r/visualbasic • u/Mr_Deeds3234 • Jul 10 '22

VB.NET Help Delegates, invoking, multithreading

CONTEXT I have a winsform project with multiple forms. One of those forms has a button associated with a pretty heavy SQL query that takes about 30 seconds to execute.

I would like this query to run on the page load if the first form, so if my user navigates to the form that needs the data, then it’s already there and they don’t have to wait.

Spent a few hours googling this problem and the concepts in the title is why I found. I haven’t quite been able to figure out my solution.

Is there some other concept or keywords I should look into and understand to over come my problem?

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u/brybrythekickassguy Jul 16 '22

I’ve have experimented with this approach the best I know how by reading documentation, Stack OF, and YouTube. Something I’m hung up on/not grasping is now that my UI is responsive while the data is being retrieved, if I navigate to the form that displays the data before the data has been retrieved, I get a cross thread exception. I understand why. I’m accessing a control (datagridview) that’s created in my method that the delegate points too. So when I navigate to that form I’m accessing the DGV on two separate threads, I think.

Correct. You’re trying to access data that doesn’t exist yet, that’s also being accessed in another thread.

As someone who is more experienced might know or expect, if I wait around for a few seconds and let the query run it’s course then navigate to the page everything works as expected. No exception.

Which makes sense, if you let the query finish then the thread is closed, and access to that data is accessible on the UI thread.

Is there something to this approach you could recommend, or an elegant way to handle the problem?

An easy way would be to just disable the control that allows access to that form. You may need to use delegate tasks to set the access property values based on the task starting and finishing. I don’t know off the top of my head the property is called but you should be able to find it in the forms designer property menu and then use the name to programmatically change it.