How to use System.Net.HttpClient to post a complex type?

You should use the SendAsync method instead, this is a generic method, that serializes the input to the service

Widget widget = new Widget()
widget.Name = "test"
widget.Price = 1;

HttpClient client = new HttpClient();
client.BaseAddress = new Uri("http://localhost:44268/api/test");
client.SendAsync(new HttpRequestMessage<Widget>(widget))
    .ContinueWith((postTask) => postTask.Result.EnsureSuccessStatusCode() );

If you don't want to create the concrete class, you can make it with the FormUrlEncodedContent class

var client = new HttpClient();

// This is the postdata
var postData = new List<KeyValuePair<string, string>>();
postData.Add(new KeyValuePair<string, string>("Name", "test"));
postData.Add(new KeyValuePair<string, string>("Price ", "100"));

HttpContent content = new FormUrlEncodedContent(postData); 

client.PostAsync("http://localhost:44268/api/test", content).ContinueWith(
    (postTask) =>
    {
        postTask.Result.EnsureSuccessStatusCode();
    });

Note: you need to make your id to a nullable int (int?)

The generic HttpRequestMessage<T> has been removed. This :

new HttpRequestMessage<Widget>(widget)

will no longer work.

Instead, from this post, the ASP.NET team has included some new calls to support this functionality:

HttpClient.PostAsJsonAsync<T>(T value) sends “application/json”
HttpClient.PostAsXmlAsync<T>(T value) sends “application/xml”

So, the new code (from dunston) becomes:

Widget widget = new Widget()
widget.Name = "test"
widget.Price = 1;

HttpClient client = new HttpClient();
client.BaseAddress = new Uri("http://localhost:44268");
client.PostAsJsonAsync("api/test", widget)
    .ContinueWith((postTask) => postTask.Result.EnsureSuccessStatusCode() );