Understanding of How to Create a Fluent Interface

Question

Hi i'm trying to understand how i could build a readable and also error preventing Fluent-API without to much restriction for the User

to hold it simple let's say we want to change the following class to be fluent

public class Car
{
    public int Gallons { get; private set; }
    public int Tons { get; private set; }
    public int Bhp { get; private set; }
    public string Make { get; private set; }
    public string Model { get; private set; }

    public Car(string make, string model)
    {
        Make = make;
        Model = model;
    }

    public void WithHorsePower(int bhp)
    {
        Bhp = bhp;
        return this;
    }

    public void WithFuel(int gallons)
    {
        Gallons = gallons;
    }

    public void WithWeight(int tons)
    {
        Tons = tons;
    }

    public int Mpg()
    {
        return Gallons/Tons;
    }
}

the problem in this case the user should only be able to access Mpg() if Weight() and Fuel() got called first also the position of HorsePower() is irrelevant.

Samples:

int mpg =Car.Create().HorsePower().Fuel().Weight().Mpg();
int mpg =Car.Create().Fuel().HorsePower().Weight().Mpg();
int mpg =Car.Create().HorsePower().Fuel().HorsePower().Weight().Mpg();// <- no typo
int mpg =Car.Create().Fuel().Weight().HorsePower().Mpg();
int mpg =Car.Create().Weight().HorsePower().Fuel().Mpg();
int mpg =Car.Create().Weight().Fuel().Mpg();

Is there a easy way to do this without a big bunch of interfaces?
I also doesn't how to implement this nested interfaces in the right way

Here are the interfaces i currently created

interface Start
{
    IFuelWeight1 HorsePower();

    IHorsePowerWeight1 Fuel();

    IHorsePowerFuel1 Weight();
}

interface IFuelWeight1 // Start.HorsePower()
{
    IHorsePowerWeight1 Fuel();

    IHorsePowerFuel1 Weight();
}

interface IHorsePowerWeight1 // Start.Fuel()
{
    IHorsePowerWeight1 HorsePower();

    IHorsePowerFuelMpg Weight();
}

interface IHorsePowerFuel1 // Start.Weight()
{
    IHorsePowerFuel1 HorsePower();

    IHorsePowerWeightMpg Fuel();
}

#region End

interface IHorsePowerFuelMpg
{
    IFuelWeightMpg HorsePower();

    IHorsePowerWeightMpg Fuel();

    int Mpg();
}

interface IHorsePowerWeightMpg
{
    IFuelWeightMpg HorsePower();

    IHorsePowerFuelMpg Weight();

    int Mpg();
}

interface IFuelWeightMpg
{
    IHorsePowerWeightMpg Fuel();

    IHorsePowerFuelMpg Weight();

    int Mpg();
}

#endregion

---

EDIT for Adam Houldsworth :-)

• Is the Interface above a good one or is there a easier way to do this but hold the restriction for Mpg()?

• How to Implement the interface above to do this?:

      var k = myMiracle as Start;
      k.Fuel().Weight();
      k.Weight().Fuel();
      k.HorsePower().Fuel().Weight();
      k.HorsePower().Weight().Fuel();
      k.Fuel().HorsePower().Weight();
      k.Weight().HorsePower().Fuel();
  

Answer 1

One alternative could be to invoke all operations on Mpg() which will allow the other operations to be conditional.

This is already answered in SO with a code sample. Please refer to Conditional Builder Method Chaining Fluent Interface

The post indicates, instead of Interfaces, the same might be achieved using constructors, with a calling method that makes all others operations conditional.