Add a Converter to serialize empty strings as null values

Allows these values to be excluded when `IgnoreNullValues` is True

ClassLibraries/Macross.Json.Extensions/Code/System.Text.Json.Serialization/JsonEmptyStringToNullConverter.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Macross.Json.Extensions.System.Text.Json.Serialization
+{
+	/// <summary>
+	/// <see cref="JsonConverter"/> to output empty strings as null values.
+	/// </summary>
+	public class JsonEmptyStringToNullConverter : JsonConverter<string>
+	{
+		/// <inheritdoc/>
+		public override string Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) => reader.GetString();
+
+		/// <inheritdoc/>
+		public override void Write(Utf8JsonWriter writer, string value, JsonSerializerOptions options)
+		{
+#pragma warning disable CA1062 // Don't perform checks for performance. Trust our callers will be nice.
+			if (string.IsNullOrEmpty(value))
+				writer.WriteNullValue();
+			else
+				writer.WriteStringValue(value);
+#pragma warning restore CA1062 // Validate arguments of public methods
+		}
+	}
+}

ClassLibraries/Macross.Json.Extensions/Test/JsonEmptyStringToNullConverterTests.cs

@@ -0,0 +1,73 @@
+using System.Text.Json;
+
+using Macross.Json.Extensions.System.Text.Json.Serialization;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace Macross.Json.Extensions.Tests
+{
+	[TestClass]
+	public class JsonEmptyStringToNullConverterTests
+	{
+		private const string? NullString = null;
+
+		[TestMethod]
+		public void TestNullIsOutputForEmptyString()
+		{
+			JsonSerializerOptions options = new JsonSerializerOptions();
+			options.Converters.Add(new JsonEmptyStringToNullConverter());
+
+			string Json = JsonSerializer.Serialize("Test", options);
+			Assert.AreEqual(@"""Test""", Json);
+
+			Json = JsonSerializer.Serialize(string.Empty, options);
+			Assert.AreEqual("null", Json);
+
+			Json = JsonSerializer.Serialize(NullString, options);
+			Assert.AreEqual("null", Json);
+
+			Json = JsonSerializer.Serialize(new TestClass(), options);
+			Assert.AreEqual("{\"Str1\":null,\"Str2\":null}", Json);
+
+			Json = JsonSerializer.Serialize(new TestClass() { Str1 = "One" }, options);
+			Assert.AreEqual("{\"Str1\":\"One\",\"Str2\":null}", Json);
+
+			Json = JsonSerializer.Serialize(new TestClass() { Str1 = "One", Str2 = "Two" }, options);
+			Assert.AreEqual("{\"Str1\":\"One\",\"Str2\":\"Two\"}", Json);
+
+			Json = JsonSerializer.Serialize(new TestClass() { Str2 = "Two" }, options);
+			Assert.AreEqual("{\"Str1\":null,\"Str2\":\"Two\"}", Json);
+
+			options = new JsonSerializerOptions();
+			options.Converters.Add(new JsonEmptyStringToNullConverter());
+			options.IgnoreNullValues = true;
+
+			Json = JsonSerializer.Serialize("Test", options);
+			Assert.AreEqual(@"""Test""", Json);
+
+			Json = JsonSerializer.Serialize(string.Empty, options);
+			Assert.AreEqual("null", Json);
+
+			Json = JsonSerializer.Serialize(NullString, options);
+			Assert.AreEqual("null", Json);
+
+			Json = JsonSerializer.Serialize(new TestClass(), options);
+			Assert.AreEqual("{}", Json);
+
+			Json = JsonSerializer.Serialize(new TestClass() { Str1 = "One" }, options);
+			Assert.AreEqual("{\"Str1\":\"One\"}", Json);
+
+			Json = JsonSerializer.Serialize(new TestClass() { Str1 = "One", Str2 = "Two" }, options);
+			Assert.AreEqual("{\"Str1\":\"One\",\"Str2\":\"Two\"}", Json);
+
+			Json = JsonSerializer.Serialize(new TestClass() { Str2 = "Two" }, options);
+			Assert.AreEqual("{\"Str2\":\"Two\"}", Json);
+		}
+
+		private class TestClass
+		{
+			public string? Str1 { get; set; }
+
+			public string? Str2 { get; set; }
+		}
+	}
+}