diff --git a/src/UniTask.NetCore/NetCore/UniTask.AsValueTask.cs b/src/UniTask.NetCore/NetCore/UniTask.AsValueTask.cs
index 834968a..e780c62 100644
--- a/src/UniTask.NetCore/NetCore/UniTask.AsValueTask.cs
+++ b/src/UniTask.NetCore/NetCore/UniTask.AsValueTask.cs
@@ -1,123 +1,19 @@
 #pragma warning disable 0649
 
-using System;
-using System.Runtime.CompilerServices;
 using System.Threading.Tasks;
-using System.Threading.Tasks.Sources;
 
 namespace Cysharp.Threading.Tasks
 {
     public static class UniTaskValueTaskExtensions
     {
-        public static ValueTask AsValueTask(this UniTask task)
+        public static ValueTask AsValueTask(this in UniTask task)
         {
-            ref var core = ref Unsafe.As<UniTask, UniTaskToValueTask>(ref task);
-            if (core.source == null)
-            {
-                return default;
-            }
-            
-            return new ValueTask(new UniTaskValueTaskSource(core.source), core.token);
+            return task;
         }
 
-        public static ValueTask<T> AsValueTask<T>(this UniTask<T> task)
+        public static ValueTask<T> AsValueTask<T>(this in UniTask<T> task)
         {
-            ref var core = ref Unsafe.As<UniTask<T>, UniTaskToValueTask<T>>(ref task);
-            if (core.source == null)
-            {
-                return new ValueTask<T>(core.result);
-            }
-
-            return new ValueTask<T>(new UniTaskValueTaskSource<T>(core.source), core.token);
-        }
-
-        struct UniTaskToValueTask
-        {
-            public IUniTaskSource source;
-            public short token;
-        }
-
-        class UniTaskValueTaskSource : IValueTaskSource
-        {
-            readonly IUniTaskSource source;
-
-            public UniTaskValueTaskSource(IUniTaskSource source)
-            {
-                this.source = source;
-            }
-
-            public void GetResult(short token)
-            {
-                source.GetResult(token);
-            }
-
-            public ValueTaskSourceStatus GetStatus(short token)
-            {
-                var status = source.GetStatus(token);
-                switch (status)
-                {
-                    case UniTaskStatus.Pending:
-                        return ValueTaskSourceStatus.Pending;
-                    case UniTaskStatus.Succeeded:
-                        return ValueTaskSourceStatus.Succeeded;
-                    case UniTaskStatus.Faulted:
-                        return ValueTaskSourceStatus.Faulted;
-                    case UniTaskStatus.Canceled:
-                        return ValueTaskSourceStatus.Canceled;
-                    default:
-                        return (ValueTaskSourceStatus)status;
-                }
-            }
-
-            public void OnCompleted(Action<object> continuation, object state, short token, ValueTaskSourceOnCompletedFlags flags)
-            {
-                source.OnCompleted(continuation, state, token);
-            }
-        }
-
-        struct UniTaskToValueTask<T>
-        {
-            public IUniTaskSource<T> source;
-            public T result;
-            public short token;
-        }
-
-        class UniTaskValueTaskSource<T> : IValueTaskSource<T>
-        {
-            readonly IUniTaskSource<T> source;
-
-            public UniTaskValueTaskSource(IUniTaskSource<T> source)
-            {
-                this.source = source;
-            }
-
-            public T GetResult(short token)
-            {
-                return source.GetResult(token);
-            }
-
-            public ValueTaskSourceStatus GetStatus(short token)
-            {
-                var status = source.GetStatus(token);
-                switch (status)
-                {
-                    case UniTaskStatus.Pending:
-                        return ValueTaskSourceStatus.Pending;
-                    case UniTaskStatus.Succeeded:
-                        return ValueTaskSourceStatus.Succeeded;
-                    case UniTaskStatus.Faulted:
-                        return ValueTaskSourceStatus.Faulted;
-                    case UniTaskStatus.Canceled:
-                        return ValueTaskSourceStatus.Canceled;
-                    default:
-                        return (ValueTaskSourceStatus)status;
-                }
-            }
-
-            public void OnCompleted(Action<object> continuation, object state, short token, ValueTaskSourceOnCompletedFlags flags)
-            {
-                source.OnCompleted(continuation, state, token);
-            }
+            return task;
         }
     }
 }
diff --git a/src/UniTask.NetCore/UniTask.NetCore.csproj b/src/UniTask.NetCore/UniTask.NetCore.csproj
index 23b0cda..6a9b15d 100644
--- a/src/UniTask.NetCore/UniTask.NetCore.csproj
+++ b/src/UniTask.NetCore/UniTask.NetCore.csproj
@@ -1,8 +1,9 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFramework>netstandard2.0</TargetFramework>
+        <TargetFramework>netstandard2.1</TargetFramework>
         <AssemblyName>UniTask</AssemblyName>
+        <LangVersion>8.0</LangVersion>
         <RootNamespace>Cysharp.Threading.Tasks</RootNamespace>
     </PropertyGroup>
 
diff --git a/src/UniTask.NetCoreSandbox/Program.cs b/src/UniTask.NetCoreSandbox/Program.cs
index 32de3a0..ee1989e 100644
--- a/src/UniTask.NetCoreSandbox/Program.cs
+++ b/src/UniTask.NetCoreSandbox/Program.cs
@@ -21,6 +21,25 @@ namespace NetCoreSandbox
         public string text { get; set; }
     }
 
+    public class ZeroAllocAsyncAwaitInDotNetCore
+    {
+        public ValueTask<int> NanikaAsync(int x, int y)
+        {
+            return Core(this, x, y);
+
+            static async UniTask<int> Core(ZeroAllocAsyncAwaitInDotNetCore self, int x, int y)
+            {
+                // nanika suru...
+                await Task.Delay(TimeSpan.FromSeconds(x + y));
+
+                return 10;
+            }
+        }
+    }
+
+
+
+
     public static partial class UnityUIComponentExtensions
     {
         public static void BindTo(this IUniTaskAsyncEnumerable<string> source, Text text)
@@ -88,8 +107,10 @@ namespace NetCoreSandbox
 
 
 
-        static void Main(string[] args)
+        static async Task Main(string[] args)
         {
+            var foo = await new ZeroAllocAsyncAwaitInDotNetCore().NanikaAsync(1, 2);
+            Console.WriteLine(foo);
 
             var channel = Channel.CreateSingleConsumerUnbounded<int>();
 
@@ -99,7 +120,7 @@ namespace NetCoreSandbox
 
             var token = cts.Token;
 
-            FooAsync(token).ForEachAsync(x => { }, token);
+            await FooAsync(token).ForEachAsync(x => { }, token);
 
 
             // Observable.Range(1,10).CombineLatest(
diff --git a/src/UniTask/Assets/Plugins/UniTask/Runtime/IUniTaskSource.cs b/src/UniTask/Assets/Plugins/UniTask/Runtime/IUniTaskSource.cs
index 6b7e26c..4904d14 100644
--- a/src/UniTask/Assets/Plugins/UniTask/Runtime/IUniTaskSource.cs
+++ b/src/UniTask/Assets/Plugins/UniTask/Runtime/IUniTaskSource.cs
@@ -19,17 +19,75 @@ namespace Cysharp.Threading.Tasks
 
     // similar as IValueTaskSource
     public interface IUniTaskSource
+#if !UNITY_2018_3_OR_NEWER
+        : System.Threading.Tasks.Sources.IValueTaskSource
+#pragma warning disable CS0108
+#endif
     {
         UniTaskStatus GetStatus(short token);
         void OnCompleted(Action<object> continuation, object state, short token);
         void GetResult(short token);
 
         UniTaskStatus UnsafeGetStatus(); // only for debug use.
+
+#if !UNITY_2018_3_OR_NEWER
+#pragma warning restore CS0108
+
+        System.Threading.Tasks.Sources.ValueTaskSourceStatus System.Threading.Tasks.Sources.IValueTaskSource.GetStatus(short token)
+        {
+            return (System.Threading.Tasks.Sources.ValueTaskSourceStatus)(int)((IUniTaskSource)this).GetStatus(token);
+        }
+
+        void System.Threading.Tasks.Sources.IValueTaskSource.GetResult(short token)
+        {
+            ((IUniTaskSource)this).GetResult(token);
+        }
+
+        void System.Threading.Tasks.Sources.IValueTaskSource.OnCompleted(Action<object> continuation, object state, short token, System.Threading.Tasks.Sources.ValueTaskSourceOnCompletedFlags flags)
+        {
+            // ignore flags, always none.
+            ((IUniTaskSource)this).OnCompleted(continuation, state, token);
+        }
+
+#endif
     }
 
     public interface IUniTaskSource<out T> : IUniTaskSource
+#if !UNITY_2018_3_OR_NEWER
+        , System.Threading.Tasks.Sources.IValueTaskSource<T>
+#endif
     {
         new T GetResult(short token);
+
+#if !UNITY_2018_3_OR_NEWER
+
+        new public UniTaskStatus GetStatus(short token)
+        {
+            return ((IUniTaskSource)this).GetStatus(token);
+        }
+
+        new public void OnCompleted(Action<object> continuation, object state, short token)
+        {
+            ((IUniTaskSource)this).OnCompleted(continuation, state, token);
+        }
+
+        System.Threading.Tasks.Sources.ValueTaskSourceStatus System.Threading.Tasks.Sources.IValueTaskSource<T>.GetStatus(short token)
+        {
+            return (System.Threading.Tasks.Sources.ValueTaskSourceStatus)(int)((IUniTaskSource)this).GetStatus(token);
+        }
+
+        T System.Threading.Tasks.Sources.IValueTaskSource<T>.GetResult(short token)
+        {
+            return ((IUniTaskSource<T>)this).GetResult(token);
+        }
+
+        void System.Threading.Tasks.Sources.IValueTaskSource<T>.OnCompleted(Action<object> continuation, object state, short token, System.Threading.Tasks.Sources.ValueTaskSourceOnCompletedFlags flags)
+        {
+            // ignore flags, always none.
+            ((IUniTaskSource)this).OnCompleted(continuation, state, token);
+        }
+
+#endif
     }
 
     public static class UniTaskStatusExtensions
diff --git a/src/UniTask/Assets/Plugins/UniTask/Runtime/UniTask.cs b/src/UniTask/Assets/Plugins/UniTask/Runtime/UniTask.cs
index 35bf8b6..36d5b6e 100644
--- a/src/UniTask/Assets/Plugins/UniTask/Runtime/UniTask.cs
+++ b/src/UniTask/Assets/Plugins/UniTask/Runtime/UniTask.cs
@@ -68,6 +68,20 @@ namespace Cysharp.Threading.Tasks
             return new UniTask<bool>(new IsCanceledSource(source), token);
         }
 
+#if !UNITY_2018_3_OR_NEWER
+
+        public static implicit operator System.Threading.Tasks.ValueTask(in UniTask self)
+        {
+            if (self.source == null)
+            {
+                return default;
+            }
+
+            return new System.Threading.Tasks.ValueTask(self.source, self.token);
+        }
+
+#endif
+
         public override string ToString()
         {
             if (source == null) return "()";
@@ -414,6 +428,20 @@ namespace Cysharp.Threading.Tasks
             return self.AsUniTask();
         }
 
+#if !UNITY_2018_3_OR_NEWER
+
+        public static implicit operator System.Threading.Tasks.ValueTask<T>(in UniTask<T> self)
+        {
+            if (self.source == null)
+            {
+                return new System.Threading.Tasks.ValueTask<T>(self.result);
+            }
+
+            return new System.Threading.Tasks.ValueTask<T>(self.source, self.token);
+        }
+
+#endif
+
         /// <summary>
         /// returns (bool IsCanceled, T Result) instead of throws OperationCanceledException.
         /// </summary>