C# Multi-Level sorting using generic IComparer

IComparer<T> is commonly used multi-level sorting (also called secondary sorting, compound sorting, or lexicographical sorting).

The idea is:

1. Compare by the primary field
2. If equal (result == 0), compare by the secondary field
3. Continue until a difference is found

Example: Sort Employee by Department → Name → Salary

class Employee
{
    public string Name { get; set; } = "";
    public string Department { get; set; } = "";
    public double Salary { get; set; }
}

class EmployeeComparer : IComparer<Employee>
{
    public int Compare(Employee? x, Employee? y)
    {
        if (x == null && y == null) return 0;
        if (x == null) return -1;
        if (y == null) return 1;

        // Level 1: Department
        int result = string.Compare(
            x.Department,
            y.Department,
            StringComparison.OrdinalIgnoreCase);

        if (result != 0)
            return result;

        // Level 2: Name
        result = string.Compare(
            x.Name,
            y.Name,
            StringComparison.OrdinalIgnoreCase);

        if (result != 0)
            return result;

        // Level 3: Salary
        return x.Salary.CompareTo(y.Salary);
    }
}

class Program
{
    static void Main()
    {
        List<Employee> employees = new List<Employee>
        {
            new Employee { Name = "Alice", Department = "HR", Salary = 50000 },
            new Employee { Name = "Bob", Department = "IT", Salary = 60000 },
            new Employee { Name = "Charlie", Department = "HR", Salary = 55000 },
            new Employee { Name = "David", Department = "IT", Salary = 65000 },
            new Employee { Name = "Eve", Department = "HR", Salary = 50000 }
        };
        employees.Sort(new EmployeeComparer());
        foreach (var employee in employees)
        {
            Console.WriteLine($"{employee.Department} - {employee.Name} - ${employee.Salary}");
        }
    }
}

OUTPUT

HR - Alice - $50000

HR - Charlie - $55000

HR - Eve - $50000

IT - Bob - $60000

IT - David - $65000

Tips. Practically we use LINQ which is often cleaner:

var sorted = employees

    .OrderBy(e => e.Department)

    .ThenBy(e => e.Name)

    .ThenBy(e => e.Salary);