Question

Here is two my variants of method, which returns a string, associated with enum value (kept in dictionary). First variant is slower, but thread-safe, second is faster, but i don’t know, whether it is thread-safe or not. First:

string GetStringForEnum (SomeEnum e)
{
   string str = null;
   lock (someDictionary) //someDictionary is not used anywhere else (only in this method)
   { if (!someDictionary (e, out str)) { someDictionary.Add (e, "somehowCreatedString"); }
   return str;
}

Second variant:

string GetStringForEnum (SomeEnum e)
{
   string str = null;
   if (!someDictionary (e, out str))
   {
     lock (someDictionary) //someDictionary is not used anywhere else (only in this method)
     { if (!someDictionary (e, out str)) { someDictionary.Add (e, "somehowCreatedString"); }
   }
   return str;
}

Second variant is not used “lock” each time, but is it thread-safe or not?

Answer 1

There are 2 problems here:

• lock (someDictionary) – this is dis-advised, even if the dictionary is not used elsewhere. It is a theoretical argument but the (future) code of the Dictionary class could lock on itself.

• if (!someDictionary (e, out str)) without a lock. I assume this is a call to TryGetValue(). This simply is not thread-safe, your Read could be interrupted by a Write in another thread. This could end in all sorts of errors (index out of range, null reference). The errors will be very rare (= hard to reproduce).