Dictionary add causes an exception when the index is out of range.

In the case of high concurrency, the index in the dictionary add operation is out of the range.

If the lock is a class field, it is not static. The locks between multiple requests are not the same.

Solution,
First, change to static locks.
Second, change "Dictionary" to "concurrentdictionary.

The second solution is recommended.

If some data is loaded for the first time, the lock/unlock model can be used.

After testing, 1000 threads on my machine add item at the same time. dictionary threw indexoutofrange in more than a dozen seconds! Concurrentdictionary performs well and can always stick to outofmemoery.

Make sure to cache data using dictionaries in IIS.