Demonstrates the use of parameter customization attributes

AutoFixtureParameterCustomization.cs

using AutoFixture.Dsl;
using AutoFixture.Kernel;
using AutoFixture.Xunit2;
using System;
using System.Reflection;
using Xunit;

namespace AutoFixture.Issue1302
{
    public class ParameterDecoratorTests
    {
        [Theory, AutoData]
        public void CustomizesDecoratedOrder([BlackFridayOrder(20d)] Order order, Order order1)
        {
            Assert.Equal(0, order.Tax);
            Assert.Equal(20, order.Discount);

            Assert.NotEqual(0, order1.Tax);
            Assert.NotEqual(20, order1.Discount);
        }
    }

    public class Order
    {
        public decimal Tax { get; set; }
        public decimal Discount { get; set; }
    }

    [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false)]
    public class BlackFridayOrderAttribute : Attribute, IParameterCustomizationSource
    {
        public BlackFridayOrderAttribute(double discount)
        {
            Discount = (decimal)discount;
        }

        public decimal Discount { get; }

        public ICustomization GetCustomization(ParameterInfo parameter)
        {
            var builder = Customize<Order>(x => x
                            .With(x => x.Discount, this.Discount)
                            .With(x => x.Tax, 0));

            return new ParameterRelay(
                new ParameterSpecification(
                    new ExactParameterSpecification(parameter)),
                builder)
            .ToCustomization();
        }

        public IPostprocessComposer<T> Customize<T>(Func<IPostprocessComposer<T>, IPostprocessComposer<T>> func)
        {
            return func(SpecimenBuilderNodeFactory.CreateComposer<T>().WithAutoProperties(true));
        }
    }

    public class ExactParameterSpecification : IEquatable<ParameterInfo>
    {
        private readonly ParameterInfo parameterInfo;

        public ExactParameterSpecification(ParameterInfo parameterInfo)
        {
            this.parameterInfo = parameterInfo;
        }

        public bool Equals(ParameterInfo other)
        {
            return this.parameterInfo?.Equals(other) == true;
        }
    }

    public class ParameterRelay : ISpecimenBuilder
    {
        private readonly IRequestSpecification specification;
        private readonly ISpecimenBuilder builder;

        public ParameterRelay(IRequestSpecification specification, ISpecimenBuilder builder)
        {
            this.specification = specification;
            this.builder = builder;
        }

        public object Create(object request, ISpecimenContext context)
        {
            if (!this.specification.IsSatisfiedBy(request))
            {
                return new NoSpecimen();
            }

            if (request is not ParameterInfo parameter)
                return new NoSpecimen();

            return this.builder.Create(parameter.ParameterType, context);
        }
    }
}