Fluent Interface: Simple Concept To Code Legibility

A fluent interface is a method for designing object-oriented APIs that rely extensively on method chaining. Its goal is to increase code legibility by creating a domain-specific language. It was coined in 2005 by Eric Evans and Martin Fowler.

A fluent interface is normally implemented by using method chaining to implement method cascading (in languages that do not natively support cascading), concretely by having each method return this (self). Stated more abstractly, a fluent interface relays the instruction context of a subsequent call in method chaining, where generally the context is

• Defined through the return value of a called method
• Self-referential, where the new context is equivalent to the last context
• Terminated through the return of a void context

Note that a "fluent interface" means more than just method cascading via chaining; it entails designing an interface that reads like a DSL, using other techniques like "nested functions and objects scoping"

Examples(C#):

// Defines the data context
class Context
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Sex { get; set; }
    public string Address { get; set; }
}

class Customer
{
    private Context _context = new Context(); // Initializes the context

    // set the value for properties
    public Customer FirstName(string firstName)
    {
        _context.FirstName = firstName;
        return this;
    }

    public Customer LastName(string lastName)
    {
        _context.LastName = lastName;
        return this;
    }

    public Customer Sex(string sex)
    {
        _context.Sex = sex;
        return this;
    }

    public Customer Address(string address)
    {
        _context.Address = address;
        return this;
    }

    // Prints the data to console
    public void Print()
    {
        Console.WriteLine("First name: {0} \nLast name: {1} \nSex: {2} \nAddress: {3}", _context.FirstName, _context.LastName, _context.Sex, _context.Address);
    }
}

class Program
{
    static void Main(string[] args)
    {
        // Object creation
        Customer c1 = new Customer();
        // Using the method chaining to assign & print data with a single line
        c1.FirstName("vinod").LastName("srivastav").Sex("male").Address("bangalore").Print();
    }
}