Help with using .Invoke()

Zygis

Hello again,

Here's the thing: I get an error "Cross-thread operation not valid: Control 'pictureBox1' accessed from a thread other than the thread it was created on.";

Here's the situation: I start System.Timers.Timer in Class33(Form), when the timer elapses it calls a method which makes another class - Class1(Not a Form, just a class). When Class1 code is being executed it "wants" to change picture on the Class33(Form) and then i get that error.

Here's what i found:
1) i could use Control.CheckForIllegalCrossThreadCalls, but it's not very good choice;
2) I should use .Invoke() method which is good;

Here's the problem: I don't understand how to use it. What should I invoke: Class33 or Class1 or something else...?

Hope you have understood my problem, thank you.

4 Replies

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Sam Hobbs15y ago
If you search the MSDN and/or the internet for key words in that error message you will get answers. I did.

Microsoft has attempted to make it simple using the Control.InvokeRequired Property. You can search this web site's articles and forums for help with that. For example Accessing Windows Forms Controls across Threads and Updating the UI on different thread.
Vulpes15y ago
Well, in theory, whenever you access a member of any control from a thread other than the thread on which the control was created, then you should marshal the call to the creating thread using Invoke() or BeginInvoke().

Despite this, a call may sometimes appear to work without marshalling but, as it may only be working intermittently, the safest thing to do is to marshal all such calls so you don't get any nasty surprises later.

The marshalling process is completely encapsulated by the Invoke call - you don't need to do anything else to complete it.

The difference between Invoke() and BeginInvoke() is that the former is synchronous and the latter asynchronous. So, Invoke() will block the calling thread until it completes but BeginInvoke() returns immediately and queues the marshalling operation until it can next be processed by the UI thread. You should therefore use BeginInvoke if Class1 is carrying out some process that you can't afford to block.
Zygis15y ago
ok, thank you, it helped. But I have more questions now:
1) why in Class1 "class33.pictureBox1.Visible = false;" needs to be invoked, but later code "class33.pictureBox1.Image = b;" doesnt need to be invoked? Is the .Invoke() method invoked whole pictureBox1 so it doesnt need to be invoked later?
2) should I end that invoking proccess somehow or does end as soon as I dispose class1?
3) maybe it is better to use BeginInvoke instead?
Vulpes15y ago
Suppose that at the moment you have code like this in Class1, where 'class33' is a a reference to your Class33 form instance:

class33.pictureBox1.Image = someImage;

Then you can call Invoke() like this (.NET 2.0 or later):

class33.Invoke(new MethodInvoker(delegate()
{
class33.pictureBox1.Image = someImage;
}));