如何为显示的第二种形式获取同步上下文 [英] How to get a Synchronization Context for the second form shown

问题描述

改写并简化了整个帖子 [/EDIT]

Rephrased and Simplified whole post [/EDIT]

在此博客中，下面(我简化了一点)给出了一个示例，该示例使用SynchronizationContext对象在UI线程上运行Task:

In this blog, the following (I simplified it a bit) is given as an example of using a SynchronizationContext object to run a Task on the UI thread:

Task.Factory.StartNew(() =>"Hello World").ContinueWith(
            task => textBox1.Text = task.Result,
            TaskScheduler.FromCurrentSynchronizationContext());

我可以在一个新项目中重复这些结果，安全地更新UI，但是由于我当前项目中的任何原因(即使它正在工作)，我都不能这样做.我收到了标准的不允许您从错误的线程更新UI"例外.

I can repeat these results in a fresh project, updating the UI safely, but for whatever reason in my current project (even though it's been working) I can't. I get the standard "You're not allowed to update the UI from the wrong thread" exception.

我的代码(在MainForm_Load(...)中)是这样的，它可以在新项目中工作，但要在主窗体中添加一个textBox1，但不能在我当前的项目中使用

My code (in MainForm_Load(...)) is like this, which works in a fresh Project w/ a textBox1 added to the main form, but does not work in my current project:

var one = Task.Factory.StartNew(
        () => "Hello, my name is Inigo Montoya");
var two = one.ContinueWith(
        task => textBox1.Text = one.Result,
        TaskScheduler.FromCurrentSynchronizationContext());

任何人都对可能发生的事情有任何想法.

Anyone have any thoughts on what might be gong on.

我将错误追溯到对象的实例化，该对象使用一种形式来提示用户输入登录信息.仅在显示表单后才会发生错误.(如果我在表单的 Show 发生之前返回了一个硬编码的值，那么整个程序就可以正常工作.)

I've traced the error back to the instantiation of an object which uses a form to prompt the user for login information. The error only happens when the form has been shown. (If I return a hardcoded value before that Form's Show happens the whole thing works fine).

新问题:如果我自己正在构造的表单在其显示之前先显示另一种表单，该如何获取SynchronizationContext?重现正在发生的事情的方法如下:

New question: How can I get the SynchronizationContext for the form which I'm constructing if its own constructor displays another form before it has been shown? Here's how you can reproduce what's happening:

1)创建两个表单:带有 TextBox 的Form1和带有 Button

1) Create two forms: Form1 with a TextBox, and Form2 with a Button

2)创建一个类 OwnedBy1Uses2

Form1 :

public partial class Form1 : Form
{
    OwnedBy1Uses2 member;
    public Form1()
    {
        InitializeComponent();
        member = new OwnedBy1Uses2();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        var ui = TaskScheduler.FromCurrentSynchronizationContext();
        Task<string> getData = Task.Factory.StartNew(
            () => "My name is Inigo Montoya...");
        Task displayData = getData.ContinueWith(
            t => textBox1.Text = t.Result, ui);
    }
}

Form2 :

public partial class Form2 : Form
{
    public Form2()
    {
        InitializeComponent();
        DialogResult = System.Windows.Forms.DialogResult.Cancel;
    }

    private void button1_Click(object sender, EventArgs e)
    {
        DialogResult = System.Windows.Forms.DialogResult.OK;
        Hide();
    }
}

OwnedBy1Uses2 :

class OwnedBy1Uses2
{
    int x;
    public OwnedBy1Uses2()
    {
        using (Form2 form = new Form2())
        {
            if (form.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                x = 1;
            }
            else
            {
                x = 2;
            }
        }
    }
}

推荐答案

仅在主线程上是不够的.您需要有一个有效的 SynchronizationContext.Current (在 FromCurrentSynchronizationContext 行上设置一个断点，并检查 SynchronizationContext.Current 的值；如果它是 null ，则出了点问题).

Just being on the main thread isn't sufficient. You need to have a valid SynchronizationContext.Current (set a breakpoint on the FromCurrentSynchronizationContext line and examine the value of SynchronizationContext.Current; if it's null, then something's wrong).

最干净的解决方案是从UI消息循环内执行任务代码，包括 FromCurrentSynchronizationContext -即从诸如WinForms的 Form.Load 之类的代码或WPF的Window.Loaded .

The cleanest fix is to execute your task code including FromCurrentSynchronizationContext from within the UI message loop - that is, from something like Form.Load for WinForms or Window.Loaded for WPF.

WinForms中存在一个错误，即将其放入 Form.Load 还是不够的-实际上，您必须通过读取 Handle 属性来强制Win32句柄的创建.我以为该错误已得到修复，但我可能是错的.

There was a bug in WinForms where putting it in Form.Load wasn't sufficient either - you actually had to force Win32 handle creation by reading the Handle property. I was under the impression that this bug had been fixed, but I could be wrong.

编辑2(从评论复制):

我怀疑您的问题是您在 Application.Run 之外调用 ShowDialog . ShowDialog 是一个嵌套的消息循环，但是在这种情况下，没有父消息循环.如果在 SynchronizationContext.Current 上设置监视并逐步执行 ShowDialog ，您将在显示对话框之前看到它是 WindowsFormsSynchronizationContext ，但是显示对话框后，更改为非WinForms SynchronizationContext .将成员创建(包括 ShowDialog )移动到 Load 事件可解决此问题.

I suspect your problem is that you're calling ShowDialog outside of Application.Run. ShowDialog is a nested message loop, but in this case there's no parent message loop. If you set a watch on SynchronizationContext.Current and step through the ShowDialog, you'll see that it's a WindowsFormsSynchronizationContext before the dialog is shown but changes to a non-WinForms SynchronizationContext after the dialog is shown. Moving the member creation (including the ShowDialog) to the Load event fixes the problem.

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