LINQ查询未返回唯一行

Question

I have this LINQ query which queries SQL Express database through Entity Framework but doesn't return unique rows when written this way in Visual Studio:

var q = from s in _db.SD
        where s.ID == ID && s.Time >= startDate && s.Time <= endDate
        select s

It returns correct number of rows but each row has same data as first one. However, when I tested the same query in LinqPad the result returned is fine ie correct number of rows and unique data in each row.

Secondly, if I try to change the statment to this:

var q = from s in _db.SD
        where s.ID == ID && s.Time >= startDate && s.Time <= endDate
        select s.Data

Then I got correct number of rows and unique value in each row. Can someone please help me find out the problem with code?

Thanks in advance. EDIT: Here's full function:

public List<SD> GetHistory(int ID, DateTime startDate, DateTime endDate)
{
    var q = (from s in _db.SD
            where s.ID == ID &&
            s.Time >= startDate && s.Time <= endDate
            select s).ToList();

   return q;
}

Answer 1

As you are getting correct distinct data using select s.Data , then you can write a Comparer class implementing IEqualityComparer<> inside the SD class as follows:

public class SD
{
    public Int16 ID { get; set; }

    public Byte MT { get; set; }
    public Byte Data { get; set; }
    public Byte SS { get; set; }
    public DateTime Time { get; set; }

    public class Comparer : IEqualityComparer<SD>
    {
        public bool Equals(SD x, SD y)
        {
            return x.Data == y.Data; // look...here I m using 'Data' field for distinction
        }

        public int GetHashCode(SD obj)
        {
            unchecked  // overflow is fine
            {
                int hash = 17;
                hash = hash * 23 + obj.Data.GetHashCode();
                return hash;
            }
        }
    }
}

Then you can write the GetHistory() method passing the Comparer in the Distinct() method as follows:

public List<SD> GetHistory(int ID, DateTime startDate, DateTime endDate)
{
    var list = new List<SD>();
    var q = (from s in list
                where s.ID == ID &&
                s.Time >= startDate && s.Time <= endDate
                select s).Distinct(new SD.Comparer()).ToList();

    return q;
}

---

EDIT

The regular query uses the default (reference equality) for checking whether two objects are equal or not.

So, in order to make Distinct() work the way you want, you have to implement IEqualityComparer<T> . Otherwise, it will use the default (reference equality) for checking, which means it would only determine the elements were not distinct if they were the same exact instance. See the reference here.