如何解決問題「無法將表達式...轉換爲SQL，無法將其視爲本地表達式」。

Question

我有下面的代碼：

void Main() 
{ 
    var q = from a in Applicants 
      where(a.Claims.Any()) 
      select a.Claims.Sum(c => c.TotalClaimAmount()); 


    q.Dump(); 
} 

public static class MyExt 
{ 
    public static decimal TotalClaimAmount(this Claim c) 
    { 
     var t = c.Accommodations.Sum(a => a.AmountClaimed) + 
       c.MealAllowances.Sum(ma => ma.AmountClaimed) + 
       c.Meals.Sum(m => m.AmountClaimed) + 
       c.Mileages.Sum(mi => mi.AmountClaimed) + 
       c.Others.Sum(o => o.AmountClaimed) + 
       c.ParkingTransits.Sum(pt => pt.AmountClaimed) + 
       c.Travels.Sum(tr => tr.AmountClaimed); 

     return (decimal)t; 
    } 
} 

當我運行它在LinqPad得到以下問題：

出現InvalidOperationException：無法翻譯表達「a.Claims.Sum（C => c.TotalClaimAmount（）） '轉換爲SQL，不能將其視爲本地表達式。

請幫我一把。非常感謝

Answer 1

在您的自定義方法沒有相應的命令在SQL中。所以編譯器無法翻譯你的表達式。相反，先得到你的項目，然後選擇您的結果：

var items = Applicants.Where(a => a.Claims.Any()).ToList(); 
var results = items.Select(a => a.Claims.Sum(c => c.TotalClaimAmount()); 

Answer 2

LINQ是無法將TotalClaimAmount方法調用轉換爲SQL表達式。

你可以做的是從TotalClaimAmount方法中提取表達式並直接在Sum方法調用中使用它。

public static class MyExt { 
    public static Func<Claim, decimal> TotalClaimAmountFunc = c => 
     c.Accommodations.Sum(a => a.AmountClaimed) + 
     c.MealAllowances.Sum(ma => ma.AmountClaimed) + 
     c.Meals.Sum(m => m.AmountClaimed) + 
     c.Mileages.Sum(mi => mi.AmountClaimed) + 
     c.Others.Sum(o => o.AmountClaimed) + 
     c.ParkingTransits.Sum(pt => pt.AmountClaimed) + 
     c.Travels.Sum(tr => tr.AmountClaimed); 

    public static decimal TotalClaimAmount(this Claim c) { 
     return TotalClaimAmountFunc(c); 
    } 
} 

void Main() { 
    var q = from a in Applicants 
     where(a.Claims.Any()) 
     select a.Claims.Sum(MyExt.TotalClaimAmountFunc); 

    q.Dump(); 
}