如何从此列表中检索列数据

Question

I only have the id for the specific line of data which is saved in item. I am trying to retrieve extra data to use in an insert statement to keep a log of who deletes on the system.

    public static void DeleteFromClass(this Entities context, List<long> ids, string deletedBy)
    {
        if (ids.Count == 0) return;

        foreach (var item in ids)
        {
            var result = (from details in context.Class
                          where details.ID == item
                          select new
                          {
                              id = item,
                              studNumber = details.studNumber,
                              name = details.name,
                              lastName = details.lastName,
                          }).ToList();

            var InsertQuery = "INSERT INTO  DeletedStudents (studNumber, name, lastName, DeletedBy, DeletedOn, stage) " +
                "VALUES (" + result.Where(e => e.id == item).Select(e => e.studNumber) + "," + result.Where(e => e.id == item).Select(e => e.name) + "," + result.Where(e => e.id == item).Select(e => e.lastName) + "," + deletedBy + "," + DateTime.Now + ", 1) ";                                      
        }
    }

Answer 1

This code doesn't do what you think.

result.Where(e => e.id == item).Select(e => e.studNumber)

^ That code returns an object representing zero or more records. You probably did not mean to concatenate it as a string.

If you want to get a single string from a single record, you could do something like this:

result.Single(e => e.id == item).studNumber

If you think there may be more than one record returned, perhaps you just want to use the first one:

result.First(e => e.id == item).studNumber

And if you're not sure there is a record, you could do something like this:

result.FirstOrDefault(e => e.id == item)?.studNumber ?? ""

Or if you want an insert statement for each record:

string[] sql = result.Select( e => "INSERT blah blah VALUES (" + e.studNumber + ").ToArray();

That all being said, you probably should not concatenate strings to form SQL statements . You should use a stored procedure with strongly-typed arguments.

Or better yet, don't mix and match EF and SQL. Use the EF to do the inserts instead.