using System;

using System.Linq;

using System.Collections.Generic;

using System.Linq.Expressions;

using System.Reflection;

​

public class User

    {

        public int ID {get;set;}

    }

    public class Absence

    {

        public int ID {get;set;}

        public User Author {get;set;}

    }

​

public class Program

{

    public static void Main()

    {

        var user1 = new User { ID = 1 };

        var user2 = new User { ID = 2 };

        int currentUserID = 2;

        var absences = (new List<Absence>() { new Absence { ID = 100, Author = user2 }, new Absence { ID = 200, Author = user1 } }).AsQueryable();

        Expression<Func<Absence, bool>> a = (x) => x.Author.ID == currentUserID;

        ParameterExpression pe = Expression.Parameter(typeof(Absence), "x");

        Expression accessAuthor = Expression.Property(pe, typeof(Absence).GetProperty("Author"));

        Expression accessID = Expression.Property(accessAuthor, typeof(User).GetProperty("ID"));

        Expression currentUserIDExpression = Expression.Constant(currentUserID, typeof(int));

        Expression equalExpression = Expression.Equal(accessID, currentUserIDExpression);

        Expression lambda = Expression.Lambda<Func<Absence, bool>>(equalExpression, new ParameterExpression[] { pe });

        Console.WriteLine(a.ToString());

        Console.WriteLine("---------------------");

        Console.WriteLine("accessAuthor:  " + accessAuthor.ToString());

        Console.WriteLine("accessID:  " + accessID.ToString());

        Console.WriteLine("currentUserIDExpression:  " + currentUserIDExpression.ToString());

        Console.WriteLine("equalExpression:  " + equalExpression.ToString());

        Console.WriteLine("lambda:  " + lambda.ToString());

        Console.WriteLine("---------------------");

        var result1 = absences.FirstOrDefault(a);

        Console.WriteLine("result1: " + result1.ID);

        var result2 = absences.FirstOrDefault(lambda as Expression<Func<Absence, bool>>);

        Console.WriteLine("result2: " + result2.ID);

    }

}