在.Where（）中使用.SingleOrDefault（）将引发以下异常：-用户代码未处理System.NotSupportedException

Question

I am working on an asp.net mvc-4 web application, and i am using Entity Framework 5. now i have the following model method:-

 public IQueryable<TMSStorageDevice> CustomerSD(int customerid)
        {


            var customerSiteIds = entities.SiteDefinitions.Where(a => a.AccountDefinitions1.SingleOrDefault().ORG_ID == customerid).Select(a2 => a2.SITEID).ToList();
        }

now this will raise the following error:-

System.NotSupportedException was unhandled by user code
  HResult=-2146233067
  Message=The methods 'Single' and 'SingleOrDefault' can only be used as a final query operation. Consider using the method 'FirstOrDefault' in this instance instead.
  Source=System.Data.Entity
  StackTrace:
       at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SingleTranslatorBase.TranslateUnary(ExpressionConverter parent, DbExpression operand, MethodCallExpression call)
       at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.UnarySequenceMethodTranslator.Translate(ExpressionConverter parent, MethodCallExpression call)
       at System.Data.Objects.ELinq.ExpressionConver

i was able to fix this by replacing ( SingleOrDefault() ):-

a.AccountDefinitions1.SingleOrDefault().ORG_ID

with ( FirstOrDefault() ):-

a.AccountDefinitions1.FirsteOrDefault().ORG_ID

so can anyone advice on this behavior ? i mean why FirstOrDefault() will work in this case unlike SingleorDefault() ?

Answer 1

Because SingleOrDefault would require it to generate SQL which ensures that there is not more than 1 foreign row. Essentially, it would have to create a join which returns no rows if the count > 1

FirstOrDefault , however, simply allows it to generate a join and SELECT TOP 1 (or equivalent) which is vastly easier.

It just simply isn't implemented in EF.