[RESOLVED] Thread Safe ListView

[nolim8ts]

Hi All,

I currently have a form with a listview. On another form, I would like get all the items from that listview. This I can achieve but I always get the "Cross-Thread operation not allowed exception" message.

I've tried the following code to avail. Any suggestions?

Delegate Sub ListView_Delegate(ByVal [tslvw] As ListView)
 
    Private Sub ListView_ThreadSafe(ByVal tslvw As ListView)
 
        If [tslvw].InvokeRequired Then
            Dim MyDelegate As New ListView_Delegate(AddressOf ListView_ThreadSafe)
            Me.Invoke(MyDelegate, New Object() {[tslvw]})
        End If
 
    End Sub

[jmcilhinney]

Your method doesn't do anything if an invocation is NOT required. The idea is that if InvokeRequired is True you call Invoke, otherwise you do the work. You've got the first part but not the second. You might want to follow the CodeBank link in my signature and find my Controls & Multithreading submission, which is dedicated to this topic.

[chris128]

As well as what JMC has said about correcting the way your code works, I would also question whether or not you actually need to pass in the entire listview object. If you just need to access the items then pass in the Items property of the ListView.

Also, just accessing the items of a listview from another form should not cause cross-thread exceptions unless your calling code is being run from a background thread. You say you just want to get these items "from another form" so I'm wondering if you are actually showing that form on a background thread (which is generally not a good idea)

[jmcilhinney]

If you just need to access the items then pass in the Items property of the ListView.

You can't do that from another thread though, which is the whole point.

[chris128]

I know - I didnt say just pass the items property of the listview and that will solve your threading problems... I just said as well as following your guide for how to access controls from a worker thread, if you only need to access the items then why bother passing the entire listview.

[jmcilhinney]

I know - I didnt say just pass the items property of the listview and that will solve your threading problems... I just said as well as following your guide for how to access controls from a worker thread, if you only need to access the items then why bother passing the entire listview.

I think you'll find that that method from the first post is the one that's being called from the secondary thread, so all you can pass is the control itself. The whole point of that method is to use that parameter as a way to access members of the control in a thread safe way. You can't pass the Items property to a method that is being called in a secondary thread. I think what you may be missing is the fact that that method is simply not going to work as is. It needs to be rewritten as a function that actually returns the items. Basically you pass in a reference to the ListView, delegate to the UI thread, get the items and return them, which will return them to the original thread via the delegate.

[chris128]

Yeah thats what I meant but I guess I didnt explain that it needed to be rewritten, my bad.

[jmcilhinney]

my bad.

Possibly my dumb, but I'll go with your bad.

[chris128]

Touché

[nolim8ts]

Hi All,

Thanks for the suggestions. I ended up saving each listview item into a collection. I then passed that collection across to the other form for processing.