c# 跨线程错误

Question

我编写了一个程序来保存传感器的 3 个不同位置。如果传感器回到保存的位置，我希望我的文本框的背景颜色为绿色。因为这些值的变化非常快速和精确，所以我决定只比较前 3 个值。所以我启动了我的程序并通过单击保存了一个位置。该位置已保存，文本框立即变为绿色，这很好，因为传感器仍处于此位置。但在那之后我得到了一个跨线程的例外，我不明白。我是 C# 的新手，我认为我在函数开始时通过调用解决了这个问题。

        private void Safe_Position1(TextBox tBr1, TextBox tBi1, TextBox tBj1, TextBox tBk1, string[] text)
    {
        if (button3clicked == true)
        {
            if (!Dispatcher.CheckAccess())
            {
                textBox5.Dispatcher.BeginInvoke(new Action(() => Safe_Position1(tBr1, tBi1, tBj1, tBk1, text)));
            }
            if (!Dispatcher.CheckAccess())
            {
                textBox7.Dispatcher.BeginInvoke(new Action(() => Safe_Position1(tBr1, tBi1, tBj1, tBk1, text)));
            }
            if (!Dispatcher.CheckAccess())
            {
                textBox8.Dispatcher.BeginInvoke(new Action(() => Safe_Position1(tBr1, tBi1, tBj1, tBk1, text)));
            }
            if (!Dispatcher.CheckAccess())
            {
                textBox9.Dispatcher.BeginInvoke(new Action(() => Safe_Position1(tBr1, tBi1, tBj1, tBk1, text)));
            }

            else
            {
                tBr1.Text = text[0];
                tBi1.Text = text[1];
                tBj1.Text = text[2];
                tBk1.Text = text[3];
                button3clicked = false;
            }

            string firstthreetBr1 = new string(text[0].Take(3).ToArray());
            string firstthreetBi1 = new string(text[1].Take(3).ToArray());
            string firstthreetBj1 = new string(text[2].Take(3).ToArray());
            string firstthreetBk1 = new string(text[3].Take(3).ToArray());


            if (firstthreetBr1 == tBr1.Text.Substring(0,3)) <------ EXCEPTION HERE
            {
                tBr1.Background = Brushes.Green;
            }
            if (firstthreetBi1 == tBi1.Text.Substring(0, 3))
            {
                tBi1.Background = Brushes.Green;
            }
            if (firstthreetBj1 == tBj1.Text.Substring(0, 3))
            {
                tBj1.Background = Brushes.Green;
            }
            if (firstthreetBk1 == tBk1.Text.Substring(0, 3))
            {
                tBk1.Background = Brushes.Green;
            }
        }
    }

谁能给我解释一下？

Answer 1

我为你做了类似（不一样——你应该明白不要复制粘贴）的情况。

单击按钮时，我正在创建 3 个线程，它们正在更新文本框的文本（在 UI 线程中创建）

private void btnStart_Click(object sender, EventArgs e)
{
    Thread t1 = new Thread(() => ThreadRunner(1));
    Thread t2 = new Thread(() => ThreadRunner(2));
    Thread t3 = new Thread(() => ThreadRunner(3));
    t1.Start();
    t2.Start();
    t3.Start();
}
private void ThreadRunner(int threadNum)
{
    while(true)
    {
        TextBoxUpdater(threadNum);
        Thread.Sleep(100 * threadNum);
    }
}

以及被调用来更新文本框的方法，

private void TextBoxUpdater(int num)
{
    //here i m checking, if it is from different thread then UI thread
    //if yes then Invoke is used, 
    //      so it will call the same method but in UI thread
    if(this.InvokeRequired)
    {
        txtBox1.BeginInvoke(new Action(() => TextBoxUpdater(num)));
        txtBox2.BeginInvoke(new Action(() => TextBoxUpdater(num)));
        txtBox3.BeginInvoke(new Action(() => TextBoxUpdater(num)));
    }
    else
    {
        //If it is UI thread (meaning this call is coming from If part of this method)
        //and as it is UI thread now, we can update textbox's text
        txtBox1.Text = num.ToString();
        txtBox2.Text = num.ToString();
        txtBox3.Text = num.ToString();
    }

    //but after all above checks, we are leving below code without any checking
    //so once if part of this code will call this function again and else part of code runs
    // and then after completing that call, 
    //      flow will retun back to if part (not that it is not UI thread), 
    //      then flow will directly checks below codition and throw an exception
    if (txtBox1.Text.Substring(1) == "1")
    {

    }
}

您可以像下面这样进行最少的更改来防止异常。

private void TextBoxUpdater(int num)
{
    //here i m checking, if it is from different thread then UI thread
    //if yes then Invoke is used, 
    //      so it will call the same method but in UI thread
    if(this.InvokeRequired)
    {
        txtBox1.BeginInvoke(new Action(() => TextBoxUpdater(num)));
        txtBox2.BeginInvoke(new Action(() => TextBoxUpdater(num)));
        txtBox3.BeginInvoke(new Action(() => TextBoxUpdater(num)));
    }
    else
    {
        //If it is UI thread (meaning this call is coming from If part of this method)
        //and as it is UI thread now, we can update textbox's text
        txtBox1.Text = num.ToString();
        txtBox2.Text = num.ToString();
        txtBox3.Text = num.ToString();

        //perform this logic too insdie of else block
        if (txtBox1.Text.Substring(1) == "1")
        {
        }
    }
}

但是同样，你的方法最终会调用太多不必要的调用 TextBoxUpdater 。 你应该想知道我为什么要这么说。

所以要修复这部分，你应该修改你的方法，如下所示。

private void TextBoxUpdater(int num)
{
    if (this.InvokeRequired)
    {
        //if current call is form other than UI thread invoke to UI thread
        this.BeginInvoke(new Action(() => TextBoxUpdater(num)));
    }
    else
    {
        //If it is UI thread (meaning this call is coming from If part of this method)
        //and as it is UI thread now, we can update textbox's text
        txtBox1.Text = num.ToString();
        txtBox2.Text = num.ToString();
        txtBox3.Text = num.ToString();
        if (txtBox1.Text.Substring(1) == "1")
        {

        }
    }
}

Answer 2

正如 Matthew 所说，您必须在每个 BeginInvoke() 之后编写 return 语句。此代码引发异常，因为在您的方法从 BeginInvoke() 返回后，它会更深入地进入代码。