CallingClass.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Engine.Questions.Question1
{
    class CallingClass
    {
        public void MyFunction()
        {
            var report1 =
                ReportGenerator
                .CreateStandardSalesReport(DateTime.UtcNow.AddMonths(-1), DateTime.UtcNow)
                .GroupBy(ReportGenerator.GroupingMethod.Department)
                .SortBy(ReportGenerator.SortMethod.DateAscending)
                .BuildReport();

            var report2 =
                ReportGenerator
                .CreateSalesReport(DateTime.UtcNow.AddMonths(-1), DateTime.UtcNow)
                .IncludeAllSalespeople()
                .IncludeAllCategories()
                .ExcludeReturnedOrders()
                .IncludeUnshippedOrders()
                .GroupBy(ReportGenerator.GroupingMethod.Department)
                .SortBy(ReportGenerator.SortMethod.DateAscending)
                .BuildReport();
        }
    }
}

ReportGenerator.cs

using System;
using System.Collections.Generic;

namespace Engine.Questions.Question1
{
    public class ReportGenerator : ICanSetAllOrOneSalesperson,
        ICanSetAllOrOneCategory, ICanAddSalespersonIDOrSetCategories,
        ICanSetReturnedOrdersInclusion,
        ICanAddCategoryIDOrSetReturnedOrdersInclusion,
        ICanSetUnshippedOrdersInclusion,
        ICanSetGroupBy, ICanSetSortBy, ICanBuildReport
    {
        #region Enums

        public enum GroupingMethod
        {
            None,
            Salesperson,
            Office,
            Department
        }

        public enum SortMethod
        {
            DateAscending,
            DateDescending,
            OrderTotalAscending,
            OrderTotalDescending
        }

        #endregion

        private bool _includeAllSalespeople;
        private List<int> _includedSalespeopleIDs = new List<int>();
        private bool _includeAllCategories;
        private List<int> _includedCategoryIDs = new List<int>();
        private bool _includeReturnedOrders;
        private bool _includeUnshippedOrders;
        private DateTime _from;
        private DateTime _to;
        private GroupingMethod _groupBy;
        private SortMethod _sortBy;

        private ReportGenerator(DateTime from, DateTime to)
        {
            _from = from;
            _to = to;
        }

        // Instantiating functions
        public static ICanSetGroupBy CreateStandardSalesReport(DateTime from, DateTime to)
        {
            var standardReport = new ReportGenerator(from, to);
            standardReport._includeAllSalespeople = true;
            standardReport._includeAllCategories = true;
            standardReport._includeReturnedOrders = false;
            standardReport._includeUnshippedOrders = true;
            return standardReport;
        }

        public static ICanSetAllOrOneSalesperson CreateSalesReport(DateTime from, DateTime to)
        {
            return new ReportGenerator(from, to);
        }

        // Chaining functions
        public ICanSetAllOrOneCategory IncludeAllSalespeople()
        {
            _includeAllSalespeople = true;
            return this;
        }

        public ICanAddSalespersonIDOrSetCategories IncludeSalespersonID(int id)
        {
            _includedSalespeopleIDs.Add(id);
            return this;
        }

        public ICanSetReturnedOrdersInclusion IncludeAllCategories()
        {
            _includeAllCategories = true;
            return this;
        }

        public ICanAddCategoryIDOrSetReturnedOrdersInclusion IncludeCategoryID(int id)
        {
            _includedCategoryIDs.Add(id);
            return this;
        }

        public ICanSetUnshippedOrdersInclusion IncludeReturnedOrders()
        {
            _includeReturnedOrders = true;
            return this;
        }

        public ICanSetUnshippedOrdersInclusion ExcludeReturnedOrders()
        {
            _includeReturnedOrders = false;
            return this;
        }

        public ICanSetGroupBy IncludeUnshippedOrders()
        {
            _includeUnshippedOrders = true;
            return this;
        }

        public ICanSetGroupBy ExcludeUnshippedOrders()
        {
            _includeUnshippedOrders = false;
            return this;
        }

        public ICanSetSortBy GroupBy(GroupingMethod groupBy)
        {
            _groupBy = groupBy;
            return this;
        }

        public ICanBuildReport SortBy(SortMethod sortBy)
        {
            _sortBy = sortBy;
            return this;
        }

        // Ending functions
        public object BuildReport()
        {
            // This would have the report-generating code here.
            return new object();
        }

    }

    public interface ICanSetAllOrOneSalesperson
    {
        ICanSetAllOrOneCategory IncludeAllSalespeople();
        ICanAddSalespersonIDOrSetCategories IncludeSalespersonID(int id);
    }

    public interface ICanSetAllOrOneCategory
    {
        ICanSetReturnedOrdersInclusion IncludeAllCategories();
        ICanAddCategoryIDOrSetReturnedOrdersInclusion IncludeCategoryID(int id);
    }

    public interface ICanAddSalespersonIDOrSetCategories
    {
        ICanAddSalespersonIDOrSetCategories IncludeSalespersonID(int id);
        ICanSetReturnedOrdersInclusion IncludeAllCategories();
        ICanAddCategoryIDOrSetReturnedOrdersInclusion IncludeCategoryID(int id);
    }

    public interface ICanSetReturnedOrdersInclusion
    {
        ICanSetUnshippedOrdersInclusion IncludeReturnedOrders();
        ICanSetUnshippedOrdersInclusion ExcludeReturnedOrders();
    }

    public interface ICanAddCategoryIDOrSetReturnedOrdersInclusion 
    {
        ICanAddCategoryIDOrSetReturnedOrdersInclusion IncludeCategoryID(int id);
        ICanSetUnshippedOrdersInclusion IncludeReturnedOrders();
        ICanSetUnshippedOrdersInclusion ExcludeReturnedOrders();
    }

    public interface ICanSetUnshippedOrdersInclusion
    {
        ICanSetGroupBy IncludeUnshippedOrders();
        ICanSetGroupBy ExcludeUnshippedOrders();
    }

    public interface ICanSetGroupBy
    {
        ICanSetSortBy GroupBy(ReportGenerator.GroupingMethod groupBy);
    }

    public interface ICanSetSortBy
    {
        ICanBuildReport SortBy(ReportGenerator.SortMethod sortBy);
    }

    public interface ICanBuildReport
    {
        object BuildReport();
    }
}