Another interview question which was expecting a true / false answer and I wasn't too sure.
finally
is executed most of the time. It's almost all cases. For instance, if an async exception (like StackOverflowException
, OutOfMemoryException
, ThreadAbortException
) is thrown on the thread, finally
execution is not guaranteed. This is why constrained execution regions exist for writing highly reliable code.
For interview purposes, I expect the answer to this question to be false (I won't guarantee anything! The interviewer might not know this herself!).
Generally the finally
block is guaranteed to execute.
However, a few cases forces the CLR to shutdown in case of an error. In those cases, the finally
block is not run.
One such example is in the presence of a StackOverflow exception.
E.g. in the code below the finally
block is not executed.
static void Main(string[] args) {
try {
Foo(1);
} catch {
Console.WriteLine("catch");
} finally {
Console.WriteLine("finally");
}
}
public static int Foo(int i) {
return Foo(i + 1);
}
The other case I am aware of is if a finalizer throws an exception. In that case the process is terminated immediately as well, and thus the guarantee doesn't apply.
The code below illustrates the problem
static void Main(string[] args) {
try {
DisposableType d = new DisposableType();
d.Dispose();
d = null;
GC.Collect();
GC.WaitForPendingFinalizers();
} catch {
Console.WriteLine("catch");
} finally {
Console.WriteLine("finally");
}
}
public class DisposableType : IDisposable {
public void Dispose() {
}
~DisposableType() {
throw new NotImplementedException();
}
}
In both cases the process terminates before both catch
and finally
.
I'll admit that the examples are very contrived, but they are just made to illustrate the point.
Fortunately neither happens very often.
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