How to handle errors in a cross thread operation - vb.net

Question

I have a webbrowser control created at runtime and used via a background thread. The following is an example of the code used:
If Me.InvokeRequired Then Me.Invoke(Sub() webbroswers(3).Document.GetElementById("ID").InnerText = TextBox4.Text)
This works great! But sometime the webbrowser doesn't have the element "ID" (for example). So I'd like a method to basically allow the code to continue if an error occurs. I've tried the try - catch block but this doesn't catch it!

Answer 1

You can use a multi-line lambda expression, like this:
If Me.InvokeRequired Then Me.Invoke(
Sub()
Dim element As HtmlElement = webbroswers(3).Document.GetElementById("ID")
If element IsNot Nothing Then
element.InnerText = TextBox4.Text
End If
End Sub
)
Checking if it's Nothing, like that, is more efficient than letting it fail and catching the exception. However, if you need to do a Try/Catch for any other reason, you can also do that easily in a multi-line lambda expression, for instance:
If Me.InvokeRequired Then Me.Invoke(
Sub()
Try
webbroswers(3).Document.GetElementById("ID").InnerText = TextBox4.Text
Catch ex As Exception
' ...
End Try
End Sub
)
However, if the lambda expression gets too long, or if you'd like to have more meaningful stack traces in your exceptions, you can use a delegate to an actual method, like this:
If Me.InvokeRequired Then Me.Invoke(AddressOf UpdateId)
'...
Private Sub UpdateId()
Try
webbroswers(3).Document.GetElementById("ID").InnerText = TextBox4.Text
Catch ex As Exception
' ...
End Try
End Sub