Returning nulls

[anonymous.user0@gmail.com]

Whats the common/best practice for returning a "Does not exist/not
found" result from a function. Imagine I've got a function:

myObjectType GetById(long id);

where the object may or may not be found. Currently, I just return a
null. But this muddies up the calling code with null checks and stuff.
Example:
I've got an Appoinment class, that has a property Doctor of class
Doctor. This property is lazily loaded, so I only store the id of the
doctor in the Appointment class (doctor_id).

public property Doctor Doctor{
get{
if(doctor == null){doctor =
DoctorManager.G etById(doctor_i d)}
return doctor;
}
set{
doctor = value;
if(value != null){doctor_id = value.id;}
else{doctor_id = 0;}
}
}

Now, when using the Doctor property in Appointment class, I always have
to check if the Doctor is null. This can be ugly when doing something
like displaying the Appointment information.

private void showAppointment (Appointment apt){
txtTime.Text = ...
if(apt.Doctor != null){
txtRoom.Text = apt.Doctor.Offi ceNumber;
txtDoctorName.T ext = apt.Doctor.Name ;
}
etc...
}

I was thinking of creating a static Doctor instance in the Doctor class
that represents a Null Doctor like this:
public class Doctor{
public static readonly Doctor NullDoctor = new Doctor("Null
Doctor name", "Null Doctor Office",...);
...
}

and in the GetById() functions, returning the NullDoctor if none is
found. This of course, would cause me to change my Doctor Property to
something like this:
public property Doctor Doctor{
get{
if(doctor == null || doctor = Doctor.NullDoct or)
{doctor = DoctorManager.G etById(doctor_i d)}
return doctor;
}
set{
doctor = value;
if(value != null){doctor_id = value.id;}
else{doctor_id = 0;}
}
}

Pros/Cons for each approch?