCharacteristicsCompleted 
{
    [MethodImpl(MethodImplOptions.Synchronized)]
    add
    {
        _findProductsByCharacteristicsCompleted += value;
    }
    [MethodImpl(MethodImplOptions.Synchronized)]
    remove
    {
        _findProductsByCharacteristicsCompleted -= value;
    }
}

And now, implement your method like this:

var handler = _findProductsByCharacteristicsCompleted;
if(handler == null)
    return;

handler(this, new FindProductsByCharacteristicsCompletedEventArgs(...));

This has the advantage that it is thread-safe.

Even if someone detached the last handler from the event after you checked for null but before you actually raised the event, you would not get an exception, because you are operating on the unchanged local variable.