dotnet-dynamic-linq/PoweredSoft.DynamicLinq.ConsoleApp/BetterProto2.cs

using PoweredSoft.DynamicLinq.Dal.Pocos;
using PoweredSoft.DynamicLinq.Helpers;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Threading.Tasks;

namespace PoweredSoft.DynamicLinq.ConsoleApp
{
    public class BetterProto2
    {
        /*
         *  (parent, innerExpression, innerExpressionLambda) =>
                {
                    var listGenericArgumentType = parent.Type.GetGenericArguments().First();
                    Expression ret = null;
                    if (selectCollectionHandling == SelectCollectionHandling.LeaveAsIs || !QueryableHelpers.IsGenericEnumerable(innerExpression))
                        ret = Expression.Call(typeof(Enumerable), "Select", new Type[] { listGenericArgumentType, innerExpression.Type }, parent, innerExpressionLambda);
                    else
                        ret = Expression.Call(typeof(Enumerable), "SelectMany", new Type[] { listGenericArgumentType, innerExpression.Type.GenericTypeArguments.First() }, parent, innerExpressionLambda);

                    return ret;
                }
         */
        public static void Run()
        {
            //Case1();
            Case2();
        }

        public static void Case1()
        {
            // the expression parser.
            var ep = new ExpressionParser(typeof(Author), "Posts.Comments.Id");

            // the builder.
            var per = new PathExpressionResolver(ep);
            per.Resolve();

            // the result expression.
            var result = per.Result;
        }

        public static void Case2()
        {
            // the expression parser.
            var ep = new ExpressionParser(typeof(Post), "Author.Posts.Author.FirstName");

            /*
            var esl = new List<string>();
            new List<Post>().AsQueryable().Select(t => new 
            {
                FirstNames = (t.Author == null ? esl : (t.Author.Posts == null ? esl : t.Author.Posts.Select(t2 => t2.Author == null ? string.Empty : t2.Author.FirstName)))
            });*/

            // the builder.
            var per = new PathExpressionResolver(ep);
            per.Resolve();

            // the result expression.
            var result = per.Result;
        }
    }

    public class ExpressionParserPiece
    {
        public Type Type { get; set; }
        public bool IsGenericEnumerable { get; set; }
        public Type EnumerableType { get; set; }
        public ExpressionParserPiece Parent { get; set; }
    }

    public class PathExpressionResolver
    {
        public SelectNullHandling NullChecking { get; set; } = SelectNullHandling.LeaveAsIs;
        public SelectCollectionHandling CollectionHandling { get; set; } = SelectCollectionHandling.LeaveAsIs;
        public ExpressionParser Parser { get; protected set; }
        public Expression Result { get; protected set; }

        protected Expression NullCheckValueExpression { get; set; }

        public PathExpressionResolver(ExpressionParser parser)
        {
            Parser = parser;
        }

        public void Resolve()
        {
            Result = null;

            // parse the expression.
            Parser.Parse();

            // resolve the expression to use during a conditional if null checking is enabled.
            ResolveNullCheckingRightType();

            // reverse foreach
            Parser.Pieces.ReversedForEach((piece, index) =>
            {
                if (piece.Parent == null)
                    return;

                if (piece.Parent.IsGenericEnumerable)
                    HandleCollection(piece, index);
            });
        }

        private void ResolveNullCheckingRightType()
        {
            if (NullChecking == SelectNullHandling.LeaveAsIs)
                return;
            
            // last piece.
            var lastPiece = Parser.Pieces.Last();

            // the subject type
            Type subjectType = null;
            if (lastPiece.IsGenericEnumerable)
                subjectType = typeof(List<>).MakeGenericType(lastPiece.EnumerableType);

            if (NullChecking == SelectNullHandling.Default)
                NullCheckValueExpression = Expression.Default(subjectType);
            else
                NullCheckValueExpression = Expression.New(subjectType);
        }

        private void HandleCollection(ExpressionParserPiece piece, int index)
        {
            Parser.Pieces.

            /*
            if (Result == null)
                HandleNoPreviousResultCollection(piece, index);
            else
                HandlePreviousResultCollection(piece, index);*/
        }

        /*
        public void HandleNoPreviousResultCollection(ExpressionParserPiece piece, int index)
        {
            MethodCallExpression result = null;

            // create the lambda.
            var lambda = ResolveLambda(piece);

            if (CollectionHandling == SelectCollectionHandling.LeaveAsIs || !piece.IsGenericEnumerable)
                result = Expression.Call(typeof(Enumerable),
                    "Select",
                    new Type[] { piece.Parent.EnumerableType, piece.MemberExpression.Type },
                    piece.Parent.MemberExpression, lambda);
            else
                result = Expression.Call(typeof(Enumerable),
                    "SelectMany",
                    new Type[] { piece.Parent.EnumerableType, piece.EnumerableType },
                    piece.Parent.MemberExpression, lambda);

            Result = result;
        }

        public void HandlePreviousResultCollection(ExpressionParserPiece piece, int index)
        {
            var pieceParameter = ResolvePieceParameter(piece);

            MethodCallExpression result = null;
            var lambda = Expression.Lambda(Result, ResolvePieceParameter(piece));
            if (CollectionHandling == SelectCollectionHandling.LeaveAsIs)
                result = Expression.Call(typeof(Enumerable), "Select", 
                    new Type[] { piece.Parent.EnumerableType, Result.Type }, 
                    piece.Parent.MemberExpression, lambda);
            else
                result = Expression.Call(typeof(Enumerable),
                    "SelectMany",
                    new Type[] { piece.Parent.EnumerableType, Result.Type.GenericTypeArguments.First() },
                    piece.Parent.MemberExpression, lambda);

            Result = result;
        }*/
    }

    public class ExpressionParser
    {
        public ParameterExpression Parameter { get; protected set; }
        public string Path { get; set; }
        public List<ExpressionParserPiece> Pieces { get; set; } = new List<ExpressionParserPiece>();

        public ExpressionParser(Type type, string path) : this(Expression.Parameter(type), path)
        {
            
        }

        public ExpressionParser(ParameterExpression parameter, string path)
        {
            Parameter = parameter;
            Path = path;
        }

        public void Parse()
        {
            Pieces.Clear();

            var pathPieces = Path.Split('.').ToList();
            var param = Parameter;
            ExpressionParserPiece parent = null;

            pathPieces.ForEach(pp =>
            {
                var memberExpression = Expression.PropertyOrField(param, pp);
                var current = new ExpressionParserPiece
                {
                    Type = memberExpression.Type,
                    IsGenericEnumerable = QueryableHelpers.IsGenericEnumerable(memberExpression),
                    EnumerableType = memberExpression.Type.GenericTypeArguments.FirstOrDefault(),
                    Parent = parent
                };

                Pieces.Add(current);

                // for next iteration.
                param = Expression.Parameter(current.IsGenericEnumerable ? current.EnumerableType : current.Type);
                parent = current;
            });
        }
    }
}