IComparable

14y2kVisual Basic .NET

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
{
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 list = new List();
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();
}
}

13 Replies

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14y ago
Thank you very much for your explanation.
Vulpes14y ago
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();
}
}
/*
500000,Bill
10000,Andrew
10000,Janet
10000,Steve
8000,Lucy
*/
14y ago
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();
}
}
/*
500000,Bill
10000,Andrew
10000,Janet
10000,Steve
8000,Lucy
*/
Vulpes14y ago
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.
14y ago
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.
Vulpes14y ago
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.
14y ago
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.
Vulpes14y ago
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.
14y ago
But program in the above website is executing without public constructor.
Vulpes14y ago
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.
14y ago
If object initialize is used, is it no need for public constructor?
14y ago
Thank you very much for your explanation.
Vulpes14y ago
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);