LINQ to SQL lambda，其中[可为空的对象必须具有值错误]

Question

I'm getting the error

[nullable object must have a value]

And it's point at the following part of the code

.Min(x => x.Time)

Here's the whole function.

public DateTime GetFirstRecord(long userId)
        {
            DateTime firstRecordDate;

            using (DeviceExerciseDataDataContext context = new DeviceExerciseDataDataContext())
            {
                firstRecordDate = (DateTime)context.Datas.Where(x => x.UserID.Equals(userId)).Min(x => x.Time);
            }

            return firstRecordDate;
        }

Also, how can I debug this functions lambda? When I hover over the x's, It shows no value. When when I try to print in the immediate window, it says "the name 'x' does not exist in the current context"

Edit:

So I've updated my code to this:

public DateTime? GetFirstRecord(long userId)
        {
            DateTime? firstRecordDate;
            firstRecordDate = DateTime.Now;

            using (DeviceExerciseDataDataContext context = new DeviceExerciseDataDataContext())
            {
                var test = context.Datas.Where(x => x.UserID.Equals(userId));
                if (test != null)
                    firstRecordDate = (DateTime)test.Min(x => x.Time);
                else
                    firstRecordDate = null;   
            }

            return firstRecordDate;
        }

But I'm still getting the save error.

In the DB, There are no null value's anywhere . Oh, and it doesn't go to the else, in the if/else block.

Answer 1

There is a good chance that context.Datas could be null or any of the Data's userId could be null or x.Time is null and you are trying to cast it as (DateTime) instead of (DateTime?).

You will have to add conditions to capture these null values in your assignment statement something like:

DateTime? datevalue = context.Datas == null
    ? null
    : (DateTime?)context.Datas.Where(x => x.UserId != null && x.UserID.Equals(userId))
                              .Min(x => x.Time);

Answer 2

If it is possible for firstRecordDate to be null, the .Min isn't allowed.

Can you try this?

firstRecordDate = (DateTime)context.Datas.Where(x => x.UserID.Equals(userId));
if (firstRecordDate.Count() > 0) firstRecordDate = firstRecordDate.Min(x => x.Time); 

if that works, then that's your problem. There are probably more elegant ways to deal with this but at least you will have a workaround to start with.

Answer 3

If you can be assured that at least one row has a valid time you could filter out the entries that have null as Time (or handle the case that DateTime.MaxValue is returned)

firstRecordDate = context.Datas.Where(x => x.UserID == userId)
                               .Select( x=> x.Time ?? DateTime.MaxValue)
                               .Min();

Answer 4

Try

(DateTime)context.Datas.Where(x => x.UserId != null && x.UserID.Equals(userId)).Min(x => x.Time)

The userID != null should evaluate first which will prevent nulls when you get to the Min. I haven't tested that but I think I remember doing stuff like it in the past.

Answer 5

The Min function is attempting to return the minimum value, and I've encountered this message before when no elements were being returned from the IQueryable - you need to check for the existance of elements before invoking, or declare firstRecordDate as a DateTime? .

Debugging lambda's is not supported in the watch window. If you want to debug it, I would suggest instead making it a long form lambda over multi line in order to debug

.Where(x =>
   {
       return x.ToString();
   });

You would be able to debug the inner method code then.

Answer 6

You will get that error if context.Datas.Where(x => x.UserID.Equals(userId)) does not return any values.

If you use DateTime? instead of DateTime it will avoid this error by returning null.