Attempted to access an unloaded app domain

Hi ,

 

While am working today,i suddendly got the error as :attempted to access an unloaded appdomain  in visual studio .

 

when i google it ,i got solution like just close the visual studio and reopen it .

 

 

Its works fine for me ,but i would  like to know the root cause for the problem  ,so that next time i can prevent it. can anyone please guide me in that briefly?

 

Thanks in advance

Karthik