Why is cross thread operation exception not thrown while running exe in bin\Debug

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Yes, this is only checked when a debugger is attached. This was necessary because there was lots of .NET 1.x code that violated this rule. It is not an obvious one.

The bigger problem is that such code got away with it. Either by luck, not thinking too much about the occasional painting problems or by thinking that aborting the app when it deadlocked and restarting it once a day was acceptable. Because the programmer had no real hope of discovering the problem without a diagnostic.

Microsoft cares a lot about backward compat, even if it is buggy compat. The fix is excellent, even though it is sometimes wrong (Show(owner) is checked when it shouldn’t). And sometimes overlooks to check when it is code in the framework that violates the rule. Which happens when the thread dependency is indirect. Most common cases of that are updating the data source of a data-bound control in a worker thread (unbind first!) and using a control that listens for the SystemEvents.UserPreferenceChanged event (don’t create UI on a second thread!)

For reference, the relevant code is present in the static constructor of the Control class:

static Control()
{
    //...
    checkForIllegalCrossThreadCalls = Debugger.IsAttached;
    //...
}

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