VB/C#.net Dynamically Add Control Items with Background Worker

Question

I'm creating a Windows Form application which Dynamically creates controls based on data pulled from a Database.

I have the code working great in the background which loads the data from the database and applies it to variables, the problem I am having is when trying to create the controls using this data, I get a multi-threading error ( Additional information: Cross-thread operation not valid: Control 'flowpanelMenuRules' accessed from a thread other than the thread it was created on.)

I'm using the BackgroundWorker_DoWork event and the code that fails is the following:

Me.flowpanelMenuRules.Controls.Add(PanelRule(i))

The code before is a simple loop going through the variable (which is pulled from the database) and gathering the information.

Has anybody had any experience in safely invoking the above line? I just can't seem to get it to work at all :(

Thanks for the help, I can post more code if needed.

Answer 1

My recommendation is to have your BackgroundWorker simply create the controls, but not add them to the form. Instead, return the prepared controls to the calling/UI thread via the RunWorkerCompleted event. Then add the controls to your form at this point, possibly in conjunction with the SuspendLayout()/ResumeLayout() methods.

Answer 2

The native way for WinForms application is to use System.Winfows.Forms.Control methods like Invoke() and InvokeRequired property to accessing UI thread:

if(this.flowpanelMenuRules.InvokeRequired)
{
    this.flowpanelMenuRules.Invoke(() => AddPanelRule());
}
else
{
    AddPanelRule();
}

You can also use Dispatcher class.

If you are sure to be on a UI thread (e.g. in an button.Click handler), Dispatcher.CurrentDispatcher gives you the UI thread dispatcher that you can later use to dispatch from background threads to the UI thread as usual.

Dispatcher.CurrentDispatcher.Invoke(() => this.flowpanelMenuRules.Controls.Add(PanelRule(i)));