IComparable

Question

This orignal program is in the following website (http://www.dotnetperls.com/icomparable).List of name and salary have been added in following way list.Add(new Employee() { Name = "Steve", Salary = 10000 });

If we remove the curly braces, Name, and Salary and we write like as follows list.Add(new Employee("Steve", 10000)); Program is requesting a public constructor. And we add a public constructor. The output is as follows. Please explain the reason. Modification is highlighted in the program.

/*
8000,Lucy
500000,Bill
10000,Andrew
10000,Janet
10000,Steve
*/

using System;
using System.Collections.Generic;

class Employee : IComparable<Employee>
{
public int Salary { get; set; }
public string Name { get; set; }

public Employee(string Name, int Salary)
{
this.Name = Name;
this.Salary = Salary;
}

public int CompareTo(Employee other)
{
// Alphabetic sort if salary is equal. [A to Z]
if (this.Salary == other.Salary)
{
return this.Name.CompareTo(other.Name);
}
// Default to salary sort. [High to low]
return other.Name.CompareTo(this.Name);
}

public override string ToString()
{
// String representation.
return this.Salary.ToString() + "," + this.Name;
}
}

class Program
{
static void Main()
{
List<Employee> list = new List<Employee>();
list.Add(new Employee("Steve", 10000));
list.Add(new Employee("Janet", 10000));
list.Add(new Employee("Andrew", 10000));
list.Add(new Employee("Bill", 500000));
list.Add(new Employee("Lucy", 8000));

// Uses IComparable.CompareTo()
list.Sort();

// Uses Employee.ToString
foreach (var element in list)
{
Console.WriteLine(element);
}
Console.ReadKey();
}
}

Maha · Answer

Thank you very much for your explanation.

Vulpes · Answer

It's not compiling because whilst Employee has a public constructor which takes 2 parameters it doesn't have one which takes 0 parameters which is needed for the object initializers. If you provide one then it compiles and works fine: using System; using System.Collections.Generic; class Employee : IComparable { public int Salary; public string Name; public Employee() // public parameterless constructor { } public Employee(int Salary string Name) //public constructor { this.Salary = Salary; this.Name = Name; } public int CompareTo(Employee other) { // Alphabetic sort if salary is equal. [A to Z] if (this.Salary == other.Salary) { return this.Name.CompareTo(other.Name); } // Default to salary sort. [High to low] return other.Salary.CompareTo(this.Salary); } public override string ToString() { // String representation. return this.Salary.ToString() +  + this.Name; } } class Program { static void Main() { List list = new List (); list.Add(new Employee() { Name = Steve Salary = 10000 }); list.Add(new Employee() { Name = Janet Salary = 10000 }); list.Add(new Employee() { Name = Andrew Salary = 10000 }); list.Add(new Employee() { Name = Bill Salary = 500000 }); list.Add(new Employee() { Name = Lucy Salary = 8000 }); // Uses IComparable.CompareTo() list.Sort(); // Uses Employee.ToString foreach (var element in list) { Console.WriteLine(element); } Console.ReadKey(); } } /* 500000Bill 10000Andrew 10000Janet 10000Steve 8000Lucy */

Maha · Answer

Original program in the above website. For the modification purpose it has been supplied with a public constructor but the program is not executing. What is the reason? using System; using System.Collections.Generic; class Employee : IComparable { public int Salary; public string Name; public Employee(int Salary string Name) //public constructor { this.Salary = Salary; this.Name = Name; } public int CompareTo(Employee other) { // Alphabetic sort if salary is equal. [A to Z] if (this.Salary == other.Salary) { return this.Name.CompareTo(other.Name); } // Default to salary sort. [High to low] return other.Salary.CompareTo(this.Salary); } public override string ToString() { // String representation. return this.Salary.ToString() +  + this.Name; } } class Program { static void Main() { List list = new List (); list.Add(new Employee() { Name = Steve Salary = 10000 }); list.Add(new Employee() { Name = Janet Salary = 10000 }); list.Add(new Employee() { Name = Andrew Salary = 10000 }); list.Add(new Employee() { Name = Bill Salary = 500000 }); list.Add(new Employee() { Name = Lucy Salary = 8000 }); // Uses IComparable.CompareTo() list.Sort(); // Uses Employee.ToString foreach (var element in list) { Console.WriteLine(element); } Console.ReadKey(); } } /* 500000Bill 10000Andrew 10000Janet 10000Steve 8000Lucy */

Vulpes · Answer

I think you need to look at it the other way around. If a class doesn't have a public constructor or only has a public parameterless constructor then it makes sense to use an object initializer to initialize property values. If a class has a public constructor which already initializes the properties you're interested in then it doesn't make sense to use an object initializer. If you're designing a class yourself then you might decide to supply several constructors including a parameterless one for users who like to use object initializers.

Maha · Answer

Therefore I can come to a conclusion that if we use object initialize there is no need for public constructor to initialize the parameters if we are not using object initializer there is a need for public constructor to initialize parameters.

Vulpes · Answer

In the original program in the website the Employee class didn't have a public constructor and so it made sense to use an object initializer with the default parameterless constructor. In contrast in the program in your opening post to this thread the Employee class does have a single public constructor which takes two parameters which initialize the Name and Salary fields. Consequently it makes no sense to use an object initializer in this program.

Maha · Answer

But in this particular program if we modified in the above way with use of list.Add(new Employee( Steve 10000 ); that means without object initializer. We have to provide a public constructor the program to execute otherwise program is not executing.

Vulpes · Answer

It's still using a public constructor but it's the default one provided by the system when (as here) the program does not provide one itself. If you recall the default constructor takes no parameters and does nothing apart from calling the base class's parameterless constructor.

Maha · Answer

But program in the above website is executing without public constructor.

Vulpes · Answer

Yes you still need a public constructor because an object initializer consists of a call to a constructor followed by a property initialization list enclosed in curly brackets.

Maha · Answer

If object initialize is used is it no need for public constructor?

Maha · Answer

Thank you very much for your explanation.

Vulpes · Answer

The expression on the right hand side of the equals sign: Employee emp = new Employee() { Name = Steve Salary = 10000 }; is what's known as an object initializer and was introduced in C# 3.0. Where a parameterless constructor is being used the () can be omitted: Employee emp = new Employee { Name = Steve Salary = 10000 }; It's shorthand for this: Employee emp = new Employee(); emp.Name = Steve; emp.Salary = 10000; So the idea is that you create a new Employee object and initialize its Name and Salary properties in the same expression. There's not much point in using this syntax if the Employee class actually has a constructor which takes two parameters to initialize the Name and Salary properties as it's shorter to write this: Employee emp = new Employee(Steve 10000); If a class has a lot of different constructors some developers may still prefer to use an object initializer in the interests of clarity though (in C# 4.0) you can also used 'named parameters' for the same purpose : Employee emp = new Employee(Name : Steve Salary : 10000);