Add snake_case support for System.Text.Json

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

@@ -137,13 +137,15 @@
     <Compile Include="System\Text\Json\Serialization\JsonConverterOfT.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonDefaultNamingPolicy.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonIgnoreCondition.cs" />
+    <Compile Include="System\Text\Json\Serialization\JsonKebabCaseNamingPolicy.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonNamingPolicy.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonNumberHandling.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonParameterInfo.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonParameterInfoOfT.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonPropertyInfo.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonPropertyInfoOfT.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonResumableConverterOfT.cs" />
+    <Compile Include="System\Text\Json\Serialization\JsonSeparatedCaseNamingPolicy.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Read.HandleMetadata.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Read.HandlePropertyName.cs" />
@@ -161,6 +163,7 @@
     <Compile Include="System\Text\Json\Serialization\JsonSerializerDefaults.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializerOptions.Converters.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializerOptions.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\MetadataPropertyName.cs" />

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonKebabCaseNamingPolicy.cs

@@ -0,0 +1,10 @@
+// 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
+{
+    internal class JsonKebabCaseNamingPolicy : JsonSeparatedCaseNamingPolicy
+    {
+        public override string ConvertName(string name) => ToSeparatedCase(name, '-');
+    }
+}

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

@@ -20,6 +20,15 @@ protected JsonNamingPolicy() { }
 
         internal static JsonNamingPolicy Default { get; } = new JsonDefaultNamingPolicy();
 
+        /// <summary>
+        /// Returns the naming policy for snake-casing.
+        /// </summary>
+        public static JsonNamingPolicy SnakeCase { get; } = new JsonSnakeCaseNamingPolicy();
+
+        // KebabCase was added together with SnakeCase because sharing implementation.
+        // Once approved it can be made public.
+        internal static JsonNamingPolicy KebabCase { get; } = new JsonKebabCaseNamingPolicy();
+
         /// <summary>
         /// When overridden in a derived class, converts the specified name according to the policy.
         /// </summary>

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSeparatedCaseNamingPolicy.cs

@@ -0,0 +1,82 @@
+// 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
+{
+    internal abstract class JsonSeparatedCaseNamingPolicy : JsonNamingPolicy
+    {
+        private enum SeparatedCaseState
+        {
+            Start,
+            NewWord,
+            Upper,
+            Lower
+        }
+
+        protected static string ToSeparatedCase(string s, char separator)
+        {
+            if (string.IsNullOrEmpty(s))
+            {
+                return s;
+            }
+
+            StringBuilder sb = new StringBuilder();
+            SeparatedCaseState state = SeparatedCaseState.Start;
+
+            for (int i = 0; i < s.Length; i++)
+            {
+                if (s[i] == ' ')
+                {
+                    if (state != SeparatedCaseState.Start)
+                    {
+                        state = SeparatedCaseState.NewWord;
+                    }
+                }
+                else if (char.IsUpper(s[i]))
+                {
+                    switch (state)
+                    {
+                        case SeparatedCaseState.Upper:
+                            bool hasNext = (i + 1 < s.Length);
+                            if (i > 0 && hasNext)
+                            {
+                                char nextChar = s[i + 1];
+                                if (!char.IsUpper(nextChar) && nextChar != separator)
+                                {
+                                    sb.Append(separator);
+                                }
+                            }
+                            break;
+                        case SeparatedCaseState.Lower:
+                        case SeparatedCaseState.NewWord:
+                            sb.Append(separator);
+                            break;
+                    }
+
+                    char c;
+                    c = char.ToLowerInvariant(s[i]);
+                    sb.Append(c);
+
+                    state = SeparatedCaseState.Upper;
+                }
+                else if (s[i] == separator)
+                {
+                    sb.Append(separator);
+                    state = SeparatedCaseState.Start;
+                }
+                else
+                {
+                    if (state == SeparatedCaseState.NewWord)
+                    {
+                        sb.Append(separator);
+                    }
+
+                    sb.Append(s[i]);
+                    state = SeparatedCaseState.Lower;
+                }
+            }
+
+            return sb.ToString();
+        }
+    }
+}

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

@@ -0,0 +1,10 @@
+// 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
+{
+    internal class JsonSnakeCaseNamingPolicy : JsonSeparatedCaseNamingPolicy
+    {
+        public override string ConvertName(string name) => ToSeparatedCase(name, '_');
+    }
+}

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

@@ -0,0 +1,48 @@
+// 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 class SnakeCaseUnitTests
+    {
+        [Fact]
+        public static void ToCamelCaseTest()
+        {
+            // 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/libraries/System.Text.Json/tests/System.Text.Json.Tests.csproj

@@ -114,6 +114,7 @@
     <Compile Include="Serialization\ReferenceHandlerTests.Serialize.cs" />
     <Compile Include="Serialization\SampleTestData.OrderPayload.cs" />
     <Compile Include="Serialization\SerializationWrapper.cs" />
+    <Compile Include="Serialization\SnakeCaseUnitTests.cs" />
     <Compile Include="Serialization\SpanTests.cs" />
     <Compile Include="Serialization\Stream.Collections.cs" />
     <Compile Include="Serialization\Stream.ReadTests.cs" />