Well, based on the text, I think there was a mistake...
>> Alternatively, it might be better to use more elaborate code to
>> create rhs outside of the lock, as in the following example. Then,
>> you can use an interlocked compare exchange to update x only if it
>> is still null. This assumes that creation of duplicate rhs values
>> does not cause negative side effects.
>>
>> if (x == null)
>> {
>> lock (this)
>> {
>> if (x == null)
>> {
>> // Perform some elaborate code to create rhs.
>> x = rhs;
>> }
>> }
>> }
However, it does make sense with a slight alteration -- and one that
would be almost unnoticed due to positional similarities:
>> if (x == null)
>> {
//Perform the rhs calculations here.
>> lock (this)
>> {
>> if (x == null)
>> {
>> x = rhs;
>> }
>> }
>> }
In other words, after the first x == null check, not the second.
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