如何在lambda表達式的選擇方法中編寫多個表達式

Question

我正在嘗試使用XDocument準備一個XML文檔，同時我將項目添加到特定的字典中，但是我遇到了以下錯誤。

方法'System.Linq.Enumerable.Select（System.Collections.Generic.IEnumerable，System.Func）'的類型參數不能根據用法推斷出來。嘗試明確指定類型參數。

和代碼片段

Dictionary<string, string> userguid = new Dictionary<string, string>(); 

    XDocument XMLDoc = 
       new XDocument(new XDeclaration("1.0", "utf-8", "yes"), 
       new XElement("NewDataSet", 
        new XElement("Users", 
         new XElement("UserID", userid), 
         new XElement("FullName", "anyone"), 
         new XElement("UserName", "anyone"), 
         new XElement("Password", 123), 
         new XElement("Description", "anyone"), 
         new XElement("DomainName", string.Empty), 
         new XElement("Mailbox_Size", 20), 
         new XElement("Enabled", "True"), 
         new XElement("Permissions", 14), 
         new XElement("CreationTime", "2011-07-19T17:45:58.53125+05:30") 
        ), 
       userNames.Select((item, value) => 
        { 
         new XElement("Users", 
          new XElement("UserID", Guid.NewGuid().ToString("N")), 
          new XElement("FullName", item.Trim()), 
          new XElement("UserName", item.Trim()), 
          new XElement("Password", passwords[value].Trim()), 
          new XElement("Description", item.Trim()), 
          new XElement("DomainName", string.Empty), 
          new XElement("Mailbox_Size", 20), 
          new XElement("Enabled", "True"), 
          new XElement("Permissions", 14), 
          new XElement("CreationTime", "2011-07-19T17:45:58.53125+05:30")); 
         userguid.Add(userid, emailAddresses[value].Trim()); 
        } 
         ) 
        )); 

Answer 1

你幾乎做的是正確的，但使用語句塊（{…}）時，你需要明確return一個合適的值，因爲Select需要一個返回值的委託。

（記住，簡稱單式拉姆達：

x => expr(x) 

真的只是短暫的更全面的版本：

x => { return expr(x); } 

）

Answer 2

當您使用的是塊而不是lambda（在select方法中，我的意思是），你應該返回結果。我的意思是userNames.Select((item, value) => { /* return the result here... */ });：

Dictionary<string, string> userguid = new Dictionary<string, string>(); 

XDocument XMLDoc = 
    new XDocument(
     new XDeclaration("1.0", "utf-8", "yes"), 
     new XElement(
      "NewDataSet", 
      new XElement(
       "Users", 
       new XElement("UserID", userid), 
       new XElement("FullName", "anyone"), 
       new XElement("UserName", "anyone"), 
       new XElement("Password", 123), 
       new XElement("Description", "anyone"), 
       new XElement("DomainName", string.Empty), 
       new XElement("Mailbox_Size", 20), 
       new XElement("Enabled", "True"), 
       new XElement("Permissions", 14), 
       new XElement("CreationTime", "2011-07-19T17:45:58.53125+05:30") 
       ), 
      userNames.Select(
          (item, value) =>{ 
           userguid.Add(userid, emailAddresses[value].Trim()); 
           return new XElement(
            "Users", 
            new XElement("UserID", Guid.NewGuid().ToString("N")), 
            new XElement("FullName", item.Trim()), 
            new XElement("UserName", item.Trim()), 
            new XElement("Password", passwords[value].Trim()), 
            new XElement("Description", item.Trim()), 
            new XElement("DomainName", string.Empty), 
            new XElement("Mailbox_Size", 20), 
            new XElement("Enabled", "True"), 
            new XElement("Permissions", 14), 
            new XElement("CreationTime", "2011-07-19T17:45:58.53125+05:30")); 
          } 
       ) 
      ));