Updating Variable in Multithreading in VB.NET - vb.net

Question

I've wrote a program which on startup loads the computer list from Active Directory. This takes about 10 seconds. If the user has started the program with a specific host as parameter, it should be usable immediately.
So to don't interrupt the user I want to load the computer list in a different thread. The problem is that it writes to a variable (the computer list) which is also used in the main thread.
You may think, I could simply use a temporary variable and when its done overwrite the main variable. But I have to keep existing data of the main variable.
'hosts list
Private Shared hosts As New SortedDictionary(Of String, HostEntry)
'Get all computers in Active Directory
'Will run in a extra thread
Private Delegate Sub GetADcomputersDelegate()
Private Sub GetADcomputers()
If Me.InvokeRequired Then
Me.Invoke(New GetADcomputersDelegate(AddressOf GetADcomputers), Nothing)
Else
lblStatusAD.Text = "Getting Computers..."
Try
Dim search As New DirectorySearcher(ActiveDirectory.Domain.GetCurrentDomain().GetDirectoryEntry(), "(objectClass=computer)")
For Each host As SearchResult In search.FindAll()
'AddHost creates a new HostEntry object and adds it to my "global" hosts variable
'It also checks if a host is already present in the list and only updates it.
AddHost(host.GetDirectoryEntry().Properties("cn").Value.ToLower(), host.GetDirectoryEntry().Properties("description").Value)
Next
Catch ex As Exception
Debug.WriteLine("GetADcomputers() Exception: " & ex.Message)
End Try
ThreadPool.SetMaxThreads(hosts.Count, hosts.Count)
Dim ah As String = activehost
'Fill my ListBox with the computers
lstHosts.DataSource = New BindingSource(hosts, Nothing)
'Select the computer that was selected before
UseHost(ah)
lblStatusAD.Text = ""
End If
End Sub
So when GetADcomputers() runs in its own thread, the main thread is also blocked. I guess because auf the hosts variable.
So what could I change to make the thread do it's work and after that apply the updated computer list without losing data of entries in old hosts list? And all this in a fast and efficient way.

Answer 1

That code is very wrong. If you call that method on a secondary thread then it immediately marshals a call back to the UI thread and does EVERYTHING on the UI thread. What you should be doing is executing all the background work on the secondary thread and then marshalling to the UI thread ONLY to update the UI.
Get rid of that If...Else block and just make the entire body of the method what's current ly in the Else block. Next, identify all the lines that specifically interact with the UI and remove each of those to their own method. You then add If...Else blocks to each of those methods so that only the code that actually touches the UI is executed on the UI thread.
Here's a start:
Private Sub GetADcomputers()
UpdateStatusADLabel("Getting Computers...")
Try
Dim search As New DirectorySearcher(ActiveDirectory.Domain.GetCurrentDomain().GetDirectoryEntry(), "(objectClass=computer)")
For Each host As SearchResult In search.FindAll()
'AddHost creates a new HostEntry object and adds it to my "global" hosts variable
'It also checks if a host is already present in the list and only updates it.
AddHost(host.GetDirectoryEntry().Properties("cn").Value.ToLower(), host.GetDirectoryEntry().Properties("description").Value)
Next
Catch ex As Exception
Debug.WriteLine("GetADcomputers() Exception: " & ex.Message)
End Try
ThreadPool.SetMaxThreads(hosts.Count, hosts.Count)
Dim ah As String = activehost
'Fill my ListBox with the computers
lstHosts.DataSource = New BindingSource(hosts, Nothing)
'Select the computer that was selected before
UseHost(ah)
lblStatusAD.Text = ""
End Sub
Private Sub UpdateStatusADLabel(text As String)
If lblStatusAD.InvokeRequired Then
lblStatusAD.Invoke(New Action(Of String)(AddressOf UpdateStatusADLabel), text)
Else
lblStatusAD.Text = text
End If
End Sub