Getting a POST endpoint to work in self-hosted (WebServiceHost) C# webservice?

I think a simple code can answer all your questions

Task.Factory.StartNew(()=>StartServer());
Thread.Yield();
StartClient();

---

void StartServer()
{
    Uri uri = new Uri("http://localhost:8080/test");
    WebServiceHost host = new WebServiceHost(typeof(WCFTestServer), uri);
    host.Open();
}

void StartClient()
{
    try
    {
        WebClient wc = new WebClient();

        //GET
        string response1 = wc.DownloadString("http://localhost:8080/test/PutMessageGET/abcdef");
        //returns: "fedcba"

        //POST with UriTemplate
        string response2 = wc.UploadString("http://localhost:8080/test/PutMessagePOSTUriTemplate/abcdef",
                                            JsonConvert.SerializeObject(new { str = "12345" }));
        //returns: fedcba NOT 54321


        //POST with BodyStyle=WebMessageBodyStyle.WrappedRequest
        //Request: {"str":"12345"}
        wc.Headers["Content-Type"] = "application/json";
        string response3 = wc.UploadString("http://localhost:8080/test/PutMessagePOSTWrappedRequest",
                                            JsonConvert.SerializeObject(new { str="12345" }));

        //POST with BodyStyle=WebMessageBodyStyle.Bare
        wc.Headers["Content-Type"] = "application/json";
        string response4 = wc.UploadString("http://localhost:8080/test/PutMessagePOSTBare", "12345" );

    }
    catch (WebException wex)
    {
        Console.WriteLine(wex.Message);
    }
}

---

[ServiceContract]
public class WCFTestServer
{
    [OperationContract]
    [WebInvoke(Method = "GET", UriTemplate = "/PutMessageGET/{str}", BodyStyle = WebMessageBodyStyle.Bare, ResponseFormat = WebMessageFormat.Json, RequestFormat = WebMessageFormat.Json)]
    public string PutMessageGET(string str)
    {
        return String.Join("", str.Reverse());
    }

    [OperationContract]
    [WebInvoke(Method = "POST", UriTemplate = "/PutMessagePOSTUriTemplate/{str}", BodyStyle = WebMessageBodyStyle.WrappedRequest, ResponseFormat = WebMessageFormat.Json, RequestFormat = WebMessageFormat.Json)]
    public string PutMessagePOSTUriTemplate(string str)
    {
        return String.Join("", str.Reverse());
    }

    [OperationContract]
    [WebInvoke(Method = "POST", BodyStyle=WebMessageBodyStyle.WrappedRequest, ResponseFormat = WebMessageFormat.Json, RequestFormat = WebMessageFormat.Json)]
    public string PutMessagePOSTWrappedRequest(string str)
    {
        return String.Join("", str.Reverse());
    }

    [OperationContract]
    [WebInvoke(Method = "POST", BodyStyle = WebMessageBodyStyle.Bare, ResponseFormat = WebMessageFormat.Json, RequestFormat = WebMessageFormat.Json)]
    public string PutMessagePOSTBare(string str)
    {
        return String.Join("", str.Reverse());
    }
}

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PS: You can find the JsonConvert here