From 524ba0f7bafa206dcc1cbf571ec6dd61eb0b2e45 Mon Sep 17 00:00:00 2001
From: Vladimir Pecanac <vladimir.pecanac@gmail.com>
Date: Sun, 1 Nov 2026 13:29:46 +0100
Subject: [PATCH] Filter a list based on another list: .NET 10, RemoveAll,
 HashSet lookup, Join and IntersectBy demos

---
 ...lterAListBasedOnAnotherListInCSharp.csproj |  2 +-
 .../Program.cs                                | 18 +++++
 .../School.cs                                 |  2 +-
 .../Student.cs                                |  4 +-
 .../Utilities.cs                              | 27 +++++++
 ...AListBasedOnAnotherListInCSharpUnitTest.cs | 72 ++++++++++++++++++-
 .../Tests/Tests.csproj                        | 10 +--
 7 files changed, 125 insertions(+), 10 deletions(-)

diff --git a/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp.csproj b/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp.csproj
index 2150e3797b..ed9781c223 100644
--- a/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp.csproj
+++ b/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp.csproj
@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net8.0</TargetFramework>
+    <TargetFramework>net10.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
   </PropertyGroup>
diff --git a/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/Program.cs b/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/Program.cs
index 8fbaeb8b69..377455432e 100644
--- a/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/Program.cs
+++ b/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/Program.cs
@@ -26,6 +26,12 @@
 Console.WriteLine(Utilities.PrintList(filteredContainedInts));
 Console.WriteLine();
 
+Console.WriteLine("REMOVEALL");
+List<int> intsToChange = [1, 2, 3, 4, 4, 4, 5, 6, 7, 7, 8, 8, 9];
+int removedCount = Utilities.RemoveNotContained(intsToChange, filteringIntList);
+Console.WriteLine($"Removed {removedCount}, left: {Utilities.PrintList(intsToChange)}");
+Console.WriteLine();
+
 Console.WriteLine("DUPLICATES");
 Console.WriteLine("Unique elements from list 1 that are contained in list 2:");
 filteredContainedInts = Utilities.FilterContainedUnique(intsToFilter, filteringIntList);
@@ -78,6 +84,13 @@
 Console.WriteLine(Utilities.PrintList(filteredStudents));
 Console.WriteLine();
 
+Console.WriteLine("INTERSECTBY");
+Console.WriteLine(Utilities.PrintList(Utilities.FilterStudentsBySchoolCityUsingIntersectBy(studentsToFilter, schoolsToFilter)));
+Console.WriteLine();
+Console.WriteLine("JOIN");
+Console.WriteLine(Utilities.PrintList(Utilities.FilterStudentsBySchoolCityUsingJoin(studentsToFilter, schoolsToFilter)));
+Console.WriteLine();
+
 Console.WriteLine("SPECIFIC PROPERTIES");
 List<int> schoolIds = [1, 3];
 Console.WriteLine("Names of students from list 1 that attend schools with IDs from list 2:");
@@ -100,6 +113,11 @@
 Console.WriteLine(Utilities.PrintList(filteredContainedInts));
 Console.WriteLine();
 
+Console.WriteLine("HASHSET LOOKUP");
+Console.WriteLine(Utilities.PrintList(Utilities.FilterContainedUsingHashSetLookup(intsToFilter, filteringIntList)));
+Console.WriteLine(Utilities.PrintList(Utilities.FilterContainedUsingHashSetLookup([1, 2, 3, 4, 4, 4, 5, 6, 7, 7, 8, 8, 9], [4, 4, 7, 9, 10])));
+Console.WriteLine();
+
 Console.WriteLine("MASKS");
 intsToFilter = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15];
 List<bool> mask = [true, false, false, false];
diff --git a/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/School.cs b/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/School.cs
index cb2bee3c3e..1643498fc8 100644
--- a/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/School.cs
+++ b/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/School.cs
@@ -3,5 +3,5 @@
 public class School
 {
     public int Id { get; set; }
-    public string City { get; set; }
+    public string City { get; set; } = string.Empty;
 }
diff --git a/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/Student.cs b/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/Student.cs
index f708ebf85a..0d15ce2659 100644
--- a/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/Student.cs
+++ b/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/Student.cs
@@ -3,9 +3,9 @@
 public class Student
 {
     public int Id { get; set; }
-    public string Name { get; set; }
+    public string Name { get; set; } = string.Empty;
     public int SchoolId { get; set; }
-    public string City { get; set; }
+    public string City { get; set; } = string.Empty;
 
     override public string ToString()
     {
diff --git a/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/Utilities.cs b/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/Utilities.cs
index 22f7916fc8..00c9bc029f 100644
--- a/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/Utilities.cs
+++ b/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/HowToFilterAListBasedOnAnotherListInCSharp/Utilities.cs
@@ -97,4 +97,31 @@ public static List<int> FilterUsingMask(List<int> listToFilter, List<bool> mask)
     { 
         return listToFilter.Where((item, index) => mask[index % mask.Count]).ToList();
     }
+
+    public static int RemoveNotContained(List<int> listToFilter, List<int> filteringList)
+    {
+        return listToFilter.RemoveAll(item => !filteringList.Contains(item));
+    }
+
+    public static List<int> FilterContainedUsingHashSetLookup(List<int> listToFilter, List<int> filteringList)
+    {
+        HashSet<int> lookup = filteringList.ToHashSet();
+
+        return listToFilter.Where(lookup.Contains).ToList();
+    }
+
+    public static List<Student> FilterStudentsBySchoolCityUsingIntersectBy(List<Student> listToFilter, List<School> filteringList)
+    {
+        return listToFilter.IntersectBy(filteringList.Select(school => school.City), student => student.City)
+            .ToList();
+    }
+
+    public static List<Student> FilterStudentsBySchoolCityUsingJoin(List<Student> listToFilter, List<School> filteringList)
+    {
+        return listToFilter.Join(filteringList,
+                student => student.City,
+                school => school.City,
+                (student, school) => student)
+            .ToList();
+    }
 }
diff --git a/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/Tests/HowToFilterAListBasedOnAnotherListInCSharpUnitTest.cs b/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/Tests/HowToFilterAListBasedOnAnotherListInCSharpUnitTest.cs
index 413b7ff333..6f59860a5b 100644
--- a/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/Tests/HowToFilterAListBasedOnAnotherListInCSharpUnitTest.cs
+++ b/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/Tests/HowToFilterAListBasedOnAnotherListInCSharpUnitTest.cs
@@ -231,4 +231,74 @@ public void Given2Lists_WhenFilterUsingMaskMethodCalled_ThenOnlyElementsCorrespo
         // Assert
         Assert.Equal([50, 7], output);
     }
-}
\ No newline at end of file
+
+    [Fact]
+    public void Given2Lists_WhenRemoveNotContainedMethodCalled_ThenListIsChangedInPlace()
+    {
+        // Arrange
+        List<int> numbers = [17, 34, 50, 68, 121, 7, 145];
+        List<int> otherNumbers = [50, 44, 121];
+
+        // Act
+        var removed = Utilities.RemoveNotContained(numbers, otherNumbers);
+
+        // Assert
+        Assert.Equal(5, removed);
+        Assert.Equal([50, 121], numbers);
+    }
+
+    [Fact]
+    public void Given2Lists_WhenFilterContainedUsingHashSetLookupMethodCalled_ThenOrderAndDuplicatesAreKept()
+    {
+        // Arrange
+        List<int> numbers = [121, 17, 50, 50, 68, 121, 7];
+        List<int> otherNumbers = [50, 44, 121];
+
+        // Act
+        var output = Utilities.FilterContainedUsingHashSetLookup(numbers, otherNumbers);
+
+        // Assert
+        Assert.Equal([121, 50, 50, 121], output);
+    }
+
+    [Fact]
+    public void Given2Lists_WhenFilterStudentsBySchoolCityUsingIntersectByMethodCalled_ThenOneStudentPerCityIsKept()
+    {
+        // Arrange
+        List<Student> students =
+        [
+            new Student { Id = 1, Name = "Alice", SchoolId = 1, City = "Paris" },
+            new Student { Id = 2, Name = "Bob", SchoolId = 2, City = "Reims" },
+            new Student { Id = 5, Name = "Eve", SchoolId = 2, City = "Reims" }
+        ];
+        List<School> schools = [new School { Id = 1, City = "Paris" }, new School { Id = 2, City = "Reims" }];
+
+        // Act
+        var output = Utilities.FilterStudentsBySchoolCityUsingIntersectBy(students, schools)
+            .Select(student => student.Name).ToList();
+
+        // Assert
+        Assert.Equal(["Alice", "Bob"], output);
+    }
+
+    [Fact]
+    public void Given2Lists_WhenFilterStudentsBySchoolCityUsingJoinMethodCalled_ThenEveryMatchingStudentIsKept()
+    {
+        // Arrange
+        List<Student> students =
+        [
+            new Student { Id = 1, Name = "Alice", SchoolId = 1, City = "Paris" },
+            new Student { Id = 2, Name = "Bob", SchoolId = 2, City = "Reims" },
+            new Student { Id = 3, Name = "Charlie", SchoolId = 1, City = "Calais" },
+            new Student { Id = 5, Name = "Eve", SchoolId = 2, City = "Reims" }
+        ];
+        List<School> schools = [new School { Id = 1, City = "Paris" }, new School { Id = 2, City = "Reims" }];
+
+        // Act
+        var output = Utilities.FilterStudentsBySchoolCityUsingJoin(students, schools)
+            .Select(student => student.Name).ToList();
+
+        // Assert
+        Assert.Equal(["Alice", "Bob", "Eve"], output);
+    }
+}
diff --git a/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/Tests/Tests.csproj b/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/Tests/Tests.csproj
index 125cc0c22b..8857323730 100644
--- a/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/Tests/Tests.csproj
+++ b/collections-lists/HowToFilterAListBasedOnAnotherListInCSharp/Tests/Tests.csproj
@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
+    <TargetFramework>net10.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
 
@@ -10,13 +10,13 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.6.0" />
-    <PackageReference Include="xunit" Version="2.4.2" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5">
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.11.0" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="4.0.1">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>
-    <PackageReference Include="coverlet.collector" Version="6.0.0">
+    <PackageReference Include="coverlet.collector" Version="10.1.0">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>