System.Text.Json で static プロパティをシリアライズする方法（JSON Contract 使用）

public class StaticTest
{
	public int NonStatic { get; set; }
	public static int Static1 { get; set; }
	public static int Static2 { get; set; }
}

	[HttpGet]
	[Route("/static/test")]
	public async Task<IActionResult> TestStatic()
	{
		var s = new StaticTest
		{
			NonStatic = 1,
		};
		StaticTest.Static1 = 1;
		StaticTest.Static2 = 1;
		return Ok(s);
	}

[AttributeUsage(AttributeTargets.Property | AttributeTargets.Class)]
public class JsonIncludeStaticAttribute : Attribute { }

	public static void IncludeStaticModifier(JsonTypeInfo typeInfo)
	{
		if (typeInfo.Kind != JsonTypeInfoKind.Object) return;
		if (!typeInfo.Type.IsDefined(typeof(JsonIncludeStaticAttribute), false)) return;
		var propertyInfoList = typeInfo.Type.GetProperties(BindingFlags.Static | BindingFlags.Public);

		foreach (var propertyInfo in propertyInfoList)
		{
			if (HasAttribute<JsonIncludeStaticAttribute>(propertyInfo))
			{
				JsonPropertyInfo jsonPropertyInfo = typeInfo.CreateJsonPropertyInfo(propertyInfo.PropertyType, propertyInfo.Name);
				jsonPropertyInfo.Get = CreateGetter(propertyInfo);
				typeInfo.Properties.Add(jsonPropertyInfo);
			}
		}
	}

	private static bool HasAttribute<T>(PropertyInfo propertyInfo)
	{
		object[]? customAttributes = propertyInfo.GetCustomAttributes(typeof(T), true);
		if (customAttributes is not null && customAttributes.Length == 1)
		{
			return true;
		}
		else
		{
			return false;
		}
	}

	static Func<object, object?>? CreateGetter(PropertyInfo propertyInfo)
	{
		if (!(propertyInfo.GetGetMethod() is { } getMethod)) return null;
        // publicでないstaticメソッドを検索対象とする
		var typedCreator = typeof(CustomJsonContracts).GetMethod(nameof(CreateGetterFunc), BindingFlags.Static | BindingFlags.NonPublic);
		// MakeGenericMethodの引数にジェネリック型に渡す型を指定する
        var getterFunc = typedCreator!.MakeGenericMethod(propertyInfo.PropertyType);
        // staticメソッドの場合、第一引数はnull
		return (Func<object, object?>?)getterFunc.Invoke(null, [getMethod]);
	}

	static Func<object, object?> CreateGetterFunc<T>(MethodInfo methodInfo)
	{
		var typedFunc = methodInfo.CreateDelegate<Func<T?>>();
		return (o => typedFunc());
	}

[JsonIncludeStatic]
public class StaticTest
{
	public int NonStatic { get; set; }
	[JsonIncludeStatic]
	public static int Static1 { get; set; }
	public static int Static2 { get; set; }
}

	[HttpGet]
	[Route("/static/test")]
	public async Task<IActionResult> TestStatic()
	{
		var s = new StaticTest
		{
			NonStatic = 1,
		};
		StaticTest.Static1 = 1;
		StaticTest.Static2 = 1;
		JsonSerializerOptions serializerOption = new()
		{
			WriteIndented = true,
			TypeInfoResolver = new DefaultJsonTypeInfoResolver
			{
				Modifiers = { CustomJsonContracts.IncludeStaticModifier }
			}
		};
		var json = JsonSerializer.Serialize(s, serializerOption);
		return Ok(json);
	}