有個需求，想在Web Service中傳遞Dictionary<string, string>參數，例如:
[WebMethod]
public Dictionary<string, string> Process(Dictionary<string, string> dct)
{
//Do something on the Dictionary
//... blah blah blah ....
return dct;
}
天不從人願，以上的寫法會產生Web Service不支援IDictionary的錯誤:
The type System.Collections.Generic.Dictionary`2[[System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]] is not supported because it implements IDictionary.
既是IDictionary的原罪，就算是換用Hashtable、ListDictionary應該也是同樣結果，測試之下果然無一倖免。
Google發現討論區有篇來自MS RD的留言，算是證實這是先天限制:
ASMX web services do not support types that implement IDictionary since V1. This is by design and was initially done since key-value constraints could not be appropriately expressed in schema.
來源: http://social.msdn.microsoft.com/Forums/en-US/asmxandxml/thread/d7cb8844-6774-4a98-8aa3-85e445af4867/
既然是By Design，就只有繞道而行。Survey了一下，找到一些建議做法:
- 改用XML
- 使用DataSet
- 另外自訂Class作為參數
- 以DictionaryEntry[]瓜代之
評估了一下，我原本想要借重的就是Dictionary Key/Value的單純資料結構，XML為開放格式不易限制成Key/Value的形式；小小需求動用到DataSet略嫌笨重；自訂Class在編譯時期就要確定Key的種類，不符本案例的前題。看來DictionaryEntry[]較合需求，因此我試寫如下: (剛好Dictionary與DirectionaryEntry的雙向轉換都有示範到)
[WebMethod]
public DictionaryEntry[] Test(System.Collections.DictionaryEntry[] entries)
{
//用ListDictionary主要是為了稍後可以直接CopyTo轉DictionaryEntry[]
//若有效率或其他考量，可改用其他Collection Class
ListDictionary dct = new ListDictionary();
foreach (DictionaryEntry de in entries)
dct.Add(de.Key, de.Value);
//Do something on the Dictionary
//... blah blah blah ....
if (dct.Contains("Kuso"))
dct["Kuso"] = "殺很大";
DictionaryEntry[] result = new DictionaryEntry[dct.Count];
dct.CopyTo(result, 0);
return result;
}
呼叫端範例如下:
protected void Page_Load(object sender, EventArgs e)
{
localhost.AFAWebService aws = new localhost.AFAWebService();
aws.Credentials = CredentialCache.DefaultCredentials;
Dictionary<string, string> dct = new Dictionary<string, string>();
dct.Add("Kuso", "你不要走");
//DictionaryEntry在Web Service傳遞時會被當成自訂類別
//因此要用namespace.DictionaryEntry而非System.Collections.DictionaryEntry
List<localhost.DictionaryEntry> lst = new List<localhost.DictionaryEntry>();
foreach (string key in dct.Keys)
{
localhost.DictionaryEntry de = new localhost.DictionaryEntry();
de.Key = key;
de.Value = dct[key];
lst.Add(de);
}
localhost.DictionaryEntry[] result = aws.Test(lst.ToArray());
Dictionary<string, string> dctRes = new Dictionary<string, string>();
foreach (localhost.DictionaryEntry de in result)
dctRes.Add(de.Key.ToString(), de.Value.ToString());
Response.Write(dct["Kuso"] + "->" + dctRes["Kuso"]);
Response.End();
}
經過這番來回折騰，這方法看來也不怎麼簡潔。
於是，我又嘗試了Paul Welter的SerializableDictionary物件，做法上要在Web Service與Client端都Reference這個自訂物件，而且使用Visual Studio的Add Web Reference時，自動產生的Proxy Class宣告中SerializableDictionary會被當成DataSet而失敗，因此得改成手動產生Proxy Class後再將DataSet改回SerializableDictionary:
Microsoft (R) Web Services Description Language Utility
[Microsoft (R) .NET Framework, Version 2.0.50727.42]
Copyright (C) Microsoft Corporation. All rights reserved.
Writing file 'AfaWebServiceProxy.cs'.
用了SerializableDictionary後，程式碼簡化許多:
[WebMethod]
public SerializableDictionary<string, string> Test(
SerializableDictionary<string, string> dct)
{
if (dct.ContainsKey("Kuso"))
dct["Kuso"] = "殺很大";
return dct;
}
呼叫端也很單純:
protected void Page_Load(object sender, EventArgs e)
{
localhost.AFAWebService aws = new localhost.AFAWebService();
aws.Credentials = CredentialCache.DefaultCredentials;
SerializableDictionary<string, string> dct =
new SerializableDictionary<string, string>();
dct.Add("Kuso", "你不要走");
SerializableDictionary<string, string> dctRes = aws.Test(dct);
Response.Write(dct["Kuso"] + "->" + dctRes["Kuso"]);
Response.End();
}
但是，這個做法需要在Web Service與Client端加入自訂元件參照、Proxy Class需要手動增加或修改，還是有些許不便。這樣看來，DataSet或XML法雖有其他缺點，但內建支援的特點，在力求簡單的場合裡，倒也值得納入考量吧!