Autofixture Custom SpecimenBuilder Example

AutoFixtureLearningTests.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using Ploeh.AutoFixture;
using Ploeh.AutoFixture.Kernel;
using Xunit;

namespace AutoFixtureSandbox.Tests
{
    public class AutoFixtureLearningTests
    {
        [Fact]
        public void CanCreateCustomRandomCarModel()
        {
            var fixture = new Fixture();
            fixture.Customizations.Add(new CarSpecimenBuilder());


            var car = fixture.Create<Car>();


            Assert.True(car.Model == "M3" || car.Model == "V70" || car.Model == "R8");
        }

    }

    public class Car
    {
        public Car(string model, string make, int productionYear)
        {
            if (model.Length >= 5)
                throw new ArgumentException();

            Model = model;
            Make = make;
            ProductionYear = productionYear;
        }


        public string Model { get; }

        public string Make { get; }

        public int ProductionYear { get; }
    }

    public class CarSpecimenBuilder : ISpecimenBuilder
    {
        private static Random randomizer = new Random();
        private List<string> carModels = new List<string> {"M3", "V70", "R8"};

        public object Create(object request, ISpecimenContext context)
        {
            var type = request as Type;
            var propInfo = type?.GetProperty("Model");

            if (!(propInfo is PropertyInfo))
                return new NoSpecimen();

            if (IsCarModelProperty(propInfo))
                return new Car(RandomCarModel(), context.Create<string>(), context.Create<int>());

            return new NoSpecimen();

        }

        private static bool IsCarModelProperty(PropertyInfo propertyInfo) 
            => propertyInfo.Name.Contains("Model") && propertyInfo.PropertyType == typeof(string);

        private string RandomCarModel()
        {
            var index = randomizer.Next(0, 2);
            return carModels.ElementAt(index);
        }
    }
}