using System;

using System.Collections.Generic;

using System.Linq;

    public class Program

    {

        public static void Main()

        {

            Thingy t = new Thingy();

​

            // Note execution is deferred until enumeration (in this case Count())

            var allData = t.GetData();

            Console.WriteLine("All Data count: {0}", allData.Count());

            // Select only valid records from data set (should be 2)

            var isValid = t.GetData();

            isValid = isValid.Where(w => w.IsValid);

            Console.WriteLine("IsValid count: {0}", isValid.Count());

            // select only records with an ID greater than 1 (should be 2)

            var gt1 = t.GetData();

            gt1 = gt1.Where(w => w.Id > 1);

            Console.WriteLine("gt 1 count: {0}", gt1.Count());

            // Here we're combining in a single statement, IsValid and gt 1 (should be 1)

            var isValidAndIdGt1 = t.GetData();

            isValidAndIdGt1 = isValidAndIdGt1.Where(w => w.IsValid && w.Id > 1);

            Console.WriteLine("IsValid and gt 1 count: {0}", isValidAndIdGt1.Count());

            // This is the same query as the one directly above, just broken up (could perhaps be some if logic in there to determine if to add the second Where

            // Note this is how you're doing it in your question (and it's perfectly valid (should be 1)

            var isValidAndIdGt1Appended = t.GetData();

            isValidAndIdGt1Appended = isValidAndIdGt1Appended.Where(w => w.IsValid);

            isValidAndIdGt1Appended = isValidAndIdGt1Appended.Where(w => w.Id > 1);

            Console.WriteLine("IsValid and gt 1 count w/ appended where: {0}", isValidAndIdGt1Appended.Count());

​

            // This is the same query as the one directly above, but note we are executing the query twice

            var isValidAndIdGt1AppendedTwice = t.GetData();

            isValidAndIdGt1AppendedTwice = isValidAndIdGt1AppendedTwice.Where(w => w.IsValid);

            Console.WriteLine("IsValid and gt 1 count w/ appended where executing twice: {0}", isValidAndIdGt1AppendedTwice.Count()); // 2 results are valid

            isValidAndIdGt1AppendedTwice = isValidAndIdGt1AppendedTwice.Where(w => w.Id > 1);

            Console.WriteLine("IsValid and gt 1 count w/ appended where executing twice: {0}", isValidAndIdGt1AppendedTwice.Count()); // 1 result is both valid and id gt 1

            // This is one of the things you were asking about - note that without assigning the additional Where criteria to the Iqueryable, you do not get the results of the where clause, but the original query - in this case there are no appended where conditions on the t.GetData() call, so you get the full result set.

            var notReallyValid = t.GetData();

            notReallyValid.Where(w => w.Name == "this name definitly does not exist");

            Console.WriteLine("where clause not correctly appended count: {0}", notReallyValid.Count());

            // vs

            var validUse = t.GetData();

            validUse = validUse.Where(w => w.Name == "this name definitly does not exist");

            Console.WriteLine("valid use count: {0}", validUse.Count());

        }

    }

    public class Thingy

    {

        private List<Foo> _testData = new List<Foo>()

        {

            new Foo()

            {

                Id = 1,

                Name = "Alpha",

                Created = new DateTime(2015, 1, 1),

                IsValid = true

            },

            new Foo()

            {

                Id = 2,

                Name = "Beta",

                Created = new DateTime(2015, 2, 1),

                IsValid = false

            },

            new Foo()

            {

                Id = 3,

                Name = "Gamma",

                Created = new DateTime(2015, 3, 1),

                IsValid = true

            },          

        };

        public IQueryable<Foo> GetData()

        {

            return _testData.AsQueryable();

        }

        public void PrintData(IEnumerable<Foo> data)

        {

            // Note calling this will enumerate the data for IQueryable

            foreach (Foo f in data)

            {

                Console.WriteLine(string.Format("id: {0}, name: {1}, created: {2}, isValid: {3}", f.Id, f.Name, f.Created, f.IsValid));

            }

        }

    }

    public class Foo

    {

        public int Id { get; set; }

        public string Name { get; set; }

        public DateTime Created { get; set; }

        public bool IsValid { get; set; }

    }