如何使用Linq获取表的最大ID或1（如果为空）？

Question

i manually insert the id of a table by selecting the max id from the table and increment it by 1. how do i do that with c# and linq if the table is empty and the max returns null i'm using the below code but it is giving the error "The null value cannot be assigned to a member with type System.Int32 which is a non-nullable value type"

var t = (from tab1 in db.questions select tab1.QuestionId1).Max();
            if (t!=null)
            {
                 qui = t + 1;
            }

            else
            {
                 qui = 1;
            }

Answer 1

Would something like this work?

// Assume we want to start with id 1
var maxId = db.Questions
    .Max(q => q.QuestionId1)
    .DefaultIfEmpty(0)
    .Max();

return maxId + 1;

DefaultIfEmpty will handle the empty table case.

Databases usually have an auto incremented id feature. If you are using Entity Framework then it should be enabled by default.

Answer 2

Definitely not the preferred method to get "new ids" (better have the database generate them), but here it goes:

var maxId = ((from tab1 in db.questions select (int?)tab1.QuestionId1).Max() ?? 0) + 1;

Or the equivalent, but IMO easier to read:

var maxId = (db.questions.Select(x => (int?)x.QuestionId1).Max() ?? 0) + 1

Your original problem is that you are calling Max() using a IEnumerable<int> parameter, and that doesn't allow it to return an empty value (it throws an exception) since the return value is int and int can't be null .

I'm casting the enumerable returned by Select() to an enumerable of Nullable<int> (or int? for short) types, so Max() returns an int? which allows it to return null if the enumerable is empty (as it's the case when no rows are selected), instead of throwing a exception.

Answer 3

You probably are looking for something like this, using LINQ's Extension Method syntax:

var maxID = db.Questions.Any() 
            ? db.Questions.Max(q => QuestionId1) + 1
            : 1

If there are any Questions in the database grab the highest QuestionId1 and increment by 1. Otherwise return 1.