顯式轉換的IEnumerable <AnonymousType＃1>

Question

我有一個方法使用LINQ在兩個DataTable上執行不匹配的查詢。它產生了一個錯誤，通過在網上查找我已經確定了我認爲發生錯誤的位置，但我無法弄清楚如何解決它。

public IEnumerable<int> UnbilledAuditKeys(DataTable audits, string keyFieldName) { 
    var billedAudits = 
     from x in this.GetBilledAudits().AsEnumerable() 
     select new { 
      k = x.Field<int>(keyFieldName) 
     }; 

    var allAudits = 
     from x in audits.AsEnumerable() 
     select new { 
      k = x.Field<int>(keyFieldName) 
     }; 

    var unbilled = 
     from a in allAudits 
     join b in billedAudits on a.k equals b.k 
      into combined 
     from c in combined.DefaultIfEmpty() 
     where c == null 
     select new { // This is what's causing the error (I think) 
      k = a.k 
     }; 

    return unbilled; // This line the compiler is rejecting 
} 

返回的錯誤是

Cannot implicitly convert type 'System.Collections.Generic.IEnumerable<AnonymousType#1>' to 'System.Collections.Generic.IEnumerable<int>'. An explicit conversion exists (are you missing a cast?) 

我在如何解決它的損失。我已經嘗試了將整個LINQ表達式轉換爲IEnumerable，但會產生運行時異常。

任何想法將不勝感激！

編輯：

的最後一個方法：

public IEnumerable<int> UnbilledAuditKeys(DataTable rosliAudits, string keyFieldName) { 
    var billed = this.GetBilledAudits().AsEnumerable().Select(x => x.Field<int>(keyFieldName)); 
    var allaudits = rosliAudits.AsEnumerable().Select(x => x.Field<int>(keyFieldName)); 
    var unbilled = allaudits.Except(billed); 
    return unbilled; 
} 

Answer 1

簡單的解決方法：

var unbilled = 
    from a in allAudits 
    join b in billedAudits on a.k equals b.k 
     into combined 
    from c in combined.DefaultIfEmpty() 
    where c == null 
    select a.k; 

而且，o其中兩個查詢似乎並不需要匿名結構，最後一個可以被極大地簡化：

public IEnumerable<int> UnbilledAuditKeys(DataTable audits, string keyFieldName) { 
    var billedAudits = 
     from x in this.GetBilledAudits().AsEnumerable() 
     select x.Field<int>(keyFieldName); 

    var allAudits = 
     from x in audits.AsEnumerable() 
     select x.Field<int>(keyFieldName); 

    var unbilled = allAudits.Except(billedAudits); // LINQ has many useful methods like this 

    return unbilled; 
} 

Answer 2

選擇你的領域，而不是直接創造新的匿名類型：

var unbilled = 
    from a in allAudits 
    join b in billedAudits on a.k equals b.k 
     into combined 
    from c in combined.DefaultIfEmpty() 
    where c == null 
    select a.k;