Fluent interfaces and inheritance in C#

Question

I'll show a problem by example. There is a base class with fluent interface:

class FluentPerson {     private string _FirstName = String.Empty;     private string _LastName = String.Empty;      public FluentPerson WithFirstName(string firstName)     {         _FirstName = firstName;         return this;     }      public FluentPerson WithLastName(string lastName)     {         _LastName = lastName;         return this;     }      public override string ToString()     {         return String.Format("First name: {0} last name: {1}", _FirstName, _LastName);     } } 

and a child class:

class FluentCustomer : FluentPerson {     private long _Id;      private string _AccountNumber = String.Empty;      public FluentCustomer WithAccountNumber(string accountNumber)     {         _AccountNumber = accountNumber;         return this;     }     public FluentCustomer WithId(long id)     {         _Id = id;         return this;     }      public override string ToString()     {         return base.ToString() + String.Format(" account number: {0} id: {1}", _AccountNumber, _Id);     } } 

The problem is that when you call customer.WithAccountNumber("000").WithFirstName("John").WithLastName("Smith") you can't add .WithId(123) in the end because return type of the WithLastName() method is FluentPerson (not FluentCustomer).

How this problem usually solved?

Answer 1

Try to use some Extension methods.

static class FluentManager {     public static T WithFirstName<T>(this T person, string firstName) where T : FluentPerson     {         person.FirstName = firstName;         return person;     }      public static T WithId<T>(this T customer, long id) where T : FluentCustomer     {         customer.ID = id;         return customer;     } }  class FluentPerson {     public string FirstName { private get; set; }     public string LastName { private get; set; }      public override string ToString()     {         return string.Format("First name: {0} last name: {1}", FirstName, LastName);     } }  class FluentCustomer : FluentPerson {     public long ID { private get; set; }     public long AccountNumber { private get; set; }      public override string ToString()     {         return base.ToString() + string.Format(" account number: {0} id: {1}", AccountNumber, ID);     } } 

after you can use like

new FluentCustomer().WithId(22).WithFirstName("dfd").WithId(32); 

Answer 2

You can use generics to achieve that.

public class FluentPerson<T>     where T : FluentPerson<T> {     public T WithFirstName(string firstName)     {         // ...         return (T)this;     }      public T WithLastName(string lastName)     {         // ...         return (T)this;     } }  public class FluentCustomer : FluentPerson<FluentCustomer> {     public FluentCustomer WithAccountNumber(string accountNumber)     {         // ...         return this;     } } 

And now:

var customer = new FluentCustomer()   .WithAccountNumber("123")   .WithFirstName("Abc")   .WithLastName("Def")   .ToString();