Dictionary as thread-safe variable

Question

I have a class (singleton) and it contains a static Dictionary

private static Dictionary<string, RepositoryServiceProvider> repositoryServices = null;

in the instance of this class I populate the dictionary (can occur from multiple threads). At first I had just

        RepositoryServiceProvider service = null; 
        repositoryServices.TryGetValue(this.Server.Name, out service);
        if (service == null) {
          service = new RepositoryServiceProvider(this.Server);
          repositoryServices.Add(this.Server.Name, service);  
        }

then I got some exceptions as Item already added so I changed it to:

        RepositoryServiceProvider service = null;    
        repositoryServices.TryGetValue(this.Server.Name, out service);
        if (service == null) {
          lock (padlock) {
            repositoryServices.TryGetValue(this.Server.Name, out service);
            if (service == null) {
              service = new RepositoryServiceProvider(this.Server);
              repositoryServices.Add(this.Server.Name, service);  
            }
          }
        }

and padlock is in the class:

private static readonly object padlock = new object();

is this thread safe? or its overcomplicated? or should I use ConcurentDictionary ?

Answer 1

IF you can use ConcurrentDictionary - it is in several situations faster than your approach because it implements most operations lock-free while being thread-safe.

EDIT - as per comments:

The term "most operations lock-free" is a bit too general...

Basically it means reduced contention ... thus in some cases more efficiency compared to a situation with one global lock, ie accessing a second bucket while the first bucket is locked works as if there was no lock from the POV of the accessing code... although that means a lock local to that bucket... in real-world applications it delivers much better performance than a global lock - esp. with multi-core.