Isn't blindly using InvokeRequired just bad practice?

问题

I am a novice programmer so I could be completely mistaken here, but this issue bugs me more then it should.

This is actually a follow-up from this question.

The accepted answer was, that you have to call InvokeRequired in order to avoid some overhead, because there is a chance you are already operating on the UI thread.

In theory, I agree that it could save some time. After some tests I found out that using Invoke takes about twice the time compared to calling an operation normally (tests like setting the text of a label n times, or placing a very, very big string in a RichTextBox).

But! Then there is practice.

MSDN documentation says:

This property can be used to determine if you must call an invoke method, which can be useful if you do not know what thread owns a control.

In most cases, you do know when you try to access a control from another thread. Actually the only situation I can think of is, when the control is accessed from a method that can be called by thread X aswell as the owner thread. And that to me is a very unlikely situation.

And even if you genuinely don't know which thread tries to manipulate the control, there is the fact that the UI thread doesn't have to be updated that frequently. Anything between 25-30 fps should be okay for your GUI. And most of the changes made in the UI-controls takes far less then milliseconds to perform.

So if I understand corrrectly, the only scenario where you have to check if an invoke is required is when you don't know which thread is accessing the control and when the GUI update takes more than about 40 ms to finish.

Then there is the answer to this question I asked on http://programmers.stackexchange.com. Which states that you shouldn't be busy with premature optimisation when you don't need it. Especially if it sacrifices code readability.

So this brings me to my question: shouldn't you just use invoke when you know a different thread accesses a control, and only when you know your UI thread can access that piece of code and you find that it should run faster, that you should check if an Invoke is required?

PS: after proofreading my question it really sounds like I am ranting. But actually I am just curious why InvokeRequired is seemingly overused by many more-experienced-than-me programmers.

回答1:

You are taking things out of context here. The first question you linked linked another question which specifically was about writing a thread-safe method to access a UI control.

If you don't need a thread-safe access to a UI control, because you know you won't update it from another thread, then certainly, you shouldn't employ this technique. Simply update your UI control without using InvokeRequired or Invoke.

On the other hand, if the call will always originate in a thread other than the UI thread, simply use Invoke without first checking for InvokeRequired.

This leads to three simple rules:

1. If you update the control only from the UI thread, use neither InvokeRequired nor Invoke
2. If you update the control only from a thread other than the UI thread, use only Invoke.
3. If you update the control from both the UI thread and other threads, use Invoke in combination with InvokeRequired.

回答2:

In practice people tend to call the same method from both the foreign and the owning thread. The usual pattern is that the method itself determines whether the thread is the owning thread. If it is, it executes the follow-up code. If it isn't the method calls its own self using Invoke this time.

One benefit of this is that it makes the code more compact, as you have one method related to the operation instead of two.

Another and probably more important benefit is that it reduces the chance that the cross thread exception will be raised. If both methods were available at any time and both threads could choose any of the two, then there would be a chance of a seemingly legitimate method call raising an exception. On the other hand, if there's only one method that adapts to the situation, it provides a safer interface.

来源：https://stackoverflow.com/questions/17421696/isnt-blindly-using-invokerequired-just-bad-practice