LINQ集團由多個參數

Question

我有一個VB.NET應用程序，並希望在多個列做分組。

階層結構：

Public Class Person 
    Public Property Name as String 
    Public Property City as String 
    Public Property Country as String 
End Class 

Dim oResult = PersonList _ 
       .GroupBy(Function(v) New With {v.City, v.Country}) _ 
       .Where(Function(grp) grp.Count > 1).ToList() 

我有多個個人記錄，其中包含同一個城市的名字&國名。但上面的查詢返回零項。如果我只使用一個城市或國家列，那麼它工作正常。

Dim oResult = PersonList _ 
       .GroupBy(Function(v) v.City) _ 
       .Where(Function(grp) grp.Count > 1).ToList() 

任何人指出我在哪裏我錯了羣由LINQ查詢與多個參數。

Answer 1

問題是隻有Key匿名類型的屬性用於VB中的相等和散列。 （在C＃匿名類型的所有屬性是有效的關鍵屬性。）所以你只需要您的查詢更改爲：

Dim oResult = PersonList _ 
       .GroupBy(Function(v) New With { Key v.City, Key v.Country}) _ 
       .Where(Function(grp) grp.Count > 1).ToList() 

見anonymous types in VB的文檔瞭解更多信息。

Answer 2

Dim q1 = (From p In Repository.Table 
          Group p By p.CreatedDate.Year, p.CreatedDate.Month Into Group 
          Select New With {.Year = Year, .Month = Month, .Count = Group.Count()}).ToList 

或者

Dim q2 = (From p In Repository.Table 
          Group p By key = New With {.Year = p.CreatedDate.Year, .Month = p.CreatedDate.Month} Into Group 
          Select New With {.Year = key.Year, .Month = key.Month, .Count = Group.Count()}).ToList