diff --git a/src/UniTask.NetCore/Linq/Append.cs b/src/UniTask.NetCore/Linq/Append.cs
deleted file mode 100644
index 4d22846..0000000
--- a/src/UniTask.NetCore/Linq/Append.cs
+++ /dev/null
@@ -1,775 +0,0 @@
-namespace Cysharp.Threading.Tasks.Linq
-{
-    internal sealed class Append
-    {
-    }
-
-
-}
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diff --git a/src/UniTask.NetCore/Linq/AppendPrepend.cs b/src/UniTask.NetCore/Linq/AppendPrepend.cs
new file mode 100644
index 0000000..8b3a135
--- /dev/null
+++ b/src/UniTask.NetCore/Linq/AppendPrepend.cs
@@ -0,0 +1,913 @@
+using Cysharp.Threading.Tasks.Internal;
+using System;
+using System.Threading;
+
+namespace Cysharp.Threading.Tasks.Linq
+{
+    public static partial class UniTaskAsyncEnumerable
+    {
+        public static IUniTaskAsyncEnumerable<TSource> Append<TSource>(this IUniTaskAsyncEnumerable<TSource> source, TSource element)
+        {
+            Error.ThrowArgumentNullException(source, nameof(source));
+
+            return new AppendPrepend<TSource>(source, element, true);
+        }
+
+        public static IUniTaskAsyncEnumerable<TSource> Prepend<TSource>(this IUniTaskAsyncEnumerable<TSource> source, TSource element)
+        {
+            Error.ThrowArgumentNullException(source, nameof(source));
+
+            return new AppendPrepend<TSource>(source, element, true);
+        }
+    }
+
+    internal sealed class AppendPrepend<TSource> : IUniTaskAsyncEnumerable<TSource>
+    {
+        readonly IUniTaskAsyncEnumerable<TSource> source;
+        readonly TSource element;
+        readonly bool append; // or prepend
+
+        public AppendPrepend(IUniTaskAsyncEnumerable<TSource> source, TSource element, bool append)
+        {
+            this.source = source;
+            this.element = element;
+            this.append = append;
+        }
+
+        public IUniTaskAsyncEnumerator<TSource> GetAsyncEnumerator(CancellationToken cancellationToken = default)
+        {
+            return new Enumerator(source, element, append, cancellationToken);
+        }
+
+        sealed class Enumerator : MoveNextSource, IUniTaskAsyncEnumerator<TSource>
+        {
+            enum State : byte
+            {
+                None,
+                RequirePrepend,
+                RequireAppend,
+                Completed
+            }
+
+            static readonly Action<object> MoveNextCoreDelegate = MoveNextCore;
+
+            readonly IUniTaskAsyncEnumerable<TSource> source;
+            readonly TSource element;
+            CancellationToken cancellationToken;
+
+            State state;
+            IUniTaskAsyncEnumerator<TSource> enumerator;
+            UniTask<bool>.Awaiter awaiter;
+
+            public Enumerator(IUniTaskAsyncEnumerable<TSource> source, TSource element, bool append, CancellationToken cancellationToken)
+            {
+                this.source = source;
+                this.element = element;
+                this.state = append ? State.RequireAppend : State.RequirePrepend;
+                this.cancellationToken = cancellationToken;
+            }
+
+            public TSource Current { get; private set; }
+
+
+            public UniTask<bool> MoveNextAsync()
+            {
+                cancellationToken.ThrowIfCancellationRequested();
+                completionSource.Reset();
+
+                if (enumerator == null)
+                {
+                    if (state == State.RequireAppend)
+                    {
+                        Current = element;
+                        state = State.None;
+                        return CompletedTasks.True;
+                    }
+
+                    enumerator = source.GetAsyncEnumerator(cancellationToken);
+                }
+
+                if (state == State.Completed)
+                {
+                    return CompletedTasks.False;
+                }
+
+                awaiter = enumerator.MoveNextAsync().GetAwaiter();
+
+                if (awaiter.IsCompleted)
+                {
+                    MoveNextCoreDelegate(this);
+                }
+                else
+                {
+                    awaiter.SourceOnCompleted(MoveNextCoreDelegate, this);
+                }
+
+                return new UniTask<bool>(this, completionSource.Version);
+            }
+
+            static void MoveNextCore(object state)
+            {
+                var self = (Enumerator)state;
+
+                if (self.awaiter.GetResult())
+                {
+                    self.Current = self.enumerator.Current;
+                }
+                else
+                {
+                    if (self.state == State.RequireAppend)
+                    {
+                        self.state = State.Completed;
+                        self.Current = self.element;
+                    }
+                    else
+                    {
+                        self.state = State.Completed;
+                        self.completionSource.TrySetResult(false);
+                        return;
+                    }
+                }
+
+                self.completionSource.TrySetResult(true);
+            }
+
+            public UniTask DisposeAsync()
+            {
+                if (enumerator != null)
+                {
+                    return enumerator.DisposeAsync();
+                }
+                return default;
+            }
+        }
+    }
+
+}
+
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diff --git a/src/UniTask.NetCore/Linq/AsyncEnumeratorBase.cs b/src/UniTask.NetCore/Linq/AsyncEnumeratorBase.cs
index 2818715..3befbef 100644
--- a/src/UniTask.NetCore/Linq/AsyncEnumeratorBase.cs
+++ b/src/UniTask.NetCore/Linq/AsyncEnumeratorBase.cs
@@ -153,6 +153,8 @@ namespace Cysharp.Threading.Tasks.Linq
     public abstract class AsyncEnumeratorAwaitSelectorBase<TSource, TResult, TAwait> : MoveNextSource, IUniTaskAsyncEnumerator<TResult>
     {
         static readonly Action<object> moveNextCallbackDelegate = MoveNextCallBack;
+        static readonly Action<object> setCurrentCallbackDelegate = SetCurrentCallBack;
+
 
         readonly IUniTaskAsyncEnumerable<TSource> source;
         protected CancellationToken cancellationToken;
@@ -274,7 +276,7 @@ namespace Cysharp.Threading.Tasks.Linq
             }
             else
             {
-                resultAwaiter.SourceOnCompleted(SetCurrentCallBack, this); // TODO:cache
+                resultAwaiter.SourceOnCompleted(setCurrentCallbackDelegate, this);
                 result = default;
                 return false;
             }
diff --git a/src/UniTask.NetCore/Linq/Concat.cs b/src/UniTask.NetCore/Linq/Concat.cs
index 327ac6f..6e3dfa4 100644
--- a/src/UniTask.NetCore/Linq/Concat.cs
+++ b/src/UniTask.NetCore/Linq/Concat.cs
@@ -1,775 +1,151 @@
-namespace Cysharp.Threading.Tasks.Linq
+using Cysharp.Threading.Tasks.Internal;
+using System;
+using System.Threading;
+
+namespace Cysharp.Threading.Tasks.Linq
 {
-    internal sealed class Concat
+    public static partial class UniTaskAsyncEnumerable
     {
+        public static IUniTaskAsyncEnumerable<TSource> Concat<TSource>(this IUniTaskAsyncEnumerable<TSource> first, IUniTaskAsyncEnumerable<TSource> second)
+        {
+            Error.ThrowArgumentNullException(first, nameof(first));
+            Error.ThrowArgumentNullException(second, nameof(second));
+
+            return new Concat<TSource>(first, second);
+        }
     }
 
-
-}
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+    internal sealed class Concat<TSource> : IUniTaskAsyncEnumerable<TSource>
+    {
+        readonly IUniTaskAsyncEnumerable<TSource> first;
+        readonly IUniTaskAsyncEnumerable<TSource> second;
+
+        public Concat(IUniTaskAsyncEnumerable<TSource> first, IUniTaskAsyncEnumerable<TSource> second)
+        {
+            this.first = first;
+            this.second = second;
+        }
+
+        public IUniTaskAsyncEnumerator<TSource> GetAsyncEnumerator(CancellationToken cancellationToken = default)
+        {
+            return new Enumerator(first, second, cancellationToken);
+        }
+
+        sealed class Enumerator : MoveNextSource, IUniTaskAsyncEnumerator<TSource>
+        {
+            static readonly Action<object> MoveNextCoreDelegate = MoveNextCore;
+
+            enum IteratingState
+            {
+                IteratingFirst,
+                IteratingSecond,
+                Complete
+            }
+
+            readonly IUniTaskAsyncEnumerable<TSource> first;
+            readonly IUniTaskAsyncEnumerable<TSource> second;
+            CancellationToken cancellationToken;
+
+            IteratingState iteratingState;
+
+            IUniTaskAsyncEnumerator<TSource> enumerator;
+            UniTask<bool>.Awaiter awaiter;
+
+            public Enumerator(IUniTaskAsyncEnumerable<TSource> first, IUniTaskAsyncEnumerable<TSource> second, CancellationToken cancellationToken)
+            {
+                this.first = first;
+                this.second = second;
+                this.cancellationToken = cancellationToken;
+                this.iteratingState = IteratingState.IteratingFirst;
+            }
+
+            public TSource Current { get; private set; }
+
+            public UniTask<bool> MoveNextAsync()
+            {
+                cancellationToken.ThrowIfCancellationRequested();
+
+                if (iteratingState == IteratingState.Complete) return CompletedTasks.False;
+
+                completionSource.Reset();
+                StartIterate();
+                return new UniTask<bool>(this, completionSource.Version);
+            }
+
+            void StartIterate()
+            {
+                if (enumerator == null)
+                {
+                    if (iteratingState == IteratingState.IteratingFirst)
+                    {
+                        enumerator = first.GetAsyncEnumerator(cancellationToken);
+                    }
+                    else if (iteratingState == IteratingState.IteratingSecond)
+                    {
+                        enumerator = second.GetAsyncEnumerator(cancellationToken);
+                    }
+                }
+
+                awaiter = enumerator.MoveNextAsync().GetAwaiter();
+
+                if (awaiter.IsCompleted)
+                {
+                    MoveNextCoreDelegate(this);
+                }
+                else
+                {
+                    awaiter.SourceOnCompleted(MoveNextCoreDelegate, this);
+                }
+            }
+
+            static void MoveNextCore(object state)
+            {
+                var self = (Enumerator)state;
+
+                if (self.awaiter.GetResult())
+                {
+                    self.Current = self.enumerator.Current;
+                    self.completionSource.TrySetResult(true);
+                }
+                else
+                {
+                    if (self.iteratingState == IteratingState.IteratingFirst)
+                    {
+                        self.RunSecondAfterDisposeAsync().Forget();
+                        return;
+                    }
+
+                    self.iteratingState = IteratingState.Complete;
+                    self.completionSource.TrySetResult(false);
+                }
+            }
+
+            async UniTaskVoid RunSecondAfterDisposeAsync()
+            {
+                try
+                {
+                    await enumerator.DisposeAsync();
+                    enumerator = null;
+                    awaiter = default;
+                    iteratingState = IteratingState.IteratingSecond;
+                }
+                catch (Exception ex)
+                {
+                    completionSource.TrySetException(ex);
+                }
+
+                StartIterate();
+            }
+
+            public UniTask DisposeAsync()
+            {
+                if (enumerator != null)
+                {
+                    return enumerator.DisposeAsync();
+                }
+
+                return default;
+            }
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/UniTask.NetCore/Linq/Prepend.cs b/src/UniTask.NetCore/Linq/Prepend.cs
deleted file mode 100644
index b5d7d8c..0000000
--- a/src/UniTask.NetCore/Linq/Prepend.cs
+++ /dev/null
@@ -1,775 +0,0 @@
-namespace Cysharp.Threading.Tasks.Linq
-{
-    internal sealed class Prepend
-    {
-    }
-
-
-}
-
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diff --git a/src/UniTask.NetCore/Linq/_FileMaker.cs b/src/UniTask.NetCore/Linq/_FileMaker.cs
index a337916..c8f6d3d 100644
--- a/src/UniTask.NetCore/Linq/_FileMaker.cs
+++ b/src/UniTask.NetCore/Linq/_FileMaker.cs
@@ -23,21 +23,12 @@ namespace ___Dummy
 
         
 
-        public static IUniTaskAsyncEnumerable<TSource> Append<TSource>(this IUniTaskAsyncEnumerable<TSource> source, TSource element)
-        {
-            throw new NotImplementedException();
-        }
-
 
         public static IUniTaskAsyncEnumerable<TResult> Cast<TResult>(this IUniTaskAsyncEnumerable<Object> source)
         {
             throw new NotImplementedException();
         }
 
-        public static IUniTaskAsyncEnumerable<TSource> Concat<TSource>(this IUniTaskAsyncEnumerable<TSource> first, IUniTaskAsyncEnumerable<TSource> second)
-        {
-            throw new NotImplementedException();
-        }
 
 
 
@@ -333,10 +324,7 @@ namespace ___Dummy
             throw new NotImplementedException();
         }
 
-        public static IUniTaskAsyncEnumerable<TSource> Prepend<TSource>(this IUniTaskAsyncEnumerable<TSource> source, TSource element)
-        {
-            throw new NotImplementedException();
-        }
+  
 
         public static IUniTaskAsyncEnumerable<TSource> Reverse<TSource>(this IUniTaskAsyncEnumerable<TSource> source)
         {
diff --git a/src/UniTask.NetCoreTests/Linq/Concat.cs b/src/UniTask.NetCoreTests/Linq/Concat.cs
new file mode 100644
index 0000000..e0e390a
--- /dev/null
+++ b/src/UniTask.NetCoreTests/Linq/Concat.cs
@@ -0,0 +1,65 @@
+using Cysharp.Threading.Tasks;
+using Cysharp.Threading.Tasks.Linq;
+using FluentAssertions;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reactive.Concurrency;
+using System.Reactive.Linq;
+using System.Threading.Tasks;
+using Xunit;
+
+
+namespace NetCoreTests.Linq
+{
+    public class Concat
+    {
+        [Theory]
+        [InlineData(0, 0)]
+        [InlineData(0, 1)]
+        [InlineData(0, 2)]
+        [InlineData(0, 10)]
+        public async Task Append(int start, int count)
+        {
+            var xs = await Enumerable.Range(start, count).ToUniTaskAsyncEnumerable().Append(99).ToArrayAsync();
+            var ys = Enumerable.Range(start, count).Append(99).ToArray();
+
+            xs.Should().BeEquivalentTo(ys);
+        }
+
+        [Theory]
+        [InlineData(0, 0)]
+        [InlineData(0, 1)]
+        [InlineData(0, 2)]
+        [InlineData(0, 10)]
+        public async Task Prepend(int start, int count)
+        {
+            var xs = await Enumerable.Range(start, count).ToUniTaskAsyncEnumerable().Prepend(99).ToArrayAsync();
+            var ys = Enumerable.Range(start, count).Prepend(99).ToArray();
+
+            xs.Should().BeEquivalentTo(ys);
+        }
+
+        public static IEnumerable<object[]> array1 = new object[][]
+        {
+            new object[] { (0, 0), (0, 0) }, // empty + empty
+            new object[] { (0, 1), (0, 0) }, // 1 + empty
+            new object[] { (0, 0), (0, 1) }, // empty + 1
+            new object[] { (0, 5), (0, 0) }, // 5 + empty
+            new object[] { (0, 0), (0, 5) }, // empty + 5
+            new object[] { (0, 5), (0, 5) }, // 5 + 5
+        };
+
+        [Theory]
+        [MemberData(nameof(array1))]
+        public async Task ConcatTest((int, int) left, (int, int) right)
+        {
+            var l = Enumerable.Range(left.Item1, left.Item2);
+            var r = Enumerable.Range(right.Item1, right.Item2);
+
+            var xs = await l.ToUniTaskAsyncEnumerable().Concat(r.ToUniTaskAsyncEnumerable()).ToArrayAsync();
+            var ys = l.Concat(r).ToArray();
+            xs.Should().BeEquivalentTo(ys);
+        }
+    }
+}