Implement robust prime generator

The generator can now be enumerated synchronously and asynchronously.
The async generator can now be cancelled abruptly (throw) or gracefully (just stop).
The generator is now in a separate library.
Tests and web app feature three scenarios:
1. Multiply first N primes synchronously, no fault handling.
2. Multiply first N primes asynchronously, fail if time limit exceeded.
3. Multiply as many of the first N primes as possible in time limit.

AvastRecruitmentPrimeMultiplication.sln

@@ -5,7 +5,9 @@ VisualStudioVersion = 16.0.30128.74
 MinimumVisualStudioVersion = 10.0.40219.1
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "UnitTestProject1", "UnitTestProject1\UnitTestProject1.csproj", "{2C1DA99E-D388-484C-A81B-07C14664C950}"
 EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "WebApplication1", "WebApplication1\WebApplication1.csproj", "{BBB20ED0-348D-4EBD-9FA7-63F5A2A28799}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "WebApplication1", "WebApplication1\WebApplication1.csproj", "{BBB20ED0-348D-4EBD-9FA7-63F5A2A28799}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ClassLibrary1", "ClassLibrary1\ClassLibrary1.csproj", "{D44BA64C-FA3B-4A8C-AF05-3251988CB5DA}"
 EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
@@ -21,6 +23,10 @@ Global
 		{BBB20ED0-348D-4EBD-9FA7-63F5A2A28799}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{BBB20ED0-348D-4EBD-9FA7-63F5A2A28799}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{BBB20ED0-348D-4EBD-9FA7-63F5A2A28799}.Release|Any CPU.Build.0 = Release|Any CPU
+		{D44BA64C-FA3B-4A8C-AF05-3251988CB5DA}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{D44BA64C-FA3B-4A8C-AF05-3251988CB5DA}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{D44BA64C-FA3B-4A8C-AF05-3251988CB5DA}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{D44BA64C-FA3B-4A8C-AF05-3251988CB5DA}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE

ClassLibrary1/ClassLibrary1.csproj

@@ -0,0 +1,7 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
+  </PropertyGroup>
+
+</Project>

ClassLibrary1/PrimeGenerator.PrimeAsyncEnumerator.cs

@@ -0,0 +1,88 @@
+namespace ClassLibrary1
+{
+    using System;
+    using System.Collections.Generic;
+    using System.Threading;
+    using System.Threading.Tasks;
+
+    public sealed partial class PrimeGenerator
+    {
+        private sealed class PrimeAsyncEnumerator : IAsyncEnumerator<int>
+        {
+            private const int notInitialised = 1;
+            private readonly PrimeGeneratorOptions options;
+            private readonly CancellationToken cancellationToken;
+            private int current = notInitialised;
+
+            internal PrimeAsyncEnumerator(PrimeGeneratorOptions options = PrimeGeneratorOptions.None, CancellationToken cancellationToken = default)
+            {
+                this.options = options;
+                this.cancellationToken = cancellationToken;
+            }
+
+            int IAsyncEnumerator<int>.Current
+            {
+                get
+                {
+                    if (this.current == notInitialised)
+                    {
+                        throw new InvalidOperationException();
+                    }
+
+                    return this.current;
+                }
+            }
+
+            private bool IsPrime
+            {
+                get
+                {
+                    for (int i = 2; i * i <= this.current; i++)
+                    {
+                        this.ThrowIfNeeded();
+
+                        if (this.current % i == 0)
+                        {
+                            return false;
+                        }
+                    }
+
+                    return true;
+                }
+            }
+
+            private bool ShouldStop =>
+                !this.ShouldThrowOnCancel && this.cancellationToken.IsCancellationRequested;
+
+            ValueTask IAsyncDisposable.DisposeAsync() =>
+                default;
+
+            ValueTask<bool> IAsyncEnumerator<int>.MoveNextAsync()
+            {
+                do
+                {
+                    if (this.ShouldStop)
+                    {
+                        return new ValueTask<bool>(false);
+                    }
+
+                    this.current++;
+
+                } while (!this.IsPrime);
+
+                return new ValueTask<bool>(true);
+            }
+
+            private void ThrowIfNeeded()
+            {
+                if (this.ShouldThrowOnCancel)
+                {
+                    this.cancellationToken.ThrowIfCancellationRequested();
+                }
+            }
+
+            private bool ShouldThrowOnCancel =>
+                this.options.HasFlag(PrimeGeneratorOptions.ThrowOnCancel);
+        }
+    }
+}

ClassLibrary1/PrimeGenerator.PrimeEnumerator.cs

@@ -0,0 +1,32 @@
+namespace ClassLibrary1
+{
+    using System;
+    using System.Collections;
+    using System.Collections.Generic;
+
+    public sealed partial class PrimeGenerator
+    {
+        private sealed class PrimeEnumerator : IEnumerator<int>
+        {
+            private readonly IAsyncEnumerator<int> primeAsyncEnumerator;
+
+            internal PrimeEnumerator(PrimeAsyncEnumerator primeAsyncEnumerator) =>
+                this.primeAsyncEnumerator = primeAsyncEnumerator;
+
+            public int Current =>
+                this.primeAsyncEnumerator.Current;
+
+            object IEnumerator.Current =>
+                this.Current;
+
+            void IDisposable.Dispose() =>
+                this.primeAsyncEnumerator.DisposeAsync().GetAwaiter().GetResult();
+
+            bool IEnumerator.MoveNext() =>
+                this.primeAsyncEnumerator.MoveNextAsync().GetAwaiter().GetResult();
+
+            void IEnumerator.Reset() =>
+                throw new InvalidOperationException();
+        }
+    }
+}

ClassLibrary1/PrimeGenerator.cs

@@ -0,0 +1,22 @@
+namespace ClassLibrary1
+{
+    using System.Collections;
+    using System.Collections.Generic;
+    using System.Threading;
+
+    public sealed partial class PrimeGenerator : IAsyncEnumerable<int>, IEnumerable<int>
+    {
+        private readonly PrimeGeneratorOptions options;
+
+        public PrimeGenerator(PrimeGeneratorOptions options = PrimeGeneratorOptions.None) =>
+            this.options = options;
+
+        public IAsyncEnumerator<int> GetAsyncEnumerator(CancellationToken cancellationToken = default) =>
+            new PrimeAsyncEnumerator(this.options, cancellationToken);
+
+        public IEnumerator<int> GetEnumerator() =>
+            new PrimeEnumerator(new PrimeAsyncEnumerator());
+
+        IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();
+    }
+}

ClassLibrary1/PrimeGeneratorOptions.cs

@@ -0,0 +1,11 @@
+namespace ClassLibrary1
+{
+    using System;
+
+    [Flags]
+    public enum PrimeGeneratorOptions
+    {
+        None = 0b_0,
+        ThrowOnCancel = 0b_1
+    }
+}

UnitTestProject1/UnitTest1.cs

@@ -5,16 +5,17 @@ namespace UnitTestProject1
     using System.Linq;
     using System.Threading;
     using System.Threading.Tasks;
+    using ClassLibrary1;
     using Microsoft.VisualStudio.TestTools.UnitTesting;
 
     [TestClass]
     public class UnitTest1
     {
         [TestMethod]
-        public void ProofOfConcept()
+        public async Task ProofOfConcept()
         {
             const int n = 10;
-            var primes = n.Primes().ToArray();
+            var primes = new PrimeGenerator().Take(n).ToArray();
 
             var largestPrime = primes.Last();
             var largestMultiple = largestPrime * largestPrime;
@@ -54,15 +55,15 @@ public async Task ProofOfConceptAsync()
         {
             using var cts = new CancellationTokenSource(100);
 
-            const int n = 2000;
+            var generator = new PrimeGenerator(PrimeGeneratorOptions.ThrowOnCancel).WithCancellation(cts.Token);
 
-            await Assert.ThrowsExceptionAsync<TaskCanceledException>(async () =>
+            await Assert.ThrowsExceptionAsync<OperationCanceledException>(async () =>
             {
                 // Header row
                 await WriteRow();
 
                 // Body
-                await foreach (var prime in n.PrimesAsync(cts.Token))
+                await foreach (var prime in generator)
                 {
                     await WriteRow(prime);
                 }
@@ -73,7 +74,42 @@ async Task WriteRow(int? rowHeader = null)
                 // Header column
                 WriteCell(rowHeader);
 
-                await foreach (var prime in n.PrimesAsync(cts.Token))
+                await foreach (var prime in generator)
+                {
+                    WriteCell(prime, rowHeader ?? 1);
+                }
+
+                Console.WriteLine(); // LF
+            }
+
+            void WriteCell(int? prime1 = null, int? prime2 = 1)
+            {
+                Console.Write("{0, 10}", prime1 * prime2);
+            }
+        }
+
+        [TestMethod]
+        public async Task ProofOfConceptAsyncDontThrow()
+        {
+            using var cts = new CancellationTokenSource(100);
+
+            var generator = new PrimeGenerator().WithCancellation(cts.Token);
+
+            // Header row
+            await WriteRow();
+
+            // Body
+            await foreach (var prime in generator)
+            {
+                await WriteRow(prime);
+            }
+
+            async Task WriteRow(int? rowHeader = null)
+            {
+                // Header column
+                WriteCell(rowHeader);
+
+                await foreach (var prime in generator)
                 {
                     WriteCell(prime, rowHeader ?? 1);
                 }

UnitTestProject1/UnitTestProject1.csproj

@@ -12,4 +12,8 @@
     <PackageReference Include="MSTest.TestFramework" Version="2.1.2" />
   </ItemGroup>
 
+  <ItemGroup>
+    <ProjectReference Include="..\ClassLibrary1\ClassLibrary1.csproj" />
+  </ItemGroup>
+
 </Project>