问题：不支持Dictionary；结果：在Web Service中傳送Dictionary

在Web Service中傳送Dictionary

有個需求，想在Web Service中傳遞Dictionary<string, string>參數，例如:

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[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)

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[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:

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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:

C:AppCodeFolder>wsdl http: //localhost/myweb/afawebservice.asmx?WSDL /l:cs /n:localhost /out:AfaWebServiceProxy.cs
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:

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[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:

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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!