Odd dynamic effect with JObject

[jamescurran]

Source/destination types

Gist: https://gist.github.com/jamescurran/fb13fa7563edf8d9e96cf04aab723f99

class Foo
{
	public int A { get; set; }
	public int? Bbb { get; set; }
	public string  C { get; set; }
	public int D { get; set; }
}

	var target = new Foo
	{
		A=123,
		Bbb=456,
		C = "Hello",
		D = 789
	};

	dynamic overlay = ExpandoObject(); //  new  JObject(); //

	overlay.A = (int) 888;
	overlay.Bbb = (int) 999;
	overlay.C = "Greetings";

Mapping configuration

	var configuration = new MapperConfiguration(cfg => { });
	var mapper = new Mapper(configuration);

	mapper.Map<dynamic, Foo>(overlay,target);

Version: 13.0.1

Expected behavior

A | 888
Bbb | 999
C | Greetings
D | 789

The value for properties A, Bbb, and C are changed, and the non-matching field D is unchanged.

Actual behavior

We get the expected behavior when using ExpandoObject. However, if we use JObject (which is what JSON.NET will give you if you deserialize to a dynamic, we get the following:

A | 123
Bbb | 456
C | Greetings
D | 0

The int properties are not changed, the string property is changed, and the non-matching field D set to zero.

Steps to reproduce

	mapper.Map<dynamic, Foo>(overlay,target);

Real-world scenario which led to this

SPA app for editing a user record. The update is made through a webservice API that receives a JSON object with just the changed fields, so object structure changes on every call, so it must be received as a dynamic.

        [HttpPost("UpdateUser")]
         public async Task<JsonResult> UpdateUser([FromBody] dynamic updates) {....}

When using JSON.NET that dynamic object is actually a JObject

Workaround (for my situation)

Force use of ExpandoObject

        [HttpPost("UpdateUser")]
         public async Task<JsonResult> UpdateUser([FromBody] ExpandoObject updates) {....}