IdentityServer4.Models.Client as httppost parameter always null in aspnet core 2.x

Question

I am sending an httpPost parameter "client" of type IdentityServer4.Models.Client via a C# console application to a C# web api and client is always null .

If I send a different object using the same code, the parameter is received just fine. This seems to be a problem specific to Identity Server 4 and I don't know what's going on.

Server code:

[HttpPost]        
public JsonResult Post(IdentityServer4.Models.Client client){   

    return Json(new { result = true });
}

Client code:

private static async Task Register(string clientName){

        var controllerName = "BasicClient";             
        var basicClientApi = string.Format("http://localhost:5100/api/{0}", controllerName);
        using (var httpClient = new HttpClient()){

            var clientData = new IdentityServer4.Models.Client();
            var client = new { client = clientData };
            client.client.ClientName = clientName;

            var json = JsonConvert.SerializeObject(client);
            var content = new StringContent(json, Encoding.UTF8, "application/json");
            content.Headers.ContentType = new MediaTypeHeaderValue("application/json");


            var response = await httpClient.PostAsync(basicClientApi, content);

            if (!response.IsSuccessStatusCode)
            {
                Console.WriteLine(response.StatusCode);
            }
            else
            {
                var rawResponse = await response.Content.ReadAsStringAsync();

                JObject o = JObject.Parse(rawResponse);
                Console.WriteLine(o.ToString());
            }
        }
    }

EDIT After applying [FromBody] and unwrapping the object, I am still getting null for client in the receiving Web API. One thing caught my eye on the console application debug screen though.. see the image below:

The actual client variable is null in the console application, yet JsonConvert was able to serialize it into something. What's that about?

Answer 1

You are wrapping your model inside an anonymous object, which will turn its JSON representation into something which has nothing in common with your original Client class.

This:

var clientData = new IdentityServer4.Models.Client();
var client = new { client = clientData };
client.client.ClientName = clientName;

var json = JsonConvert.SerializeObject(client);

Will result in a JSON similar to the following:

{
    "client": { 
        "clientName": "foo",
        "anotherProperty": "bar",
        // other properties omitted for brevity
    }
}

But what you really want is just the Client object:

{ 
    "clientName": "foo",
    "anotherProperty": "bar",
    // other properties omitted for brevity
}

Do not wrap your clientData , just serialize it directly to follow the model inside your MVC Action Method:

var clientData = new IdentityServer4.Models.Client();
clientData.ClientName = clientName;

var json = JsonConvert.SerializeObject(clientData);

For everything to be working, you have to tell the model binder explicitly where to expect the data.

Use [FromBody] attribute on the model.

[FromBody] : Use the configured formatters to bind data from the request body. The formatter is selected based on content type of the request.

[HttpPost]        
public IActionResult Post([FromBody]IdentityServer4.Models.Client client) {

    return Json(new { result = true });
}

Reference Model Binding in ASP.NET Core

Answer 2

You're not going to believe this, but ultimately the reason why IdentityServer.Models.Client was null in the Web Api post parameter was because the class is decorated with [DebuggerDisplay("{ClientId}")] and I did not provide a ClientId in my test application, so it was always showing up as null when in fact it actually WAS THERE THE WHOLE TIME. I am glad this issue is behind me, but I am very angry about this "feature".