linq query to return distinct field values from a list of objects

Question

class obj
{
    int typeId; //10 types  0-9 
    string uniqueString; //this is unique
}

Assume there is list with 100 elements of obj, but only 10

Answer 1

If just want to user pure Linq, you can use groupby:

List<obj> distinct =
  objs.GroupBy(car => car.typeID).Select(g => g.First()).ToList();

If you want a method to be used all across the app, similar to what MoreLinq does:

public static IEnumerable<TSource> DistinctBy<TSource, TKey>
    (this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
{
    HashSet<TKey> seenKeys = new HashSet<TKey>();
    foreach (TSource element in source)
    {
        if (!seenKeys.Contains(keySelector(element)))
        {
            seenKeys.Add(keySelector(element));
            yield return element;
        }
    }
}

Using this method to find the distinct values using just the Id property, you could use:

var query = objs.DistinctBy(p => p.TypeId);

you can use multiple properties:

var query = objs.DistinctBy(p => new { p.TypeId, p.Name });

Answer 2

objList.Select(o=>o.typeId).Distinct()

Answer 3

If just want to use Linq, you can override Equals and GetHashCode methods.

Product class:

public class Product
{
    public string ProductName { get; set; }
    public int Id { get; set; }


    public override bool Equals(object obj)
    {
        if (!(obj is Product))
        {
            return false;
        }

        var other = (Product)obj;
        return Id == other.Id;
    }

    public override int GetHashCode()
    {
        return Id.GetHashCode();
    }
}

Main Method:

static void Main(string[] args)
    {

        var products = new List<Product>
        {
            new Product{ ProductName="Product 1",Id = 1},
            new Product{ ProductName="Product 2",Id = 2},
            new Product{ ProductName="Product 4",Id = 5},
            new Product{ ProductName="Product 3",Id = 3},
            new Product{ ProductName="Product 4",Id = 4},
            new Product{ ProductName="Product 6",Id = 4},
            new Product{ ProductName="Product 6",Id = 4},
        };

        var itemsDistinctByProductName = products.Distinct().ToList();

        foreach (var product in itemsDistinctByProductName)
        {
            Console.WriteLine($"Product Id : {product.Id} ProductName : {product.ProductName} ");
        }

        Console.ReadKey();
    }

Answer 4

I think this is what your looking for:

    var objs= (from c in List_Objects 
orderby c.TypeID  select c).GroupBy(g=>g.TypeID).Select(x=>x.FirstOrDefault());      

Similar to this Returning a Distinct IQueryable with LINQ?

Answer 5

Sure, use Enumerable.Distinct.

Given a collection of obj (e.g. foo), you'd do something like this:

var distinctTypeIDs = foo.Select(x => x.typeID).Distinct();

Answer 6

I wanted to bind a particular data to dropdown and it should be distinct. I did the following:

List<ClassDetails> classDetails;
List<string> classDetailsData = classDetails.Select(dt => dt.Data).Distinct.ToList();
ddlData.DataSource = classDetailsData;
ddlData.Databind();

See if it helps