In today's discussion, we will discuss Aggregation and Composition in detail using C#. So, let's get started. Let me go ahead and create simple console App for demo purpose.

  1. namespace CSharpStuffs
  2. {
  3. public class Patient
  4. {
  5. public string PatientName { get; set; }
  6. public string AdmissionDate { get; set; }
  7. public string ReleaseDate { get; set; }
  9. public DoctorInfo DoctorInfo { get; set; }
  10. public LifeInsurance Insurance { get; set; }
  11. }
  12. }
  1. namespace CSharpStuffs
  2. {
  3. public class LifeInsurance
  4. {
  5. public string PolicyName;
  6. public string CompanyName;
  7. }
  8. }
Once, the class gets created, It's time to feed the value via constructor. Hence, the following is the final code for the same.
  1. namespace CSharpStuffs
  2. {
  3. public class Patient
  4. {
  5. public string PatientName { get; set; }
  6. public string AdmissionDate { get; set; }
  7. public string ReleaseDate { get; set; }
  9. public DoctorInfo DoctorInfo { get; set; }
  10. public LifeInsurance Insurance { get; set; }
  12. public Patient(string patientName,string admissionDate,string releaseDate,string doctorName,string doctorSpeciality, string workingAt)
  13. {
  14. PatientName = patientName;
  15. AdmissionDate = admissionDate;
  16. ReleaseDate = releaseDate;
  18. //Composition comes into picture now
  19. //It also means when we construct Patient, we also constructed DoctorInfo
  20. //Hence, scope of Doctorinfo is dependent on Patient
  21. DoctorInfo = new DoctorInfo
  22. {
  23. DoctorName = doctorName,
  24. DoctorSpeciality = doctorSpeciality,
  25. WorkingAt = workingAt
  26. };
  28. }
  30. //Overrideenn ToString() for convience
  31. public override string ToString()
  32. {
  33. string result = PatientName + " " +
  34. AdmissionDate + " " +
  35. ReleaseDate + " ";
  36. //Filling Other objects
  37. if (DoctorInfo != null)
  38. {
  39. result += DoctorInfo.DoctorName + " "
  40. + DoctorInfo.DoctorSpeciality + " "
  41. + DoctorInfo.WorkingAt;
  42. }
  43. if (Insurance != null)
  44. {
  45. result += " "+ Insurance.PolicyName + " "
  46. + Insurance.CompanyName;
  47. }
  48. return result;
  49. }
  50. }
  51. }
You will also notice that here, I have used Composition concept. Composition is tied with actual object which is patient here and hence DoctorInfo also got constructed.

Now, main program looks like the following:
  1. using System;
  3. namespace CSharpStuffs
  4. {
  5. class Program
  6. {
  7. static void Main(string[] args)
  8. {
  9. Patient patient = new Patient("Tim","12-08-2015","20-08-2015","Cook","Physician","Fortis");
  10. Console.WriteLine(patient.ToString());
  11. Console.ReadLine();
  12. }
  13. }
  14. }
And, when I run the program, it will produce the following output.

run

However, I can modify the main program to add more info to this via aggregation as shown below.
  1. using System;
  3. namespace CSharpStuffs
  4. {
  5. class Program
  6. {
  7. static void Main(string[] args)
  8. {
  9. Patient patient = new Patient("Tim","12-08-2015","20-08-2015","Cook","Physician","Fortis");
  10. Console.WriteLine(patient.ToString());
  13. //Now, we can add insurance by aggregation
  14. patient.Insurance = new LifeInsurance
  15. {
  16. CompanyName = "HDFC",
  17. PolicyName = "LIFE-AXA"
  18. };
  19. Console.WriteLine(patient.ToString());
  20. Console.ReadLine();
  21. }
  22. }
  23. }
With the above change in place, it will display the following info.

display

Therefore, in a nutshell, Aggregation is the stuff which can be added at later point of time to the object, but composition gets constructed with object creation itself.

Thanks!