objects are not null and exception is still raised

Answer: it was in the else code, but I thought there is no reason that code wouldn't point to that line.

Can someone offer an idea how it could be possible that开发者_如何学编程 I get "Object reference not set to an instance of an object" on this at Invoke method:

delegate void t(tabdescriptor tab);
internal void AddItem(tabdesciptor tab)
{
    if (InvokeRequired)
    {
        t inv = new t(AddItem);
        if (inv != null && tab!= null)
            Invoke(inv, tab);
    }
    else
    {
        ....
    }
}

I'm not exactly sure what the actual issue is considering your example cannot be the code that executes, but please try something like this:

internal void AddItem(tabdesciptor tab)
{
    if (InvokeRequired)
    {
        Invoke(new Action<tabdescriptor>(AddItem), tab);
    }
    else
    {
        //...
    }
}

Also please make sure that it's not actually whatever is in the else part that fails.

If I remember correctly, this exception could be coming from inside the invoked method. If you place a try/catch inside the else of the AddItem method and a breakpoint inside the catch, do you catch the exception?

internal void AddItem(tabdesciptor tab)
{
    if (InvokeRequired)
    {
        t inv = new t(AddItem);
        if (inv != null && tab!= null)
            Invoke(inv, tab);
    }
    else
    {
        try
        {
            ....
        }
        catch
        {
            // breakpoint here
        }
    }
}

It's unclear whether it's a mistake in the given example, or not, but tab is never checked for null, yet it is passed as the argument.

Either it's probably null, or:

Also, inv is checked for null right after creating it (Good in C/C++, but unnecessary in C# as it throws an OutOfMemoryException upon failure), but it is done before checking o for null.