Ho在EF查詢中將Uint轉換爲Nullable Int

Question

我試圖將Uint（i_Customer）轉換爲Nullable Int（i_Customer）ID，因爲一個接受空值而另一個ID不支持。 父表是Customer（i_Customer），子表是故障（i_customer）。兩者，我正在加入他們的EF查詢來獲得結果。但有一個nullreferenceexception was unhandled by user code例外，這是非常令人不安。我如何解決它？

這裏是EF查詢：

if (servicelevel == 3) 
      { 
       result = (from s in res 
          join cInfo in custInfo on 
          s.fault.i_customer equals Convert.ToInt32((int?)cInfo.customers.i_Customer) 
          where (s.fault.resolved == null) || (s.tasks.assignedto == agent) 
          orderby s.fault.ispriority descending, s.fault.logtime ascending 
          select new ActiveFaultResult() 
          { Company_Name = cInfo.customers.Company_Name, 
            //replies = replies, 
            idFaults = s.fault.idFaults, 
            hashvalue = s.fault.hashvalue, 
            responsible = s.fault.responsible, 
            logtime = s.fault.logtime, 
            isinternal = s.fault.isinternal, 
            ispriority = s.fault.ispriority 

          }).ToList<ActiveFaultResult>(); 

       // var limitresult = result.Take(50); 
       return result; 
      } 

Answer 1

正常投會做

s.fault.i_customer equals (int?)cInfo.customers.i_Customer 

或者試試這個

s.fault.i_customer ?? 0 equals cInfo.customers.i_Customer 

Answer 2

您需要使用DefaultIfEmpty（）來提供外連接，以便它可以處理空值。見下面的例子：

result = (from s in res 
     join cInfo in custInfo on s.fault.i_customer equals (int)cInfo.customers.i_Customer into A 
     from cInfo in A.DefaultIfEmpty() 
     where (s.fault.resolved == null) || (s.tasks.assignedto == agent) 
     orderby s.fault.ispriority descending, s.fault.logtime ascending 
     select new ActiveFaultResult() 
     { 
       Company_Name = cInfo == null? String.Empty: cInfo.customers.Company_Name, 
       idFaults = s.fault.idFaults, 
       hashvalue = s.fault.hashvalue, 
       responsible = s.fault.responsible, 
       logtime = s.fault.logtime, 
       isinternal = s.fault.isinternal, 
       ispriority = s.fault.ispriority 
     }).ToList<ActiveFaultResult>(); 
return result;