Introduction
Boolean flags are commonly used to disable a block of code while another is running: for example,
private bool in_use;
private void Process() {
in_use = true;
(...)
in_use = false;
}
private void OnEvent() {
if (in_use)
return;
(...)
}
Private InUse As Boolean;
Private Sub Process()
InUse = True
(...)
InUse = False
End Sub
Private Sub Process()
If InUse Then Return
(...)
End Sub
This design has a major drawback :
in_use blocks cannot be nested, since nesting two such blocks will turn in_use to false too early. In the following excerpt, the (...) section in the Process function will not run correctly, because in_use will have been set to false when it runs.
private void Process() {
in_use = true;
Action1();
Action2();
(...) // in_use == false here (!)
in_use = false;
}
private void Action1() {
in_use = true;
(...)
in_use = false;
}
private void Action2() {
in_use = true;
(...)
in_use = false;
}
Private Sub Process()
InUse = True
Action1()
Action2()
(...) ' InUse = False here (!)
InUse = False
End Sub
Private Sub Action1()
InUse = True
(...)
InUse = False
End Sub
Private Sub Action2()
InUse = True
(...)
InUse = False
End Sub
Such errors are difficult to spot in large applications, and often lead to hard-to-track bugs.