为什么我不能从另一个线程访问UI控件的属性？

Question

我读到不可能从另一个线程（不仅是GUI，而且还访问所有2个不同的线程）访问UI元素，并且看到了如何解决该问题的代码，但是没有找到解释为什么我不能这样做？

Answer 1

The reason MSDN gives for not allowing this:

Access to Windows Forms controls is not inherently thread safe. If you have two or more threads manipulating the state of a control, it is possible to force the control into an inconsistent state . Other thread-related bugs are possible as well, including race conditions and deadlocks. It is important to ensure that access to your controls is done in a thread-safe way.

So technically I could be possible (and maybe they left it out by-design), but the problem is the underlying code from Microsoft isn't thread safe. This has probably to do with the inner workings of Windows and it's lack of thread safety in their Win32 UI model.

Answer 2

You cant make changes to the GUI from any other thread but the GUI thread(main thread). That being said you can read the data from controls (textboxes etc) from whatever thread without problems if you want to make changes to controls from another thread you need to use something like this:

textBox1.Invoke((MethodInvoker)(() =>
{
    textBox1.Text = "text changed from another thread";
}));

The downside of this is that now the thread from which you are calling this is getting blocked until this change to the textbox is done. This isnt a problem if you are updating your controls from your thread not often but if you want to do it often you need to create a 3rd thread that handles the updates depending on what the worker thread is doing. Something like this

List<string> list = new List<string>();
Thread workThread = new Thread(dowork =>
{
    //random work
    for(int a = 0; a< 1000000;a++)
    {
        list.Add(a+" iteration");
        Thread.Sleep(10);
    }
});
workThread .Start();
bool updateThreadWorking = true;
Thread updateThread = new Thread(dowork =>
{
    while(updateThreadWorking)
    {
        while(list.Count > 0)
        {
            listBox.Invoke((MethodInvoker)(() =>
                listBox.Items.Add(list[0]);
            }));
            list.RemoveAt(0);
        }
        Thread.Sleep(200);
    }
});
updateThread .Start();

Now the workthread is working at full speed without getting blocked by the UI and the UI is still getting updated on the current status