为什么FirstOrDefault得到此异常？ 序列不包含匹配元素异常

Question

I hope you can help me to my code after I change the data type of the following I get this exception "Sequence contains no matching element exception. ". And I am sure this because of FirstOrDefault() Extension.

LandId - long
ShowMapPoint - string
Development - string
Location - string
MapPointX - string
MapPointY - string
AreaSize - from decimal? into long?
Premium - from decimal? into long?
TransactionPrice - from decimal? into long?

This is my code:

var result = _context.DwPropertyMasters.Where(x => x.ShowMapPoint == "Y")
                .Select(x => new
                {
                    x.LandId,
                    a = x.Development == null || x.Development == "" ? x.Location : x.Development,
                    x.MapPointX,
                    x.MapPointY,
                    AreaSize = x.AreaSize ?? 0,
                    Premium = x.Premium ?? 0,
                    b = (x.Premium == 0 ? null : x.Premium) * 100000000 / (x.AreaSize == 0 ? null : x.AreaSize) ?? 0,
                    c =
                    _context.DwPropertyDetails.Where(
                            z => (z.TransactionPrice > 0 || z.TransactionPrice != null) && z.LandId == x.LandId)
                        .GroupBy(z => z.LandId)
                        .Select(g =>
                            (g.Sum(p => p.TransactionPrice) == 0 ? null : g.Sum(p => p.TransactionPrice)) /
                            (g.Sum(p => p.ActualSize) == 0 ? null : g.Sum(p => p.ActualSize)) ?? 0)
                        .FirstOrDefault(),
                    d =
                    ((x.AreaSize2 == 0 ? null : x.AreaSize2) == 0
                        ? 0
                        : (x.Premium == 0 ? null : x.Premium) * 100000000 / (x.AreaSize2 == 0 ? null : x.AreaSize2)) ??
                    0,
                    x.LandType,
                    e =
                    _context.DwPropertyDetails.Where(
                            y => (y.TransactionPrice > 0 || y.TransactionPrice != null) && y.LandId == x.LandId)
                        .Select(y => new
                        {
                            a = 1
                        }).Count()
                });

Answer 1

Your problem is in the sum of the Premium and TransactionPrice

If you grouping doesn't contain an item with a value, then it will try to sum null values. That can't work.

I would change it as follows:

(g.All(p => p.TransactionPrice == null) ? null : g.Where(p => p.TransactionPrice != null).Sum(p => p.TransactionPrice))

and

(g.All(p => p.ActualSize == null) ? null : g.Where(p => p.ActualSize != null).Sum(p => p.ActualSize)))

First check if everything is null, then it is null, else sum the values.

But you need to edit your code further, your current code would enable situations like 123 / null or null / 123 . Which would also be a exception.

Answer 2

It is hard to understand what you are doing. It is way too complex with all these inline if's. And on line 13 you have:

z.TransactionPrice > 0 || z.TransactionPrice != null. 

You might as well remove z.TransactionPrice > 0, because the second part allows all values (not null), including everything smaller than 0.

The 'Sequence contains no matching element' exception is typically something I'd expect when you use .First(). Not .FirstOrDefault().

Based on your code and what I think you want to achieve I have rewritten the query:

var result = _context.DwPropertyMasters.Where(x => x.ShowMapPoint == "Y")
                .Select(x => new
                {
                    x.LandId,
                    a = x.Development == null || x.Development == "" ? x.Location : x.Development,
                    x.MapPointX,
                    x.MapPointY,
                    AreaSize = x.AreaSize ?? 0,
                    Premium = x.Premium ?? 0,
                    b = (x.AreaSize == 0) ? 0 : (x.Premium * 100000000 / x.AreaSize ?? 0),
                    c = _context.DwPropertyDetails.Where(z => z.TransactionPrice > 0 && z.LandId == x.LandId)
                        .GroupBy(z => z.LandId)
                        .Select(g => g.Sum(p => p.ActualSize) == 0 ? 0 : (g.Sum(p => p.TransactionPrice) / g.Sum(p => p.ActualSize) ?? 0)
                        .FirstOrDefault(),
                    d = (x.AreaSize2 == 0) ? 0 : (x.Premium * 100000000 / x.AreaSize2 ?? 0),
                    x.LandType,
                    e = _context.DwPropertyDetails.Count(y => y.TransactionPrice > 0 && y.LandId == x.LandId)
                });

I do not know if this works as you intended (and works at all since I didn't test this), but it is shorter and I hope more readable.

Keep in mind that this query is executed in sql server, so there is no need to check for null values. The only thing that must be prevented is to divide by zero. If we consider the next line:

b = (x.AreaSize == 0) ? 0 : (x.Premium * 100000000 / x.AreaSize ?? 0)

If:

• x.AreaSize is null then (x.Premium * 100000000 / null ?? 0) => null ?? 0 => 0.

• x.AreaSize = 0 then the result = 0.

• x.Premium is null then the result is null / value ?? 0 => null ?? 0 => 0.

• x.Premium = 0 then the result = 0.