Add property ordering feature

src/libraries/System.Text.Json/ref/System.Text.Json.cs

@@ -833,6 +833,12 @@ public sealed partial class JsonPropertyNameAttribute : System.Text.Json.Seriali
         public JsonPropertyNameAttribute(string name) { }
         public string Name { get { throw null; } }
     }
+    [System.AttributeUsageAttribute(System.AttributeTargets.Field | System.AttributeTargets.Property, AllowMultiple = false)]
+    public sealed partial class JsonPropertyOrderAttribute : System.Text.Json.Serialization.JsonAttribute
+    {
+        public JsonPropertyOrderAttribute(int order) { }
+        public int Order { get { throw null; } }
+    }
     [System.AttributeUsageAttribute(System.AttributeTargets.Class, AllowMultiple=true)]
     public sealed partial class JsonSerializableAttribute : System.Text.Json.Serialization.JsonAttribute
     {

src/libraries/System.Text.Json/src/System.Text.Json.csproj

@@ -22,7 +22,7 @@
     <Compile Include="..\Common\JsonCamelCaseNamingPolicy.cs" Link="Common\System\Text\Json\JsonCamelCaseNamingPolicy.cs" />
     <Compile Include="..\Common\JsonNamingPolicy.cs" Link="Common\System\Text\Json\JsonNamingPolicy.cs" />
     <Compile Include="..\Common\JsonAttribute.cs" Link="Common\System\Text\Json\Serialization\JsonAttribute.cs" />
-    <Compile Include="..\Common\JsonHelpers.cs" Link ="Common\System\Text\Json\JsonHelpers.cs" />
+    <Compile Include="..\Common\JsonHelpers.cs" Link="Common\System\Text\Json\JsonHelpers.cs" />
     <Compile Include="..\Common\JsonIgnoreCondition.cs" Link="Common\System\Text\Json\Serialization\JsonIgnoreCondition.cs" />
     <Compile Include="..\Common\JsonKnownNamingPolicy.cs" Link="Common\System\Text\Json\Serialization\JsonKnownNamingPolicy.cs" />
     <Compile Include="..\Common\JsonNumberHandling.cs" Link="Common\System\Text\Json\Serialization\JsonNumberHandling.cs" />
@@ -92,6 +92,7 @@
     <Compile Include="System\Text\Json\Serialization\Attributes\JsonIncludeAttribute.cs" />
     <Compile Include="System\Text\Json\Serialization\Attributes\JsonNumberHandlingAttribute.cs" />
     <Compile Include="System\Text\Json\Serialization\Attributes\JsonPropertyNameAttribute.cs" />
+    <Compile Include="System\Text\Json\Serialization\Attributes\JsonPropertyOrderAttribute.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\JsonMetadataServicesConverter.cs" />
     <Compile Include="System\Text\Json\Serialization\IgnoreReferenceResolver.cs" />
     <Compile Include="System\Text\Json\Serialization\IJsonOnDeserialized.cs" />

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Attributes/JsonPropertyOrderAttribute.cs

@@ -0,0 +1,28 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Text.Json.Serialization
+{
+    /// <summary>
+    /// Specifies the property order that is present in the JSON when serializing. Lower values are serialized first.
+    /// If the attribute is not specified, the default value is 0.
+    /// </summary>
+    /// <remarks>If multiple properties have the same value, the ordering is undefined between them.</remarks>
+    [AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
+    public sealed class JsonPropertyOrderAttribute : JsonAttribute
+    {
+        /// <summary>
+        /// Initializes a new instance of <see cref="JsonPropertyNameAttribute"/> with the specified order.
+        /// </summary>
+        /// <param name="order">The order of the property.</param>
+        public JsonPropertyOrderAttribute(int order)
+        {
+            Order = order;
+        }
+
+        /// <summary>
+        /// The serialization order of the property.
+        /// </summary>
+        public int Order { get; }
+    }
+}

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonPropertyInfo.cs

@@ -74,6 +74,12 @@ internal virtual void GetPolicies(JsonIgnoreCondition? ignoreCondition, JsonNumb
                 DeterminePropertyName();
                 DetermineIgnoreCondition(ignoreCondition);
 
+                JsonPropertyOrderAttribute? orderAttr = GetAttribute<JsonPropertyOrderAttribute>(MemberInfo);
+                if (orderAttr != null)
+                {
+                    Order = orderAttr.Order;
+                }
+
                 JsonNumberHandlingAttribute? attribute = GetAttribute<JsonNumberHandlingAttribute>(MemberInfo);
                 DetermineNumberHandlingForProperty(attribute?.Handling, declaringTypeNumberHandling);
             }
@@ -366,6 +372,11 @@ internal abstract void InitializeForTypeInfo(
 
         internal JsonSerializerOptions Options { get; set; } = null!; // initialized in Init method
 
+        /// <summary>
+        /// The property order.
+        /// </summary>
+        internal int Order { get; set; }
+
         internal bool ReadJsonAndAddExtensionProperty(
             object obj,
             ref ReadStack state,

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonTypeInfo.cs

@@ -197,6 +197,8 @@ internal JsonTypeInfo(Type type, JsonConverter converter, Type runtimeType, Json
 
                         PropertyInfo[] properties = type.GetProperties(bindingFlags);
 
+                        bool propertyOrderSpecified = false;
+
                         // PropertyCache is not accessed by other threads until the current JsonTypeInfo instance
                         //  is finished initializing and added to the cache on JsonSerializerOptions.
                         // Default 'capacity' to the common non-polymorphic + property case.
@@ -229,6 +231,7 @@ internal JsonTypeInfo(Type type, JsonConverter converter, Type runtimeType, Json
                                         propertyInfo,
                                         isVirtual,
                                         typeNumberHandling,
+                                        ref propertyOrderSpecified,
                                         ref ignoredMembers);
                                 }
                                 else
@@ -263,6 +266,7 @@ internal JsonTypeInfo(Type type, JsonConverter converter, Type runtimeType, Json
                                             fieldInfo,
                                             isVirtual: false,
                                             typeNumberHandling,
+                                            ref propertyOrderSpecified,
                                             ref ignoredMembers);
                                     }
                                 }
@@ -286,6 +290,11 @@ internal JsonTypeInfo(Type type, JsonConverter converter, Type runtimeType, Json
                             properties = currentType.GetProperties(bindingFlags);
                         };
 
+                        if (propertyOrderSpecified)
+                        {
+                            PropertyCache.List.Sort((p1, p2) => p1.Value!.Order.CompareTo(p2.Value!.Order));
+                        }
+
                         if (converter.ConstructorIsParameterized)
                         {
                             InitializeConstructorParameters(converter.ConstructorInfo!);
@@ -327,6 +336,7 @@ private void CacheMember(
             MemberInfo memberInfo,
             bool isVirtual,
             JsonNumberHandling? typeNumberHandling,
+            ref bool propertyOrderSpecified,
             ref Dictionary<string, JsonPropertyInfo>? ignoredMembers)
         {
             bool hasExtensionAttribute = memberInfo.GetCustomAttribute(typeof(JsonExtensionDataAttribute)) != null;
@@ -347,6 +357,7 @@ private void CacheMember(
             else
             {
                 CacheMember(jsonPropertyInfo, PropertyCache, ref ignoredMembers);
+                propertyOrderSpecified |= jsonPropertyInfo.Order != 0;
             }
         }
 

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/PropertyOrderTests.cs

@@ -0,0 +1,68 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Xunit;
+
+namespace System.Text.Json.Serialization.Tests
+{
+    public static class PropertyOrderTests
+    {
+        private class MyPoco_BeforeAndAfter
+        {
+            public int B { get; set; }
+
+            [JsonPropertyOrder(1)]
+            public int A { get; set; }
+
+            [JsonPropertyOrder(-1)]
+            public int C { get; set; }
+        }
+
+        [Fact]
+        public static void BeforeAndAfterDefaultOrder()
+        {
+            string json = JsonSerializer.Serialize<MyPoco_BeforeAndAfter>(new MyPoco_BeforeAndAfter());
+            Assert.Equal("{\"C\":0,\"B\":0,\"A\":0}", json);
+        }
+
+        private class MyPoco_After
+        {
+            [JsonPropertyOrder(2)]
+            public int C { get; set; }
+
+            public int B { get; set; }
+            public int D { get; set; }
+
+            [JsonPropertyOrder(1)]
+            public int A { get; set; }
+        }
+
+        [Fact]
+        public static void AfterDefaultOrder()
+        {
+            string json = JsonSerializer.Serialize<MyPoco_After>(new MyPoco_After());
+            Assert.EndsWith("\"A\":0,\"C\":0}", json);
+            // Order of B and D are not defined except they come before A and C
+        }
+
+        private class MyPoco_Before
+        {
+            [JsonPropertyOrder(-1)]
+            public int C { get; set; }
+
+            public int B { get; set; }
+            public int D { get; set; }
+
+            [JsonPropertyOrder(-2)]
+            public int A { get; set; }
+        }
+
+        [Fact]
+        public static void BeforeDefaultOrder()
+        {
+            string json = JsonSerializer.Serialize<MyPoco_Before>(new MyPoco_Before());
+            Assert.StartsWith("{\"A\":0,\"C\":0", json);
+            // Order of B and D are not defined except they come after A and C
+        }
+    }
+}

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/System.Text.Json.Tests.csproj

@@ -166,6 +166,7 @@
     <Compile Include="Serialization\OptionsTests.cs" />
     <Compile Include="Serialization\PolymorphicTests.cs" />
     <Compile Include="Serialization\PropertyNameTests.cs" />
+    <Compile Include="Serialization\PropertyOrderTests.cs" />
     <Compile Include="Serialization\PropertyVisibilityTests.cs" />
     <Compile Include="Serialization\ReadScenarioTests.cs" />
     <Compile Include="Serialization\ReadValueTests.cs" />