Question

I have a generic Dictionary that I am using as a cache in a threaded .NET project (C#). I will be doing a lot of reads on the dictionary (potentially hundreds or more per second at peak times).

I have a simple Get(int id) method that should return a CustomObject if it’s in the cache, or null otherwise.

My question:
Is it faster/more efficient to lock the dictionary, or just use a try/catch block?

Assuming the dictionary is stored in a variable named “dic”.

Lock Sample:

public CustomObject Get(int id)
{
   lock(dic)
   {
      if (dic.ContainsKey(id))
         return dic[id];
   }
   return null;
}

Try/Catch Sample:

public CustomObject Get(int id)
{
   try
   {
      return dic[id];
   }
   catch
   {
      return null;
   }
}

Answer 1

I think you should test it in your own environment. Basically:

• Lock is cheap
• Try without getting an exception is cheap, maybe even cheaper then lock
• Try and getting exception is very expensive

So now the question is, how often you expect to have cache-miss, and therefore get an exception thrown. I would go for lock() as it’s execution time is not dependent on whether you will or not get cache-hit, which means it’s more predictable and measurable, while still – very cheap. I don’t think that hundreds hits per second would be any problem.

Simple tests I’ve made indicate, that getting cache-miss with try/catch is very, very expensive.

Edit:

Simple test shows that:

• try-no throw costs about 2ms for 100k retrieves
• lock costs about 6ms
• try-throw costs about 4seconds

Which means, got for lock(), because it’s more efficient then try/catch if you’re getting more then 1 cache miss per few thousands tries, and it’s much more stable, being not depended on luck.