EF 为某些返回 null，但 EF Core 返回 0

Question

I migrate a site ASP.NET MVC to ASP.NET MVC core, with EF to EF Core.

I make a request with empty value :

 var q = bd.Prod
           .Where(o => o.xxx == yyy)
           .GroupBy(o => o.sss)
           .Select(g => new
           {
               ccc = g.Key,
               vvv = g.Sum(i => i.qqq),
               bbb = g.Sum(i => i.fff),
               nnn = g.Sum(i => i.ggg),
           });

When I execute this code with EF return null when all cell of sum are empty. But EF Core return 0 when all cell of sum are empty. How to make EF Core return null ?

Answer 1

Use DefaultIfEmpty :

var q = bd.Prod
          .Where(o => o.xxx == yyy)
          .GroupBy(o => o.sss)
          .Select(g => new
          {
            ccc = g.Key,
            vvv = g.Sum(i => i.qqq),
            bbb = g.Sum(i => i.fff),
            nnn = g.Sum(i => i.ggg),
         })
          .DefaultIfEmpty(null);

Answer 2

If you want the following:

• Empty collection -> null
• Only null values -> null
• Any combination of numbers and null values -> number

Try this code:

items.Aggregate((int?)null, (acc, item) => acc.HasValue ? acc + item.GetValueOrDefault() : item);

Answer 3

Building on @grek40's answer for EF Core 5.0, you can create an extension method so that it can be re-used easily

public static decimal? SumOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal?> selector)
    => (from s in source select selector(s))
       .Aggregate((decimal?)null, (acc, item) => acc.HasValue ? acc + item.GetValueOrDefault() : item);

or if you prefer any nulls to result in a null value

   public static decimal? SumAllOrNull<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal?> selector)
        => (from s in source select selector(s))
           .Aggregate((decimal?)null, (acc, item) => acc.HasValue ? acc + item : item.HasValue ? item : null);

and then call it such as

var q = bd.Prod
           .Where(o => o.xxx == yyy)
           .GroupBy(o => o.sss)
           .Select(g => new
           {
               ccc = g.Key,
               vvv = g.SumOrDefault(i => i.qqq),
               bbb = g.SumOrDefault(i => i.fff),
               nnn = g.SumAllOrNull(i => i.ggg),
           });

Note: This only works on Linq-To-Objects, so if on DB query you must call .ToList() or AsIEnumerable before hand, until EF Core supports a way to do this in Linq-To-Sql