how to post arbitrary json object to webapi

You can have your post method that takes in a HttpRequestMessage to by pass the model binding logic and you can read the content of the request directly:

    public HttpResponseMessage Post(HttpRequestMessage req)    {        var data = req.Content.ReadAsStringAsync().Result; // using .Result here for simplicity...        ...    }

By the way, the reason why the action that takes in JObject doesn't work is because of 'ObjectId("...")' that is used as the value of "_id" in your data...

We passed json object by jquery, and parse it in dynamic object. it works fine. this is sample code:

ajaxPost:...Content-Type: application/json,data: {          "name": "Jack",           "age": "12"      }...

webapi:

[HttpPost]public string DoJson2(dynamic data){    string name = data.name;    int age = data.age;    return name;}

similary question on stackoverflow: WebAPI Multiple Put/Post parameters

In your input, "_id": ObjectId("5069f825cd4c1d590cddf206") is what is breaking the JSON materialization on the server. Removing ObjectId and using "_id" : "5069f825cd4c1d590cddf206" works with JObject as well as Dictionary<string, object>