expression visitor example

ModifyExpression.cs

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

namespace ModifyExpression
{
    class Program
    {

        private static DateBox dtpDateStart;
        private static DateBox dtpDateEnd;
        private static TextBox txtDescription;

        static void Main(string[] args)
        {
            dtpDateStart = new DateBox { Value = new DateBox.HasDate { Date = DateTime.Now } };
            dtpDateEnd = new DateBox { Value = new DateBox.HasDate { Date = DateTime.Now.AddDays(2) } };
            txtDescription = new TextBox() { Text = "Hello World" };

            var filter = Filter();

            Console.WriteLine(filter);
            Console.ReadLine();
        }

        private static Expression<Func<Entity.Modelos.Flux, bool>> Filter()
        {
            var dateStart = dtpDateStart.Value.Date;
            var dateEnd = dtpDateEnd.Value.Date;
            var description = txtDescription.Text;

            Expression<Func<Entity.Modelos.Flux, bool>> expr = null;

            expr = f => f.DatFlux >= dateStart.Date && f.DatFlux <= dateEnd.Date;

            if (description != String.Empty)
            {
                //add filter
                Expression<Func<Entity.Modelos.Flux, bool>> other = f => f.Description == description;

                var modifier = new MyVisitor();
                expr = (Expression<Func<Entity.Modelos.Flux, bool>>)modifier.Modify((Expression)expr, (LambdaExpression)other);
            }

            return expr;
        }

    }

    public class MyVisitor : ExpressionVisitor
    {
        private LambdaExpression visitor;
        public Expression Modify(Expression expression, LambdaExpression visitor)
        {
            this.visitor = visitor;
            return Visit(expression);
        }

        protected override Expression VisitBinary(BinaryExpression b)
        {
            var binary = visitor.Body as BinaryExpression;

            return Expression.MakeBinary(ExpressionType.AndAlso, b, binary, b.IsLiftedToNull, b.Method);
        }
    }

    public class TextBox
    {
        public string Text { get; set; }
    }

    public class DateBox
    {
        public HasDate Value;

        public class HasDate
        {
            public DateTime Date { get; set; }
        }
    }
}

namespace Entity.Modelos
{
    public class Flux
    {
        public DateTime DatFlux { get; set; }

        public string Description { get; set; }
    }
}