我觉得我需要休息一下，因为我的问题真的很傻:

I think I need to take a rest, since my problem is really silly:

事实证明，我应该使用 TabItem 而不是 TabControl，因为它是我感兴趣的控件.

Turns out that instead of TabControl I should have used TabItem since it is the control I am interesting in.

所以，我的代码必须如下:

So, my code has to be as below:

 void mainTabControl_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        if (e.Source is TabItem)
        {       
            if (this.IsLoaded)
            {
                //do work when tab is changed
            }
        }
    }