Standard REST de-serialisation fails: Unexpected parameter encountered during deserialization

Question

Consider the following class:

public class Caps {
    public Sett SETTINGS {get;set;}
}
public class Sett {
    public string foo {get;set;}
}

When I POST the following JSON:

{"SETTINGS":{"foo":"bar"}}

To my own REST class:

...
@HttpPost
global static void doPost(Caps change) {
}

I get 400 Bad Request:

[
  {
    "message": "Unexpected parameter encountered during deserialization: SETTINGS at [line:1, column:14]",
    "errorCode": "JSON_PARSER_ERROR"
  }
]

However the exact same JSON can be de-serialised as follows:

String s = '{"SETTINGS":{"foo":"bar"}}';
Caps c = (Caps)JSON.deserialize(s, Caps.class);
system.debug(c);

As a workaround I can do the de-serialisation in my REST class:

@HttpPost
global static ZRest.ZRestResponse doPost() {
    String req = RestContext.request.requestBody.toString();
    Caps change = (Caps)JSON.deserialize(req, Caps.class);
}

This works fine and I get 200.

The question is why doesn't the standard REST de-serialisation of parameters work?

PS: I read possible duplicate questions but they only seem to offer a workaround or the same string in a different format (newlines, indents) which doesn't solve my issue at all.

Answer 1

This is a consequence of Salesforce's design decision that allows for example this code:

@HttpPost
global static void doPost(String x, Integer y) {
}

to accept this JSON:

{
    "x": "abc",
    "y": 123
}

The names and types of the method arguments form part of the JSON structure.

So in your case the following JSON would work:

{"change":{"SETTINGS":{"foo":"bar"}}}