Question

I am having a strange problem with the .NET TabControl in C# (Visual Studio 2010). Start a Windows Forms Application. Add a tab control and a button. Add two different labels to the two tab pages so you can differentiate them. The purpose of the button is just to act as a next button; subscribe to the its Click event with the code:

tabControl1.SelectTab(1);

Let’s assume the user entered something wrong on the first tab, so when they try to go to the second tab we want to send them back, so subscribe to the tab control’s SelectedIndexChanged event with the code:

if(tabControl1.SelectedIndex == 1)
{
    tabControl1.SelectTab(0);
}

Now run the program and click the button. You will notice that as judged by the highlighted tab at the top, the first tab page is the one that appears to be selected, as you’d expect. However, as judged by the tab page that actually appears in the body of the tab control, it’s still the second tab page that shows up! Calls to various controls’ Focus(), Update(), and Refresh() functions don’t seem to help. What is going on here?

Answer 1

I repro. This is a generic problem with event handlers, you can confuse the stuffing out the native Windows control by jerking the floor mat like that. TreeView is another control that’s very prone to this kind of trouble.

There’s an elegant and general solution for a problem like this, you can use Control.BeginInvoke() to delay the command. It will execute later after the native control is done with the event generation and all side-effects have been completed. Which solves this problem as well, like this:

    private void tabControl1_SelectedIndexChanged(object sender, EventArgs e) {
        if (tabControl1.SelectedIndex == 1) {
            this.BeginInvoke(new Action(() => tabControl1.SelectTab(0)));
        }
    }