LINQ选择不同的集合并检索不同的列

Question

I want to be able to execute the following query:

select distinct studentname, subject, grade from studenttable;

The student table has the following fields:

studentid
studentname
subject
grade

The linq query I have now is:

var students=dc.Select(s => new {s.studentname, s.subject, s.grade}).Distinct();

dc is the data context.

This query works, but how do I get the student id while still satisfying the distinct condition on the studentname, subject, grade set?

Answer 1

The issue here is that you've collected three properties of the necessary data which, while they will suffice to pass through Distinct , don't have any link back to their original data, and any such link would break the default implementation of Distinct .

What you CAN do, however, is to use the overload of distinct which takes an IEqualityComparer . This will allow you to compare for equality only on the desired fields, while running Distinct over the entire collection.

var students = dc
    .AsEnumerable() // In case you're using a linq-to-sql framework, this will ensure the query execute in-memory
    .Distinct(new SpecialtyComparer());

//...

public class SpecialtyComparer : IEqualityComparer<StudentTable>
{
    public int GetHashCode(StudentTable s)
    {
        return s.studentname.GetHashCode()
            && s.subject.GetHashCode()
            && s.grade.GetHashCode();
    }

    public bool Equals(StudentTable s1, StudenTable s2)
    {
        return s1.studentname.Equals(s2.studentname)
            && s1.subject.Equals(s2.subject)
            && s1.grade.Equals(s2.grade);
    }
}

Answer 2

I believe your design is broken, but I'll answer your specific question....

I'm assuming you're trying to group by name, subject and grade, and retrieve the first representative student of each group.

In this case, you can group by Tuples. A tuple will give you an Equals and GetHashCode method for free, so can be used in Group operations.

IEnumerable<Student> distinctStudents = students
                                           .AsEnumerable()
                                           .GroupBy(s => Tuple.Create
                                                               (
                                                                   s.studentname, 
                                                                   s.subject, 
                                                                   s.grade
                                                               )
                                                          )
                                           .Select(g => g.First()); /// Returns the first student of each group

Here is a dot net fiddle example: https://dotnetfiddle.net/7K13DJ

Answer 3

When doing a Distinct() on a list of objects, you are aggregating those rows into a smaller number of rows, discarding any duplicates. So, your result will not have the studentid anymore. To preserve the studentid property, you need to use a GroupBy . This will return to you your key (a student of studentname , subject , grade ), and a list of original rows. You can see an example in this question here: ( GroupBy and count the unique elements in a List ).

You will have a list of studentids to choose from, since there might be many rows with the same studentname , subject , and grade . (Otherwise you would not be doing a distinct -- they would be unique and the tuple { sudentname, subject, grade } would be a natural key ). So the question you may need to ask yourself is "Which studentid do I need?"