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在我之前的博文C#中只使用Invokerequired来判断是不是UI线程可靠吗？提到过除了检查InvokeRequired，还需要检查IsHandleCreated。今天再分享一段关于InvokeRequired的代码。

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public partial class Form9 : Form
{
	public Form9()
	{
		InitializeComponent();
		// CheckForIllegalCrossThreadCalls = false;
	}

	private void button1_Click(object sender, EventArgs e)
	{
		var t = new Thread(() => { textBox1.Text = "Random Text"; });
		t.Start();
	}
}

这段代码很简单，就是在非UI线程更新UI控件。如果用Visual Studio启动程序时会抛出InvalidOperationException异常。但是如果直接双击启动编译后的exe文件，则一切工作顺利，没有InvalidOperation异常抛出。就算双击exe文件启动后拿Visual Studio Attach到这个进程，设置Visual Studio在所有的异常抛出时都break，还是没有抓到。为什么双击exe文件就神奇的工作了呢？

回头看MSDN的How to: Make Thread-Safe Calls to Windows Forms Controls。

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, such as race conditions and deadlocks. It is important to make sure that access to your controls is performed in a thread-safe way.

It is unsafe to call a control from a thread other than the one that created the control without using the Invoke method.

这段写的就是说不能跨线程操作UI控件。接着往下读。

The .NET Framework helps you detect when you are accessing your controls in a manner that is not thread safe. When you are running your application in the debugger, and a thread other than the one which created a control tries to call that control, the debugger raises an InvalidOperationException with the message, “Control control name accessed from a thread other than the thread it was created on.”

This exception occurs reliably during debugging and, under some circumstances, at run time. You might see this exception when you debug applications that you wrote with the .NET Framework prior to the .NET Framework 2.0. You are strongly advised to fix this problem when you see it, but you can disable it by setting the CheckForIllegalCrossThreadCalls property to false.

这段写的就是说如果是用Visual Studio运行程序，会强制抛出一个InvalidOperationException，帮助我们发现这些问题然后修复他们。如果没有通过Visual Studio直接运行程序，不会稳定的抛出这个异常，但是并不是说没有问题。

另外，如果暂时不想修改代码，但是还想从Visual Studio中启动，可以在Form中设置属性CheckForIllegalCrossThreadCalls为false，这样Visual Studio就会忽略跨线程访问UI的情况，不抛出InvalidOperationException异常了。就是我上面列的代码注释掉的那行。CheckForIllegalCrossThreadCalls是.NET Framework 2.0引入的一个新属性。

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