How to remove duplication of flow logic? - oop

Question

I have a service/manager class where I fetch users from data source. The methods are simulare in flow but has different retrieve methods.
public User GetByUserName(string userName)
{
user = dependencyResolver.UserRepository.GetByUserName(userName);
if (user == null) return null;
AddStuffToUser();
return user;
}
public User GetById(int id)
{
user = dependencyResolver.UserRepository.GetById(id);
if (user == null) return null;
AddStuffToUser();
return user;
}
public User GetByUserName(string userName, string encryptedPassword)
{
user = dependencyResolver.UserRepository.GetByUsernameAndPassword(userName, encryptedPassword);
if (user == null) return null;
AddStuffToUser();
return user;
}
I would like to remove the duplication of the flow logic, but I can't find a satisfying design. I could do it by using OOP but I think that adds to much complexity for such a simple task. I also tried passing the fetch methods as a Func<> parameter, but I didn't get it right since the fetch methods have different parameters.
Which are my options here?

Answer 1

replace:
if (user == null) return null;
AddStuffToUser();
return user;
with:
return doXYZ(user);
and add:
private User doXYZ(User user){
if (user != null){
AddStuffToUser();
return user;}
return null;
at this point u placed 6 Lines for 9 and its better code-style (OOP-standard is usefull in any task)