Two methods that differ only in LINQ where part - delegate?

Question

I have a method:

internal List<int> GetOldDoctorsIDs
{
    var Result = from DataRow doctor in DoctorTable.Rows
                 where doctor.Age > 30
                 select doctor.ID
    List<int> Doctors = new List<int>();
    foreach (int id in Result)
    {
         //Register getting data
         Database.LogAccess("GetOldDoctorsID: " + id.ToString());
         if (Database.AllowAccess(DoctorsTable, id))
         {
             Doctors.Add(id);
         }
    }
}

So this gets old doctors and does other things. Now I would like to create method GetExpensiveDoctors. It will look like this above, but in place of:

where doctor.Age > 30

I will have:

where doctor.Cost > 30000

How to create elegant, object oriented solution for this? Should I use delegate or other thing?

Answer 1

You can add the where clause selectively, for example:

var Result = from DataRow doctor in DoctorTable.Rows
                 select doctor.ID;
if(getByAge) {
    Result=Result.Where(doctor => doctor.Age>30);
} else {
    Result=Result.Where(doctor => doctor.Cost>30000);
}

where getByAge would be for example a boolean parameter on your method.

EDIT. If you need to parametrize the where clause, something like this should work:

internal List<int> GetOldDoctorsIDs(Func<Doctor, bool> whereClause)
{
    var Result = DoctorTable.Rows.Where(d => whereClause(d)).Select(d => d.ID);
    //etc...
}

then you invoke the method the way helium has said.

Answer 2

You could parameterize your method so it takes the condition as a function from DataRow to bool.

Than you could call

GetDoctorsIDs(doctor => doctor.Age > 30);

or

GetDoctorsIDs(doctor => doctor.Cost > 30000);

Answer 3

If you modify your method so that a predicate parameter is included (see below), you can call the method with any filters you need as per helium's examples.

internal List<int> GetDoctorsIDs(Predicate<DataRow> doctorFilter)
{
    var Result = from DataRow doctor in DoctorTable.Rows
                 where doctorFilter(doctor)
                 select doctor.ID
    List<int> Doctors = new List<int>();
    foreach (int id in Result)
    {
         //Register getting data
         Database.LogAccess("GetOldDoctorsID: " + id.ToString());
         if (Database.AllowAccess(DoctorsTable, id))
         {
             Doctors.Add(id);
         }
    }
}

Answer 4

I've managed to do this in this way:

internal List<int> GetOldDoctorsIDs()
{
    return GetDoctorsIDs((doctor) => doctor.Age > 30);
}

internal List<int> GetExpensiveDoctorsIDs()
{
    return GetDoctorsIDs((doctor) => doctor.Cost > 30000);
}

internal List<int> GetDoctorsIDs(Func<DataRow, bool> Condition)
{
    var Result = from DataRow doctor in DoctorTable.Rows
                 where Condition(doctor)
                 select doctor.ID
    List<int> Doctors = new List<int>();
    foreach (int id in Result)
    {
         //Register getting data
         Database.LogAccess("GetOldDoctorsID: " + id.ToString());
         if (Database.AllowAccess(DoctorsTable, id))
         {
             Doctors.Add(id);
         }
    }
}