single thread locking to build resource and later the remaining threads can access the resource C#

Question

best way in c# to make only one thread to build the resource so that the remaining threads can wait and then later access the resource.

I have a dictionary which needs to be build by a thread for the remain threads to use.

private Dictionary<string, List<A>> _aDictionary;

I have a method which gets or creates the dictionary.

public async Task<List<A>> GetAs(string aId)
{
     if (string.IsNullOrWhiteSpace(aId))
     {
         return default;
     }
     try
     {
          if (!_isCached)
          {
              await lockSem.WaitAsync();
              if (!_isCached)
              {
                  await build(); // builds the dictionary
                  _isCached = true;
                  lockSem.Release();
              }
           }
          return _aDictionary.TryGetValue(aId, out var a) ? a: default;
      }
      catch(Exception ex)
      {
          _logger.Error("Error", ex);
          lockSem.Dispose();
          return default;
      }
  }

Others should not access this resource until the _isCached is set. So I looking for suggestions or best practices in this scenario