C# 2008 Programmer's Reference

FirstName property to the parameter's FirstName property. Here, you use the CompareTo() method of the String class (FirstName is of String type) to perform the comparison. 

The return value of the CompareTo(obj) method has the possible values as shown in the following table.

Value	Meaning
Less than zero	The current instance is less than obj.
Zero	The current instance is equal to obj.
Greater than zero	The current instance is greater than obj.

Now, when you sort the List object containing Employee objects, the Employee objects will be sorted by first name:

employees.Sort();
foreach (Employee emp in employees)
 Console.WriteLine(emp.ToString());

These statements produce the following output:

Brian, Will $3000
Howard, Mark $1500
John, Smith $4000
Margaret, Anderson $3000

To sort the Employee objects using the LastName instead of FirstName, simply change the CompareTo() method as follows:

public int CompareTo(Employee emp) {
 return this.LastName.CompareTo(emp.LastName);
}

The output becomes:

Margaret, Anderson $3000
Howard, Mark $1500
John, Smith $4000
Brian, Will $3000

Likewise, to sort by salary, you compare the Salary property:

public int CompareTo(Employee emp) {
 return this.Salary.CompareTo(emp.Salary);
}

The output is now:

Howard, Mark $1500
Margaret, Anderson $3000
Brian, Will $3000
John, Smith $4000

Instead of using the CompareTo() method of the type you are comparing, you can manually perform the comparison, like this:

public int CompareTo(Employee emp) {
 if (this.Salary < emp.Salary) return -1;
 else if (this.Salary == emp.Salary) return 0;
 else return 1;
}

How the Employee objects are sorted is fixed by the implementation of the CompareTo() method. If CompareTo() compares using the FirstName property, the sort is based on the FirstName property. To give users a choice of which field they want to use to sort the objects, you can use the IComparer<T> interface.

To do so, first declare a private class within the Employee class and call it SalaryComparer.

public class Employee : IComparable<Employee> {
 private class SalaryComparer : IComparer<Employee> {
  public int Compare(Employee e1, Employee e2) {
   if (e1.Salary < e2.Salary) return -1;
   else if (e1.Salary == e2.Salary) return 0;
   else return 1;
  }
 }
 public string FirstName { get; set; }
 public string LastName { get; set; }
 public int Salary { get; set; }
 public override string ToString() {
  return FirstName + ', ' + LastName + ' $' + Salary;
 }
 public int CompareTo(Employee emp) {
  return this.FirstName.CompareTo(emp.FirstName);
 }
}

The SalaryComparer class implements the IComparer<T> interface. IComparer<S> has one method — Compare() — that you need to implement. It compares the salary of two Employee objects.

To use the SalaryComparer class, declare the SalarySorter static property within the Employee class so that you can return an instance of the SalaryComparer class:

public class Employee : IComparable<Employee> {
 private class SalaryComparer : IComparer<Employee> {
  public int Compare(Employee e1, Employee e2) {
   if (e1.Salary < e2.Salary) return -1;
   else if (e1.Salary == e2.Salary) return 0;
   else return 1;
  }
 }