Expression Tree, C#

Expression Tree

//C# 'Expression Tree'
//Rextester.Program.Main is the entry point for your code. Don't change it.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            string[] companies = { "Consolidated Messenger", "Alpine Ski House", "Southridge Video", "City Power & Light",
                               "Coho Winery", "Wide World Importers", "Graphic Design Institute", "Adventure Works",
                               "Humongous Insurance", "Woodgrove Bank", "Margie's Travel", "Northwind Traders",
                               "Blue Yonder Airlines", "Trey Research", "The Phone Company",
                               "Wingtip Toys", "Lucerne Publishing", "Fourth Coffee" };

// The IQueryable data to query.
            IQueryable<String> queryableData = companies.AsQueryable<string>();

// Compose the expression tree that represents the parameter to the predicate.
            ParameterExpression pe = Expression.Parameter(typeof(string), "company");

// ***** Where(company => (company.ToLower() == "coho winery" || company.Length > 16)) *****
            // Create an expression tree that represents the expression 'company.ToLower() == "coho winery"'.
            Expression left = Expression.Call(pe, typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
            Expression right = Expression.Constant("coho winery");
            Expression e1 = Expression.Equal(left, right);

// Create an expression tree that represents the expression 'company.Length > 16'.
            left = Expression.Property(pe, typeof(string).GetProperty("Length"));
            right = Expression.Constant(16, typeof(int));
            Expression e2 = Expression.GreaterThan(left, right);

// Combine the expression trees to create an expression tree that represents the 
            // expression '(company.ToLower() == "coho winery" || company.Length > 16)'.
            Expression predicateBody = Expression.OrElse(e1, e2);

// Create an expression tree that represents the expression 
            // 'queryableData.Where(company => (company.ToLower() == "coho winery" || company.Length > 16))'
            MethodCallExpression whereCallExpression = Expression.Call(
                typeof(Queryable),
                "Where",
                new Type[] { queryableData.ElementType },
                queryableData.Expression,
                Expression.Lambda<Func<string, bool>>(predicateBody, new ParameterExpression[] { pe }));
            // ***** End Where *****

// ***** OrderBy(company => company) ***** 
            // Create an expression tree that represents the expression 
            // 'whereCallExpression.OrderBy(company => company)'
            MethodCallExpression orderByCallExpression = Expression.Call(
                typeof(Queryable),
                "OrderBy",
                new Type[] { queryableData.ElementType, queryableData.ElementType },
                whereCallExpression,
                Expression.Lambda<Func<string, string>>(pe, new ParameterExpression[] { pe }));
            // ***** End OrderBy *****

// Create an executable query from the expression tree.
            IQueryable<string> results = queryableData.Provider.CreateQuery<string>(orderByCallExpression);

// Enumerate the results. 
            foreach (string company in results)
                Console.WriteLine(company);

/*  This code produces the following output:

Blue Yonder Airlines
                City Power & Light
                Coho Winery
                Consolidated Messenger
                Graphic Design Institute
                Humongous Insurance
                Lucerne Publishing
                Northwind Traders
                The Phone Company
                Wide World Importers
            */
                
        }
    }
}


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