Add support for JsonSnakeNamingPolicy

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

@@ -154,6 +154,7 @@ public abstract partial class JsonNamingPolicy
     {
         protected JsonNamingPolicy() { }
         public static System.Text.Json.JsonNamingPolicy CamelCase { get { throw null; } }
+        public static System.Text.Json.JsonNamingPolicy SnakeCase { get { throw null; } }
         public abstract string ConvertName(string name);
     }
     public readonly partial struct JsonProperty

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

@@ -129,6 +129,7 @@
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Write.Utf8JsonWriter.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializerOptions.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializerOptions.Converters.cs" />
+    <Compile Include="System\Text\Json\Serialization\JsonSnakeCaseNamingPolicy.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonStringEnumConverter.cs" />
     <Compile Include="System\Text\Json\Serialization\MemberAccessor.cs" />
     <Compile Include="System\Text\Json\Serialization\PooledByteBufferWriter.cs" />

src/System.Text.Json/src/System/Text/Json/Serialization/JsonNamingPolicy.cs

@@ -19,6 +19,11 @@ protected JsonNamingPolicy() { }
         /// </summary>
         public static JsonNamingPolicy CamelCase { get; } = new JsonCamelCaseNamingPolicy();
 
+        /// <summary>
+        /// Returns the naming policy for snake-casing.
+        /// </summary>
+        public static JsonNamingPolicy SnakeCase { get; } = new JsonSnakeCaseNamingPolicy();
+
         internal static JsonNamingPolicy Default { get; } = new JsonDefaultNamingPolicy();
 
         /// <summary>

src/System.Text.Json/src/System/Text/Json/Serialization/JsonSnakeCaseNamingPolicy.cs

@@ -0,0 +1,81 @@
+// 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.
+
+namespace System.Text.Json
+{
+    internal sealed class JsonSnakeCaseNamingPolicy : JsonNamingPolicy
+    {
+        internal enum SnakeCaseState
+        {
+            Start,
+            Lower,
+            Upper,
+            NewWord
+        }
+
+        public override string ConvertName(string name)
+        {
+            if (string.IsNullOrEmpty(name))
+            {
+                return name;
+            }
+
+            var sb = new StringBuilder();
+            var state = SnakeCaseState.Start;
+
+            var nameSpan = name.AsSpan();
+
+            for (int i = 0; i < nameSpan.Length; i++)
+            {
+                if (nameSpan[i] == ' ')
+                {
+                    if (state != SnakeCaseState.Start)
+                    {
+                        state = SnakeCaseState.NewWord;
+                    }
+                }
+                else if (char.IsUpper(nameSpan[i]))
+                {
+                    switch (state)
+                    {
+                        case SnakeCaseState.Upper:
+                            bool hasNext = (i + 1 < nameSpan.Length);
+                            if (i > 0 && hasNext)
+                            {
+                                char nextChar = nameSpan[i + 1];
+                                if (!char.IsUpper(nextChar) && nextChar != '_')
+                                {
+                                    sb.Append('_');
+                                }
+                            }
+                            break;
+                        case SnakeCaseState.Lower:
+                        case SnakeCaseState.NewWord:
+                            sb.Append('_');
+                            break;
+                    }
+                    sb.Append(char.ToLowerInvariant(nameSpan[i]));
+                    state = SnakeCaseState.Upper;
+                }
+                else if (nameSpan[i] == '_')
+                {
+                    sb.Append('_');
+                    state = SnakeCaseState.Start;
+                }
+                else
+                {
+                    if (state == SnakeCaseState.NewWord)
+                    {
+                        sb.Append('_');
+                    }
+
+                    sb.Append(nameSpan[i]);
+                    state = SnakeCaseState.Lower;
+                }
+            }
+
+            return sb.ToString();
+        }
+    }
+}

src/System.Text.Json/tests/NewtonsoftTests/CamelCaseTests.cs

@@ -30,26 +30,6 @@
 
 namespace System.Text.Json.Tests
 {
-    internal class Person
-    {
-        public string Name { get; set; }
-
-        public DateTime BirthDate { get; set; }
-
-        public DateTime LastModified { get; set; }
-
-        [JsonIgnore]
-        public string Department { get; set; }
-    }
-
-    internal class Product
-    {
-        public string Name { get; set; }
-        public DateTime ExpiryDate { get; set;}
-        public decimal Price { get; set; }
-        public string[] Sizes { get; set; }
-    }
-
     public class CamelCaseTests
     {
         private static readonly JsonSerializerOptions s_camelCaseAndIndentedOption = new JsonSerializerOptions

src/System.Text.Json/tests/NewtonsoftTests/NamingTestBase.cs

@@ -0,0 +1,46 @@
+// Copyright (c) 2007 James Newton-King
+//
+// Permission is hereby granted, free of charge, to any person
+// obtaining a copy of this software and associated documentation
+// files (the "Software"), to deal in the Software without
+// restriction, including without limitation the rights to use,
+// copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following
+// conditions:
+//
+// The above copyright notice and this permission notice shall be
+// included in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+// HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+// WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+// OTHER DEALINGS IN THE SOFTWARE.
+
+using System.Text.Json.Serialization;
+namespace System.Text.Json.Tests
+{
+    internal class Person
+    {
+        public string Name { get; set; }
+
+        public DateTime BirthDate { get; set; }
+
+        public DateTime LastModified { get; set; }
+
+        [JsonIgnore]
+        public string Department { get; set; }
+    }
+
+    internal class Product
+    {
+        public string Name { get; set; }
+        public DateTime ExpiryDate { get; set;}
+        public decimal Price { get; set; }
+        public string[] Sizes { get; set; }
+    }
+}

src/System.Text.Json/tests/NewtonsoftTests/SnakeCaseTests.cs

@@ -0,0 +1,92 @@
+// Copyright (c) 2007 James Newton-King
+//
+// Permission is hereby granted, free of charge, to any person
+// obtaining a copy of this software and associated documentation
+// files (the "Software"), to deal in the Software without
+// restriction, including without limitation the rights to use,
+// copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following
+// conditions:
+//
+// The above copyright notice and this permission notice shall be
+// included in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+// HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+// WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+// OTHER DEALINGS IN THE SOFTWARE.
+
+using System.Text.Json.Serialization;
+using Xunit;
+
+namespace System.Text.Json.Tests
+{
+    public class SnakeCaseTests
+    {
+        private static readonly JsonSerializerOptions s_snakeCaseAndIndentedOption = new JsonSerializerOptions
+        {
+            PropertyNamingPolicy = JsonNamingPolicy.SnakeCase,
+            WriteIndented = true,
+        };
+
+        [Fact]
+        public void JsonSerializerSnakeCaseSettings()
+        {
+            Person person = new Person();
+            person.BirthDate = new DateTime(2000, 11, 20, 23, 55, 44, DateTimeKind.Utc);
+            person.LastModified = new DateTime(2000, 11, 20, 23, 55, 44, DateTimeKind.Utc);
+            person.Name = "Name!";
+
+            string json = JsonSerializer.Serialize(person, s_snakeCaseAndIndentedOption);
+
+            Assert.Equal(@"{
+  ""name"": ""Name!"",
+  ""birth_date"": ""2000-11-20T23:55:44Z"",
+  ""last_modified"": ""2000-11-20T23:55:44Z""
+}", json);
+
+            Person deserializedPerson = JsonSerializer.Deserialize<Person>(json, s_snakeCaseAndIndentedOption);
+
+            Assert.Equal(person.BirthDate, deserializedPerson.BirthDate);
+            Assert.Equal(person.LastModified, deserializedPerson.LastModified);
+            Assert.Equal(person.Name, deserializedPerson.Name);
+
+            json = JsonSerializer.Serialize(person, new JsonSerializerOptions { WriteIndented = true });
+            Assert.Equal(@"{
+  ""Name"": ""Name!"",
+  ""BirthDate"": ""2000-11-20T23:55:44Z"",
+  ""LastModified"": ""2000-11-20T23:55:44Z""
+}", json);
+        }
+
+        [Fact]
+        public void BlogPostExample()
+        {
+            Product product = new Product
+            {
+                ExpiryDate = new DateTime(2010, 12, 20, 18, 1, 0, DateTimeKind.Utc),
+                Name = "Widget",
+                Price = 9.99m,
+                Sizes = new[] { "Small", "Medium", "Large" }
+            };
+
+            string json = JsonSerializer.Serialize(product, s_snakeCaseAndIndentedOption);
+
+            Assert.Equal(@"{
+  ""name"": ""Widget"",
+  ""expiry_date"": ""2010-12-20T18:01:00Z"",
+  ""price"": 9.99,
+  ""sizes"": [
+    ""Small"",
+    ""Medium"",
+    ""Large""
+  ]
+}", json);
+        }
+    }
+}

src/System.Text.Json/tests/Serialization/SnakeCaseUnitTests.cs

@@ -0,0 +1,50 @@
+// 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.Reflection;
+using Xunit;
+
+namespace System.Text.Json.Serialization.Tests
+{
+    public static class SnakeCaseUnitTests
+    {
+        [Fact]
+        public static void ConvertToSnakeCaseTest()
+        {
+            // These test cases were copied from Json.NET.
+            Assert.Equal("url_value", ConvertToSnakeCase("URLValue"));
+            Assert.Equal("url", ConvertToSnakeCase("URL"));
+            Assert.Equal("id", ConvertToSnakeCase("ID"));
+            Assert.Equal("i", ConvertToSnakeCase("I"));
+            Assert.Equal("", ConvertToSnakeCase(""));
+            Assert.Null(ConvertToSnakeCase(null));
+            Assert.Equal("person", ConvertToSnakeCase("Person"));
+            Assert.Equal("i_phone", ConvertToSnakeCase("iPhone"));
+            Assert.Equal("i_phone", ConvertToSnakeCase("IPhone"));
+            Assert.Equal("i_phone", ConvertToSnakeCase("I Phone"));
+            Assert.Equal("i_phone", ConvertToSnakeCase("I  Phone"));
+            Assert.Equal("i_phone", ConvertToSnakeCase(" IPhone"));
+            Assert.Equal("i_phone", ConvertToSnakeCase(" IPhone "));
+            Assert.Equal("is_cia", ConvertToSnakeCase("IsCIA"));
+            Assert.Equal("vm_q", ConvertToSnakeCase("VmQ"));
+            Assert.Equal("xml2_json", ConvertToSnakeCase("Xml2Json"));
+            Assert.Equal("sn_ak_ec_as_e", ConvertToSnakeCase("SnAkEcAsE"));
+            Assert.Equal("sn_a__k_ec_as_e", ConvertToSnakeCase("SnA__kEcAsE"));
+            Assert.Equal("sn_a__k_ec_as_e", ConvertToSnakeCase("SnA__ kEcAsE"));
+            Assert.Equal("already_snake_case_", ConvertToSnakeCase("already_snake_case_ "));
+            Assert.Equal("is_json_property", ConvertToSnakeCase("IsJSONProperty"));
+            Assert.Equal("shouting_case", ConvertToSnakeCase("SHOUTING_CASE"));
+            Assert.Equal("9999-12-31_t23:59:59.9999999_z", ConvertToSnakeCase("9999-12-31T23:59:59.9999999Z"));
+            Assert.Equal("hi!!_this_is_text._time_to_test.", ConvertToSnakeCase("Hi!! This is text. Time to test."));
+        }
+
+        // Use a helper method since the method is not public.
+        private static string ConvertToSnakeCase(string name)
+        {
+            JsonNamingPolicy policy = JsonNamingPolicy.SnakeCase;
+            string value = policy.ConvertName(name);
+            return value;
+        }
+    }
+}

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

@@ -32,6 +32,8 @@
     <Compile Include="NewtonsoftTests\EnumConverterTests.cs" />
     <Compile Include="NewtonsoftTests\ImmutableCollectionsTests.cs" />
     <Compile Include="NewtonsoftTests\JsonSerializerTests.cs" />
+    <Compile Include="NewtonsoftTests\NamingTestBase.cs" />
+    <Compile Include="NewtonsoftTests\SnakeCaseTests.cs" />
     <Compile Include="Serialization\Array.ReadTests.cs" />
     <Compile Include="Serialization\Array.WriteTests.cs" />
     <Compile Include="Serialization\CacheTests.cs" />
@@ -68,6 +70,7 @@
     <Compile Include="Serialization\PropertyNameTests.cs" />
     <Compile Include="Serialization\PropertyVisibilityTests.cs" />
     <Compile Include="Serialization\ReadValueTests.cs" />
+    <Compile Include="Serialization\SnakeCaseUnitTests.cs" />
     <Compile Include="Serialization\SpanTests.cs" />
     <Compile Include="Serialization\Stream.ReadTests.cs" />
     <Compile Include="Serialization\Stream.WriteTests.cs" />