Aggregation And Composition In C#

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.

namespace CSharpStuffs
{
   public class Patient
    {
        public string PatientName { get; set; }
        public string AdmissionDate { get; set; }
        public string ReleaseDate { get; set; }

        public DoctorInfo DoctorInfo { get; set; }
        public LifeInsurance Insurance { get; set; }
        }
  }

namespace CSharpStuffs
{
   public class LifeInsurance
   {
       public string PolicyName;
       public string CompanyName;
   }
}

Once, the class gets created, It's time to feed the value via constructor. Hence, the following is the final code for the same.

namespace CSharpStuffs
{
   public class Patient
    {
        public string PatientName { get; set; }
        public string AdmissionDate { get; set; }
        public string ReleaseDate { get; set; }

        public DoctorInfo DoctorInfo { get; set; }
        public LifeInsurance Insurance { get; set; }

       public Patient(string patientName,string admissionDate,string releaseDate,string doctorName,string doctorSpeciality, string workingAt)
       {
           PatientName = patientName;
           AdmissionDate = admissionDate;
           ReleaseDate = releaseDate;

           //Composition comes into picture now
           //It also means when we construct Patient, we also constructed DoctorInfo
           //Hence, scope of Doctorinfo is dependent on Patient
           DoctorInfo = new DoctorInfo
           {
               DoctorName = doctorName,
               DoctorSpeciality = doctorSpeciality,
               WorkingAt = workingAt
           };

       }

       //Overrideenn ToString() for convience
       public override string ToString()
       {
           string result = PatientName + " " +
                           AdmissionDate + " " +
                           ReleaseDate + " ";
           //Filling Other objects
           if (DoctorInfo != null)
           {
               result += DoctorInfo.DoctorName + " "
                         + DoctorInfo.DoctorSpeciality + " "
                         + DoctorInfo.WorkingAt;
           }
           if (Insurance != null)
           {
               result += " "+ Insurance.PolicyName + " "
                         + Insurance.CompanyName;
           }
           return result;
       }
    }
}

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:

using System;

namespace CSharpStuffs
{
    class Program
    {
        static void Main(string[] args)
        {
            Patient patient = new Patient("Tim","12-08-2015","20-08-2015","Cook","Physician","Fortis");
            Console.WriteLine(patient.ToString());
            Console.ReadLine();
        }
    }
}

And, when I run the program, it will produce the following output.



However, I can modify the main program to add more info to this via aggregation as shown below.

using System;

namespace CSharpStuffs
{
    class Program
    {
        static void Main(string[] args)
        {
            Patient patient = new Patient("Tim","12-08-2015","20-08-2015","Cook","Physician","Fortis");
            Console.WriteLine(patient.ToString());


            //Now, we can add insurance by aggregation
            patient.Insurance = new LifeInsurance
            {
                CompanyName = "HDFC",
                PolicyName = "LIFE-AXA"
            };
            Console.WriteLine(patient.ToString());
            Console.ReadLine();
        }
    }
}

With the above change in place, it will display the following info.



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!