Interfaces

An interface defines a contract with no fields of its own — a class can implement any number of them, exactly like Java.

interface IPayable {
    double CalculatePay();
}

interface INamed {
    string GetName();
}

class Employee : IPayable, INamed {
    private string _name;
    private double _hourlyRate;
    private int _hoursWorked;

    public Employee(string name, double hourlyRate, int hoursWorked) {
        _name = name;
        _hourlyRate = hourlyRate;
        _hoursWorked = hoursWorked;
    }

    public double CalculatePay() => _hourlyRate * _hoursWorked;
    public string GetName() => _name;
}

C# convention prefixes interface names with I (IPayable, IDisposable) — a naming convention, not a language requirement, but followed almost universally in real C# code.