As a hardgainer, or "ectomorph" (if you want to be scientific) it's commonly touted that compound exercises with a barbell are the best way to build muscle.
The question I want to ask, and partially answer today however is, can you build muscle with just a set of dumbbells? There are two main issues I have with barbells:

• You need a spotter for safety purposes, and this isn't always possible with the variety of schedules my friends and family have.
  
• They're big and bulky when you take into account all the other equipment you need link a bench and a rack.
  

And that's why I'm trying to avoid them; at least until I reach a point where I feel more comfortable going to a gym where there's probably someone who could spot me.
Anyways, here's the approach I'm taking with using dumbbells:

1. Mix in with pull ups.
   The pull up is the most important exercise for hardgainers, so it's quite good for balancing out the relative ineffectiveness of dumbbells.
   They're certainly difficult, but very versatile and quite fun once you get the hang of them.
   
2. Use heavier weights.
   All weight lifting for hardgainers should involve heavy weights for high intensity but it's something that's easy to overlook.
   If you're consistently doing more than 12 reps per set it's probably a good idea to add some extra weight.
   
3. Have patience.
   Barbells are the most effective tool for building strength.
   Period.
   With that you have to accept that dumbbells will take longer to build strength with, but, at least in my experience, this downside is outweighed by the fact that using dumbbells is fun, sustainable and just requires less upfront work.
   

I am positive you can build muscle with dumbbells, and will diligently work towards proving that belief, but what it comes down to is what you want to do.
Even if you don't believe dumbbells will give you your strength aims, they're at least a simple way to form the strength training habit.