remove duplicates from linq query c#

Question

This is my linq query and I get lots of duplicates with school names. so I created a regex function to trim the text:

public static string MyTrimmings(string str)
        {
        return Regex.Replace(str, @"^\s*$\n", string.Empty, RegexOptions.Multiline).TrimEnd();
        }

the text gets trimed alright, however, the dropdown values are all duplicates! please help me eliminate duplicates, oh Linq joy!!

ViewBag.schools = new[]{new SelectListItem
            {
                Value = "",
                Text = "All"
            }}.Concat(
            db.Schools.Where(x => (x.name != null)).OrderBy(o => o.name).ToList().Select(s => new SelectListItem
            {
                Value = MyTrimmings(s.name),
                Text = MyTrimmings(s.name)
            }).Distinct()
            );

i didn't know distinct take values good to know. thanks i will try that now. — NULL, Oct 03 '13 at 19:01
it said cannot compare lambda expression to iequalitycomparer error — NULL, Oct 03 '13 at 19:02
See [morelinq's DistinctBy](https://code.google.com/p/morelinq/source/browse/MoreLinq/DistinctBy.cs?r=d4396b9ff63932be0ab07c36452a481d20f96307) — L.B, Oct 03 '13 at 19:12
Do your `SelectListItem` implement `IComparable`? If not you should, because `Distinct` uses this interface to do the comparaison. If it's not present, it uses the one from `object` — Shimrod, Oct 03 '13 at 19:14

score 4 · Answer 1 · answered Oct 03 '13 at 19:07

4

Distinct is poor, GroupBy for the win:

db.Schools.GroupBy(school => school.name).Select(grp => grp.First());

answered Oct 03 '13 at 19:07

weston

54,145
21
145
203

Esteban Elverdin · Accepted Answer · 2013-10-03T19:31:17.190

0

Assuming you have a School class you can write an IEqualityComparer

class SchoolComparer : IEqualityComparer<School>
{
    public bool Equals(School x, School y)
    {
        //Check whether the compared objects reference the same data. 
        if (Object.ReferenceEquals(x, y)) return true;

        //Check whether any of the compared objects is null. 
        if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))
            return false;

        //Check whether the school' properties are equal. 
        return x.Name == y.Name;
    }

    // If Equals() returns true for a pair of objects  
    // then GetHashCode() must return the same value for these objects. 

    public int GetHashCode(School school)
    {
        //Check whether the object is null 
        if (Object.ReferenceEquals(school, null)) return 0;

        //Get hash code for the Name field if it is not null. 
        int hashSchoolName = school.Name == null ? 0 : school.Name.GetHashCode();

        //Calculate the hash code for the school. 
        return hashSchoolName;
    }
}

Then your linq query would look like this:

db.Schools.Where(x => x.name != null)
          .OrderBy(o => o.name).ToList()
          .Distinct(new SchoolComparer())
          .Select(s => new SelectListItem
            {
                Value = MyTrimmings(s.name),
                Text = MyTrimmings(s.name)
            });

edited Oct 03 '13 at 19:31

answered Oct 03 '13 at 19:17

Esteban Elverdin

3,552
1
17
21

i get his error: LINQ to Entities does not recognize the method 'System.Linq.IQueryable`1[namespace.Models.School] Distinct[School](System.Linq.IQueryable`1[namespace.Models.School], System.Collections.Generic.IEqualityComparer`1[namespace.Models.School])' method, and this method cannot be translated into a store expression. – NULL Oct 03 '13 at 19:29
I updated the answer, I had put the Distinct in the wrong place – Esteban Elverdin Oct 03 '13 at 19:31
@EstebanElverdin Bringing in the entire result set and doing the work on the client side adds a *lot* of cost over doing the work on the DB end, which can both do it faster (usually) and reduce network traffic. Additionally, in the even that you *do* want to have a query run a LINQ to objects method you should use `AsEnumerable` not `ToList` to avoid eagerly executing the query, forcing the entire query to be in memory rather than having it streamed, and needlessly creating and then discarding a `List` object. – Servy Oct 03 '13 at 19:39
@Servy, I understand your point, I was just trying to make the code in the question works as expected. Regarding the ToList vs AsEnumerable, in this particular case I think there is no difference. – Esteban Elverdin Oct 03 '13 at 19:45
@EstebanElverdin I just listed three differences between them, all of which apply to this example. – Servy Oct 03 '13 at 20:02

score 0 · Answer 3 · edited May 23 '17 at 11:43

You could make your class implement the IEquatable<T> interface, so Distinct will know how to compare them. Like this (basic example):

public class SelectListItem : IEquatable<SelectListItem>
{
    public string Value { get; set; }
    public string Text { get; set; }

    public bool Equals(SelectListItem other)
    {
        if (other == null)
        {
            return false;
        }

        return Value == other.Value && Text == other.Text;
    }

    public override int GetHashCode()
    {
        unchecked
        {
            int hash = 17;

            if (Value != null)
            {
                hash = hash * 23 + Value.GetHashCode();
            }

            if (Text != null)
            {
                hash = hash * 23 + Text.GetHashCode();
            }

            return hash;
        }
    }
}

(GetHashCode taken fron John Skeet's answer here: https://stackoverflow.com/a/263416/249000)

remove duplicates from linq query c#

3 Answers3