I have a method that I will use in the following contexts:

1.

User user = null;

if(...)
{
      user = defaultUser;
      SetUser(a,b,user);
}
else
{
      SetUser(a,b,user);
}

SaveUser(user);

So some cases are where user may be null, while in other cases it will already be initialized.

How should I design the SetUser method?

I currently have it like so, but this causes an error when user is null.

public void SetUser(object a, object b, User user)
{
     if(user == null)
         user = new User();

     user.Security = a.security;
     user.Blah = b.type;
}