Added Enumerable.Append and Enumerable.Prepend

src/System.Linq.Expressions/tests/SequenceTests/SequenceTests.cs

@@ -1669,7 +1669,9 @@ public static void MatchSequencePattern()
                      "AsEnumerable",
                      "ToList",
                      "Fold",
-                     "LeftJoin"
+                     "LeftJoin",
+                     "Append",
+                     "Prepend",
                  }
                 ).ToList();
 

src/System.Linq/ref/System.Linq.cs

@@ -36,6 +36,8 @@ public static partial class Enumerable
         public static float Average<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, float> selector) { return default(float); }
         public static System.Collections.Generic.IEnumerable<TResult> Cast<TResult>(this System.Collections.IEnumerable source) { return default(System.Collections.Generic.IEnumerable<TResult>); }
         public static System.Collections.Generic.IEnumerable<TSource> Concat<TSource>(this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second) { return default(System.Collections.Generic.IEnumerable<TSource>); }
+        public static System.Collections.Generic.IEnumerable<TSource> Append<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, TSource element) { return default(System.Collections.Generic.IEnumerable<TSource>); }
+        public static System.Collections.Generic.IEnumerable<TSource> Prepend<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, TSource element) { return default(System.Collections.Generic.IEnumerable<TSource>); }
         public static bool Contains<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, TSource value) { return default(bool); }
         public static bool Contains<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, TSource value, System.Collections.Generic.IEqualityComparer<TSource> comparer) { return default(bool); }
         public static int Count<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { return default(int); }

src/System.Linq/ref/System.Linq.csproj

@@ -2,7 +2,7 @@
 <Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.props))\dir.props" />
   <PropertyGroup>
-    <AssemblyVersion>4.0.2.0</AssemblyVersion>
+    <AssemblyVersion>4.1.0.0</AssemblyVersion>
     <OutputType>Library</OutputType>
     <PackageTargetFramework>dotnet5.1</PackageTargetFramework>
     <NuGetTargetMoniker>.NETPlatform,Version=v5.1</NuGetTargetMoniker>

src/System.Linq/src/System.Linq.csproj

@@ -4,7 +4,7 @@
   <PropertyGroup>
     <ProjectGuid>{CA488507-3B6E-4494-B7BE-7B4EEEB2C4D1}</ProjectGuid>
     <AssemblyName>System.Linq</AssemblyName>
-    <AssemblyVersion>4.0.2.0</AssemblyVersion>
+    <AssemblyVersion>4.1.0.0</AssemblyVersion>
     <RootNamespace>System.Linq</RootNamespace>
     <PackageTargetFramework>dotnet5.4</PackageTargetFramework>
   </PropertyGroup>

src/System.Linq/src/System/Linq/Enumerable.cs

@@ -1152,6 +1152,30 @@ private static IEnumerable<TSource> ConcatIterator<TSource>(IEnumerable<TSource>
             foreach (TSource element in second) yield return element;
         }
 
+        public static IEnumerable<TSource> Append<TSource>(this IEnumerable<TSource> source, TSource element)
+        {
+            if (source == null) throw Error.ArgumentNull("source");
+            return AppendIterator<TSource>(source, element);
+        }
+
+        private static IEnumerable<TSource> AppendIterator<TSource>(IEnumerable<TSource> source, TSource element)
+        {
+            foreach (TSource e1 in source) yield return e1;
+            yield return element;
+        }
+
+        public static IEnumerable<TSource> Prepend<TSource>(this IEnumerable<TSource> source, TSource element)
+        {
+            if (source == null) throw Error.ArgumentNull("source");
+            return PrependIterator<TSource>(source, element);
+        }
+
+        private static IEnumerable<TSource> PrependIterator<TSource>(IEnumerable<TSource> source, TSource element)
+        {
+            yield return element;
+            foreach (TSource e1 in source) yield return e1;
+        }
+
         public static IEnumerable<TResult> Zip<TFirst, TSecond, TResult>(this IEnumerable<TFirst> first, IEnumerable<TSecond> second, Func<TFirst, TSecond, TResult> resultSelector)
         {
             if (first == null) throw Error.ArgumentNull("first");

src/System.Linq/tests/AppendPrependTests.cs

@@ -0,0 +1,119 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Collections.Generic;
+using Xunit;
+
+namespace System.Linq.Tests
+{
+    public class AppendPrependTests : EnumerableTests
+    {
+        [Fact]
+        public void SameResultsRepeatCallsIntQueryAppend()
+        {
+            var q1 = from x1 in new int?[] { 2, 3, null, 2, null, 4, 5 }
+                     select x1;
+
+            Assert.Equal(q1.Append(42), q1.Append(42));
+            Assert.Equal(q1.Append(42), q1.Concat(new int?[] { 42 }));
+        }
+
+        [Fact]
+        public void SameResultsRepeatCallsIntQueryPrepend()
+        {
+            var q1 = from x1 in new int?[] { 2, 3, null, 2, null, 4, 5 }
+                     select x1;
+
+            Assert.Equal(q1.Prepend(42), q1.Prepend(42));
+            Assert.Equal(q1.Prepend(42), (new int?[] { 42 }).Concat(q1));
+        }
+
+        [Fact]
+        public void SameResultsRepeatCallsStringQueryAppend()
+        {
+            var q1 = from x1 in new[] { "AAA", String.Empty, "q", "C", "#", "!@#$%^", "0987654321", "Calling Twice" }
+                     select x1;
+
+            Assert.Equal(q1.Append("hi"), q1.Append("hi"));
+            Assert.Equal(q1.Append("hi"), q1.Concat(new string[] { "hi" }));
+        }
+
+        [Fact]
+        public void SameResultsRepeatCallsStringQueryPrepend()
+        {
+            var q1 = from x1 in new[] { "AAA", String.Empty, "q", "C", "#", "!@#$%^", "0987654321", "Calling Twice" }
+                     select x1;
+
+            Assert.Equal(q1.Prepend("hi"), q1.Prepend("hi"));
+            Assert.Equal(q1.Prepend("hi"), (new string[] { "hi" }).Concat(q1));
+        }
+
+        [Fact]
+        public void EmptyAppend()
+        {
+            int[] first = { };
+            Assert.Single(first.Append(42), 42);
+        }
+
+        [Fact]
+        public void EmptyPrepend()
+        {
+            string[] first = { };
+            Assert.Single(first.Prepend("aa"), "aa");
+        }
+
+        [Fact]
+        public void PrependNoIteratingSourceBeforeFirstItem()
+        {
+            var ie = new List<int>();
+            var prepended = (from i in ie select i).Prepend(4);
+
+            ie.Add(42);
+
+            Assert.Equal(prepended, ie.Prepend(4));
+        }
+
+        [Fact]
+        public void ForcedToEnumeratorDoesntEnumeratePrepend()
+        {
+            var iterator = NumberRangeGuaranteedNotCollectionType(0, 3).Prepend(4);
+            // Don't insist on this behaviour, but check it's correct if it happens
+            var en = iterator as IEnumerator<int>;
+            Assert.False(en != null && en.MoveNext());
+        }
+
+        [Fact]
+        public void ForcedToEnumeratorDoesntEnumerateAppend()
+        {
+            var iterator = NumberRangeGuaranteedNotCollectionType(0, 3).Append(4);
+            // Don't insist on this behaviour, but check it's correct if it happens
+            var en = iterator as IEnumerator<int>;
+            Assert.False(en != null && en.MoveNext());
+        }
+
+        [Fact]
+        public void SourceNull()
+        {
+            Assert.Throws<ArgumentNullException>("source", () => ((IEnumerable<int>)null).Append(1));
+            Assert.Throws<ArgumentNullException>("source", () => ((IEnumerable<int>)null).Prepend(1));
+        }
+
+        [Fact]
+        public void Combined()
+        {
+            var v = "foo".Append('1').Append('2').Prepend('3').Concat("qq".Append('Q').Prepend('W'));
+
+            Assert.Equal(v.ToArray(), "3foo12WqqQ".ToArray());
+
+            var v1 = "a".Append('b').Append('c').Append('d');
+
+            Assert.Equal(v1.ToArray(), "abcd".ToArray());
+
+            var v2 = "a".Prepend('b').Prepend('c').Prepend('d');
+
+            Assert.Equal(v2.ToArray(), "dcba".ToArray());
+        }
+    }
+}

src/System.Linq/tests/System.Linq.Tests.csproj

@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.props))\dir.props" />
   <PropertyGroup>
@@ -20,6 +20,7 @@
     <Compile Include="AsEnumerableTests.cs" />
     <Compile Include="AverageTests.cs" />
     <Compile Include="CastTests.cs" />
+    <Compile Include="AppendPrependTests.cs" />
     <Compile Include="ConcatTests.cs" />
     <Compile Include="ContainsTests.cs" />
     <Compile Include="CountTests.cs" />
@@ -89,4 +90,4 @@
     </ProjectReference>
   </ItemGroup>
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.targets))\dir.targets" />
-</Project>
+</Project>