前一篇文章提到不靠IIS在Console/WinForm/WPF程式裡也可以執行ASP.NET Web API，接著我們更深入一點，談談Client端如何傳遞資料給ASP.NET Web API。
在ASP.NET Web API的傳統應用，Client端多是網頁，故常見範例是透過HTML Form、JavaScript、AJAX傳送參數資料給Web API；而在Self-Hosted ASP.NET Web API情境，由於Web API常被用於系統整合，呼叫端五花八門，.NET程式、VBScript、Excel VBA、Java... 都有可能，所幸Web API建構在HTTP協定之上，不管平台為何，都不難找到可用的HTTP Client元件或函式庫。
本文將示範我自己常用的兩種平台: .NET Client及Excel VBA。
首先，我們改寫前文範例，加上接受前端傳入Player物件新增資料的Insert() Action。由於ASP.NET MVC的ModelBinder已具備將JSON字串轉為Model物件的能力，我們也沒什麼好客氣的，直接宣告Player物件當成Insert()的輸入參數，JSON字串轉物件的工作就丟給ASP.NET MVC底層傷腦筋。
BlahController.cs改寫如下，程式碼很單純，唯一的小手腳是要捕捉例外，產生自訂錯誤訊息的HttpResponseMessage，再以其為基礎拋出HttpResponseException，理由容後說明。
using System;using System.Collections.Generic;using System.Linq;using System.Net;using System.Net.Http;using System.Text;using System.Web.Http;using Newtonsoft.Json;namespace SelfHostWebApi{publicclass BlahController : ApiController
{//宣告Model類別承接前端傳入資料publicclass Player
{publicint Id;
publicstring Name;
public DateTime RegDate;publicint Score;
}
[HttpPost]
publicstring Insert(Player player)
{try {//輸出資料，驗證已正確收到 Console.WriteLine("Id: {0:0000} Name: {1}", player.Id, player.Name); Console.WriteLine("RegDate: {0:yyyy-MM-dd} Score: {1:N0}",player.RegDate, player.Score);
return"Player [" + player.Id + "] Received";
}
catch (Exception ex) {//發生錯誤時，傳回HTTP 500及錯誤訊息 var resp = new HttpResponseMessage() {StatusCode = HttpStatusCode.InternalServerError,
Content = new StringContent(ex.Message), ReasonPhrase = "Web API Error"};
thrownew HttpResponseException(resp);
}
}
}
}
呼叫端的寫法很簡單，WebClient.UploadString(url, jsonString)會以jsonString為內容丟出HTTP POST請求，但有個關鍵: 必須設定ContentType為application/json，告知ModelBinder我們所POST的內容是JSON字串，ModelBinder才能正確地反序列化成Player類別。測試程式另外亂傳空字串及非JSON字串，以測試輸入錯誤時Web API的反應。
using System;using System.Collections.Generic;using System.IO;using System.Linq;using System.Net;using System.Text;using Newtonsoft.Json;namespace ApiTest{class Program {staticvoid Main(string[] args)
{ WebClient wc = new WebClient();string url = "httq://localhost:32767/blah/Insert";
//由WebException中取出Response內容 Action<string> postJson = (json) => {try {//重要: 需宣告application/json，才可正確Bind到Modelwc.Headers.Add(HttpRequestHeader.ContentType,
"application/json");var test = wc.UploadString(url, json);
Console.WriteLine("Succ: " +test);}
catch (WebException ex) { StreamReader sr = new StreamReader(ex.Response.GetResponseStream());
Console.WriteLine("Error: " + sr.ReadToEnd());}
};
//故意傳入無效資料進行測試 postJson(string.Empty); postJson("BAD THING");//利用匿名型別+Json.NET傳入Web API所需的Json格式 var player = new {Id = 1,
Name = "Jeffrey",RegDate = DateTime.Today,
Score = 32767
};
postJson(JsonConvert.SerializeObject(player));
Console.ReadLine();
}
}
}
測試結果如下:
Error: Object reference not set to an instance of an object. Error: Object reference not set to an instance of an object. Succ: "Player [1] Received"
當傳入空字串及非JSON字串，UpdateString()會發生WebException，而透過WebException.Response.GetResponseStream()可讀取Insert()方法在捕捉例外時透過HttpResponseMessage傳回的訊息內容。如果我們不捕捉例外，任由MVC內建機制處理，則Client會收到Exception經JSON序列化後的結果(如下所示)，資訊較詳細但需反序列化才能讀取。相形之下，拋回HttpResponseException可以精準地控制傳回的錯誤訊息及提示，更容易符合專案客製需求。
Error: {"Message":"An error has occurred.","ExceptionMessage":"Object reference
not set to an instance of an object.","ExceptionType":"System.NullReferenceExcep
tion","StackTrace":" at SelfHostWebApi.BlahController.InsertByBinding(Player p
layer) in x:\\Temp\\Lab0603\\SelfHostWebApi\\SelfHostWebApi\\BlahController.cs:l
ine 34\r\n at lambda_method(Closure , Object , Object[] )\r\n at System.Web.
Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c__DisplayClass1
3.<GetExecutor>b__c(Object instance, Object[] methodParameters)\r\n at System.
Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.Execute(Object
instance, Object[] arguments)\r\n at System.Threading.Tasks.TaskHelpers.RunSy
nchronously[TResult](Func`1 func, CancellationToken cancellationToken)"}
最後補上VBA寫法:
Sub SendApiRequest(body AsString)
Dim xhrSet xhr = CreateObject("MSXML2.ServerXMLHTTP")
Dim url AsString
url = "httq://localhost:32767/blah/insert"xhr.Open "POST", url, False
xhr.SetRequestHeader "Content-Type", "application/json"
OnErrorGoTo HttpError:
xhr.Send body
MsgBox xhr.responseText
ExitSub
HttpError:
MsgBox "Error: "& Err.DescriptionEndSub
Sub Test() SendApiRequest "BAD THING"SendApiRequest "{ ""Id"":1,""Name"":""Jeffrey"", "& _
"""RegDate"":""2012-12-21"", ""Score"":32767 }"
End Sub