Threadding: Update UI without blocking the thread?

Question

I have written a server program that does a lot of jobs in threads simultaneously.

In those threads, I have to update a ListView with status information, but as it is right now using invoke, the thread waits for the UI to finish updating the ListView.

Any good advice to how I can send the status to the ListView and continue the thread while ListView finish updating?

Here's my code...

Public Delegate Sub InfoDelegate(status As String)

Public Sub Info(status As String)
    If Me.InvokeRequired Then
        Dim d As New InfoDelegate(AddressOf Info)
        Me.Invoke(d, status)
    Else
        Dim item As New ListViewItem With {
            .Text = status}

        With lv
            .BeginUpdate()
            .Items.Insert(0, item)
            If .Items.Count > 500 Then
                For i As Integer = Me.lv.Items.Count - 1 To 500 Step -1
                    Me.lv.Items.RemoveAt(i)
                Next
            End If
            .EndUpdate()
        End With
    End If
End Sub

@VisualVincent are you sure? Microsoft doesn't call EndInvoke ... msdn.microsoft.com/en-us/library/a06c0dc2(v=vs.110).aspx — MojoDK
– MojoDK, Commented Dec 17, 2016 at 9:55
Yes: "No matter which technique you use, always call EndInvoke to complete your asynchronous call." - msdn.microsoft.com/en-us/library/2e08f6yc(v=vs.110).aspx — Visual Vincent
– Visual Vincent, Commented Dec 17, 2016 at 9:57
And I am also certain because I know that from experience. -- What you can do to avoid blocking your thread is calling EndInvoke in a method executed on the UI thread. — Visual Vincent
– Visual Vincent, Commented Dec 17, 2016 at 9:58

Visual Vincent · Accepted Answer · 2016-12-17 10:18:15Z

2

You can call Control.BeginInvoke() to invoke the method asynchronously. However that call needs to be followed by a EndInvoke() call, or else you will get memory and/or thread leaks.

In the .NET Framework versions 4.0 and up you can utilize lambda expressions to pass the IAsyncResult returned from the BeginInvoke call to the lambda expression itself. Thus, you can call EndInvoke without having it block since by the time that it is called the asynchronous operation is already finished.

Here's an example:

Dim iar As IAsyncResult = _
    Me.BeginInvoke(Sub()
                                   Info("Status here") 'Calling your Info() method.
                                   Me.EndInvoke(iar)
                               End Sub)

answered Dec 17, 2016 at 10:18

Visual Vincent

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9 Comments

Visual Vincent Over a year ago

@MojoDK : No problem, glad I could help!

jmcilhinney Over a year ago

I believe that you're wrong about having to call EndInvoke. This is from MSDN: "You can call EndInvoke to retrieve the return value from the delegate, if neccesary, but this is not required. EndInvoke will block until the return value can be retrieved". So, you CAN call EndInvoke to get the return value from an invoked function but you don't HAVE to otherwise.

Visual Vincent Over a year ago

@jmcilhinney : Then what about: "No matter which technique you use, always call EndInvoke to complete your asynchronous call"? It's from the MSDN too: msdn.microsoft.com/en-us/library/2e08f6yc(v=vs.110).aspx

Visual Vincent Over a year ago

@jmcilhinney : And from my experience you need to call EndInvoke. I've experienced problems such as the UI freezing when too many BeginInvoke calls were called without being followed by EndInvoke.

Trevor Over a year ago

@jmcilhinney Is correct and I agree. Personally I have never seen any issues not calling EndInvoke. Specifically if using a delegate it's blocked anyways and won't return to the caller until it's finished. Calling EndInvoke at this point doesn't matter. In the example you provided It doesn't matter either because when that method is called, it's immediately blocked until it's done and then comes back to the caller, calling EndInvoke is irrelevant at this point....

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