Sorting Lists using IComparable and IComparer Interface in .NET

Introduction

This article explains how to sort Lists by using two interfaces IComparable<> and IComparer<> provided by the .NET Framework.

Background

It's easy to sort a list of strings or integers by just calling the List.Sort() method, but how can we sort two objects and based on what field?

Let's look at a small example and see how we can solve the problem using IComparable<> and IComparer interfaces.

Example

Let's create a simple Employee class with two fields, Name and Salary.

class Employee 
{
     public string Name { get; set; }
     public int Salary { get; set; }
}

Now create a List of Employees and call Sort() method of a List.

// Use Collection Initializers( C# 3.0 ) to initialize the List             
List<Employee> empList = new List<Employee>() 
	{ new Employee { Name = "a", Salary = 14000 },
           new Employee { Name = "b", Salary = 13000 } 
         };
empList.Sort(); 

Oops! we got an exception!

The Exception says we need to implement IComparable<> interface,
List.Sort() sorts any class that implements IComparable<> Interface which has one method called CompareTo().

Let's implement IComparable<> interface to our Employee class:

class Employee : IComparable<Employee>
{
     public string Name { get; set; }
     public int Salary { get; set; }
        
     #region IComparable<Employee> Members
       
     public int CompareTo( Employee other )
     {
         if ( this.Salary < other.Salary ) return 1;
         else if ( this.Salary > other.Salary ) return -1;
         else return 0;
     }
        
     #endregion
}

In the above code, we are sorting objects based on salary of employee in descending order, by implementing CompareTo() method of IComparable interface which takes Employee reference as a parameter.
Now, calling empList.Sort() gives no exception and empList is well sorted by salary.

But sometimes, we may need to sort a list of objects when class does not implement IComparable<> interface and also we may need various kinds of sorting on that class like:

1. Sort Employees by Salary in Ascending Order
2. Sort Employees by Salary in Descending Order
3. Sort Employees by Name

To solve this problem, .NET provides a special interface called IComparer<> which has a method Compare(), takes two object parameters X, Y and returns an int.
Use of IComparer<> interface tells List how exactly you want to sort.

class Employee_SortBySalaryByAscendingOrder : IComparer<Employee>
{
    #region IComparer<Employee> Members
    
    public int Compare( Employee x, Employee y )
    {
        if ( x.Salary > y.Salary ) return 1;
        else if ( x.Salary < y.Salary ) return -1;
        else return 0;
    }
    
    #endregion
}

class Employee_SortBySalaryByDescendingOrder : IComparer<Employee>
{
    #region IComparer<Employee> Members
    
    public int Compare( Employee x, Employee y )
    {
        if ( x.Salary < y.Salary ) return 1;
        else if ( x.Salary > y.Salary ) return -1;
        else return 0;
    }
    
    #endregion
}

class Employee_SortByName : IComparer<Employee>
{
    #region IComparer<Employee> Members
    
    public int Compare( Employee x, Employee y )
    {
        return string.Compare( x.Name, y.Name );
    }
    
    #endregion
}

The above code introduces three classes by implementing Compare() method of IComparer interface.

Now, how do we use this? We just have to pass the reference of these classes as a object parameter to Sort() method as shown below:

// Use Collection Initializers( C# 3.0 ) to initialize the List 
List<Employee> empList = new List<Employee>() 
		{ new Employee { Name = "a", Salary = 14000 },
                    new Employee { Name = "b", Salary = 13000 } 
                  };
                                     
Employee_SortBySalaryByAscendingOrder eAsc = 
		new Employee_SortBySalaryByAscendingOrder(); 
// Sort Employees by salary by ascending order.   
empList.Sort( eAsc );
    
Employee_SortBySalaryByDescendingOrder eDsc = 
		new Employee_SortBySalaryByDescendingOrder();
// Sort Employees by salary by descending order. 
empList.Sort( eDsc );
    
Employee_SortByName eName = new Employee_SortByName();
// Sort Employees by their names.                                 
empList.Sort( eName );

Conclusion

Sorting Lists is simple as long as you sort basic elements like strings and integers for which comparison classes are defined.

Usage of IComparable<> and IComparer<> interface helps to sort Lists of objects on custom classes easily.

History

• v1.0 - Initial post