Restructure the C# bindings for Yoga to be based on .NET Standard 2.0 and the new .NET 6.0+ target platform work.

csharp/Facebook.Yoga/Facebook.Yoga.csproj

@@ -0,0 +1,5 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFrameworks>netstandard2.0;net6.0-ios</TargetFrameworks>
+  </PropertyGroup>
+</Project>

csharp/Facebook.Yoga/MonoPInvokeCallbackAttribute.cs

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+using System;
+
+namespace Facebook.Yoga
+{
+    // This attribute is recognized by the Mono AOT compiler by name
+    // and indicates that marshalling information must be generated for 
+    // the attributed method as if it is marshalled with the provided delegate
+    // type to unmanaged code.
+    // We define our own copy to avoid having to conditionally build this project for
+    // iOS and Unity.
+    [System.AttributeUsage(System.AttributeTargets.Method)]
+    internal sealed class MonoPInvokeCallbackAttribute : System.Attribute
+    {
+        public MonoPInvokeCallbackAttribute(Type t)
+        {
+            DelegateType = t;
+        }
+
+        public Type DelegateType { get; set; }
+    }
+}

csharp/Facebook.Yoga/Native.cs

@@ -12,7 +12,9 @@ namespace Facebook.Yoga
 {
     internal static class Native
     {
-#if (UNITY_IOS && !UNITY_EDITOR) || __IOS__
+        // Unity package references work via embedded source with their custom package format,
+        // so these Unity-specific defines will work.
+#if (UNITY_IOS && !UNITY_EDITOR) || IOS
         private const string DllName = "__Internal";
 #else
         private const string DllName = "yoga";

csharp/Facebook.Yoga/YogaConfig.cs

@@ -8,13 +8,6 @@
 using System;
 using System.Runtime.InteropServices;
 
-#if __IOS__
-using ObjCRuntime;
-#endif
-#if ENABLE_IL2CPP
-using AOT;
-#endif
-
 namespace Facebook.Yoga
 {
     public class YogaConfig
@@ -54,9 +47,7 @@ internal YGConfigHandle Handle
             }
         }
 
-#if (UNITY_IOS && !UNITY_EDITOR) || ENABLE_IL2CPP || __IOS__
         [MonoPInvokeCallback(typeof(YogaLogger))]
-#endif
         private static void LoggerInternal(
             IntPtr unmanagedConfigPtr,
             IntPtr unmanagedNodePtr,

csharp/Facebook.Yoga/YogaNode.cs

@@ -11,13 +11,6 @@
 using System.Runtime.InteropServices;
 using System.Text;
 
-#if __IOS__
-using ObjCRuntime;
-#endif
-#if ENABLE_IL2CPP
-using AOT;
-#endif
-
 namespace Facebook.Yoga
 {
     public partial class YogaNode : IEnumerable<YogaNode>
@@ -656,9 +649,7 @@ public void SetBaselineFunction(BaselineFunction baselineFunction)
                 Native.YGNodeStyleGetDirection(_ygNode));
         }
 
-#if (UNITY_IOS && !UNITY_EDITOR) || ENABLE_IL2CPP || __IOS__
         [MonoPInvokeCallback(typeof(YogaMeasureFunc))]
-#endif
         private static YogaSize MeasureInternal(
             IntPtr unmanagedNodePtr,
             float width,
@@ -674,9 +665,7 @@ public void SetBaselineFunction(BaselineFunction baselineFunction)
             return node._measureFunction(node, width, widthMode, height, heightMode);
         }
 
-#if (UNITY_IOS && !UNITY_EDITOR) || ENABLE_IL2CPP || __IOS__
         [MonoPInvokeCallback(typeof(YogaBaselineFunc))]
-#endif
         private static float BaselineInternal(
             IntPtr unmanagedNodePtr,
             float width,