How do I retrieve information from UI thread?-CodePudding

I'm quite new at vb.net and using different threads so I don't exactly know how to use the invoke method. I basically want to assign selected_item as the one that user has selected in combobox from UI thread.

Dim selected_item As String
If ComboBox1.InvokeRequired Then
   selected_item = ComboBox1.Invoke(ComboBox1.SelectedItem)
Else 
   selected_item = ComboBox1.SelectedItem
End If

I get the error cross-thread operation not valid. How can I fix this?

CodePudding user response：

When you call Invoke you need to specify a method that gets invoked on the UI thread. If you want to get a value, that method has to return that value and then Invoke will return that same value on your secondary thread. In your case:

Private Function GetComboBox1SelectedItem() As Object
    If ComboBox1.InvokeRequired Then
        Return ComboBox1.Invoke(New Func(Of Object)(AddressOf GetComboBox1SelectedItem))
    Else
        Return ComboBox1.SelectedItem
    End If
End Function

You can then call that GetComboBox1SelectedItem method on any thread and it will return you the currently selected item.

If you call that method on a secondary thread, InvokeRequired is True and execution enters the If block. In that block, the Invoke method creates a delegate, marshals it to the UI thread and invokes it. That delegate is for the same method, so it gets executed a second time. The second time, we're on the UI thread so InvokeRequired is False and execution enters the Else block. In that block, the Selecteditem is retrieved and returned. The Invoke method then returns the value returned by that second instance of the method and that gets returned by the first instance.

You might like to read this for a more rigorous explanation of how to build such methods and what they actually do.

CodePudding user response：

Little bit shorter

Dim selected_item As String
If ComboBox1.InvokeRequired Then
   selected_item = ComboBox1.Invoke(Sub() ComboBox1.SelectedItem)
Else
   selected_item = ComboBox1.SelectedItem
End If