In Collection class, when we are going to use simple Sort() function in a List, the base types all provide default comparison algorithms, such as string that provides the alphabetic comparisions and int that provides integer comparisions.
However, when we create our own class. If we want to sort the certain variable in the object list, the class need to implement the IComparable Interface.
If the elements inside the list are strings, it will sort by the first letter of the string:
Code:
List<string> strList = new List<string>() { "Tom", "Jerry", "Joab", "Edi", "Kane", "Hans" };
strList.Sort();
strList.ForEach(Console.WriteLine);
Output:
Edi
Hans
Jerry
Joab
Kane
Tom
If the elements inside the list are integers, it will sort by the integer from small to big:
Code:
List<int> nums = new List<int>() { 2, 33, 22, 1, 5, 8 };
nums.Sort();
nums.ForEach(Console.WriteLine);
Output:
1
2
5
8
22
33
However, if you create a Human class, and declare a variable called Name, you want to sort the Human by name, it will give you an error after you using Sort() function, if you didn’t implement the IComparable Interface in the Human class.
Code:
class Human
{
public string Name { get; set; }
public int Age { get; set; }
public Human(string name, int age)
{
Name = name;
Age = age;
}
}
--------------------
public static void Main(string[] args)
{
List<Human> human = new List<Human>()
{
new Human("Tom",22),
new Human("Jerry",12),
new Human("Joab",18),
new Human("Kyroll",11),
new Human("Cheng Gu",19)
};
human.Sort();
human.ForEach(Console.WriteLine);
}
Output:
System.Collections.Generic.ArraySortHelper<Human>.Sort(Human[] keys, int index, int length, System.Collections.Generic.ObjectComparer<Human> comparer) in /Users/builder/jenkins/workspace/build-package-osx-mono/2018-02/external/bockbuild/builds/mono-x64/external/corert/src/System.Private.CoreLib/src/System/Collections/Generic/ArraySortHelper.cs:66
So Only if you implement the IComparable Interface
So, lets sort Human by name Code:
class Human : IComparable<Human>
{
public string Name { get; set; }
public int Age { get; set; }
public Human(string name, int age)
{
Name = name;
Age = age;
}
public int CompareTo(Human other)
{
if (Age > other.Age) { return 1; }
else if (Age < other.Age) { return -1; }
else { return 0; }
/*
* Below is the default CompareTo() which does the same thing as above,
* used here to better illustrate how the comparator works.
*/
// return Age.CompareTo(other.Age);
}
public override string ToString()
{
return Name + " " + Age;
}
}
----------------
public static void Main(string[] args)
{
List<Human> human = new List<Human>()
{
new Human("Tom",22),
new Human("Jerry",12),
new Human("Joab",18),
new Human("Kyroll",11),
new Human("Cheng Gu",19)
};
human.Sort();
human.ForEach(Console.WriteLine);
}
Output:
Kyroll 11
Jerry 12
Joab 18
Cheng Gu 19
Tom 22
Now, The IComparable Interface solved the sorting error. There is another question pop up. What if you don’t want to Sort by age, you want to sort by human’s name, so you can use IComparer to implement a custom comparator.
Code:
class Human : IComparable<Human>
{
public string Name { get; set; }
public int Age { get; set; }
public Human(string name, int age)
{
Name = name;
Age = age;
}
public int CompareTo(Human other)
{
if (Age > other.Age) { return 1; }
else if (Age < other.Age) { return -1; }
else { return 0; }
/*
* Below is the default CompareTo() which does the same thing as above,
* used here to better illustrate how the comparator works.
*/
// return Age.CompareTo(other.Age);
}
public override string ToString()
{
return Name + " " + Age;
}
}
----------
class HumanNameComparer : IComparer<Human>
{
public int Compare(Human x, Human y)
{
return x.Name.CompareTo(y.Name);
}
}
----------
class MainClass
{
public static void Main(string[] args)
{
List<Human> human = new List<Human>()
{
new Human("Tom",22),
new Human("Jerry",12),
new Human("Joab",18),
new Human("Kyroll",11),
new Human("Cheng Gu",19)
};
human.Sort(new HumanNameComparer());
human.ForEach(Console.WriteLine);
Output:
Cheng Gu 19
Jerry 12
Joab 18
Kyroll 11
Tom 22
End –Cheng Gu