How to Use Linq Distinct() with Multiple Fields

How to use LINQ Distinct() with multiple fields

I assume that you use distinct like a method call on a list. You need to use the result of the query as datasource for your DropDownList, for example by materializing it via ToList.

var distinctCategories = product
                        .Select(m => new {m.CategoryId, m.CategoryName})
                        .Distinct()
                        .ToList();
DropDownList1.DataSource     = distinctCategories;
DropDownList1.DataTextField  = "CategoryName";
DropDownList1.DataValueField = "CategoryId";

Another way if you need the real objects instead of the anonymous type with only few properties is to use GroupBy with an anonymous type:

List<Product> distinctProductList = product
    .GroupBy(m => new {m.CategoryId, m.CategoryName})
    .Select(group => group.First())  // instead of First you can also apply your logic here what you want to take, for example an OrderBy
    .ToList();

A third option is to use MoreLinq's DistinctBy.

Distinct on multiple fields

Try with

GroupBy(
        p => new {p.ItemName, p.SubItemName}
       ).Select(g => new { g.Key.ItemName, g.Key.SubItemName });

How to LINQ Distinct by Multiple Fields without anonymous types

You can do the following:

return
    from c in AllClasses
    group c by new
    {
        c.Batch, 
        c.Degree_ID, 
        c.Specialization_ID, 
        c.CurrentSemester, 
        c.Section_ID 
    } into grp
    select grp.First();

This takes every first Class from the given group, thus returns a Entity object instead of a anonymous type.

How to select distinct based on combination of two fields with LINQ?

You can select just the name and age into an anonymous type, and then use Distinct():

var results = context.people
                     .Select(p => new { p.Name, p.Age })
                     .Distinct();

Obviously that won't give you the height, but then there is no single height. If you want all the heights as well, you need to group instead:

var results = context.people
                     .GroupBy(p => new { p.Name, p.Age });

That will give you a sequence of groupings, where each grouping contains all the people with the same name and age.

LINQ, how to select distinct with two columns

As far as I can understand you want to distinct records by subfield. If so, you can do that using GroupBy:

var pList = db.Part.Where(p => p.field != null && p.field!= "")
             .OrderByDescending(l => l.ID)
             .AsEnumerable()
             .GroupBy(x => x.subfield)
             .Select(x => x.First())
             .Select(x => new SelectListItem
             {
                 Value = x.ID.ToString(),
                 Text = x.subfield
             });

Or if you can use a third party library I recommend DistinctBy method

 var pList = db.Part.Where(p => p.field != null && p.field!= "")
             .OrderByDescending(l => l.ID)
             .AsEnumerable()
             .DistinctBy(x => x.subfield)
             .Select(x => new SelectListItem
             {
                 Value = x.ID.ToString(),
                 Text = x.subfield
             });

I optimized your code a bit by changing ToArray with AsEnumerable, if you just want to fetch result you can use AsEnumerable, it will execute the query and enumerate the results, you don't need to put the result into a collection.

Select distinct list from nested collections on multiple fields

Try This Group by with multiple Keys.

List<Instrument> ins = clientAcccounts.SelectMany(x => x.Holdings.Select(s => s.Instrument)).ToList().GroupBy(p=> new {p.InstrumentID,p.AssetTypeID},(key,group)=>group.First()).ToList<Instrument>();

Distinct on Multiple Columns Entity Framework LINQ

Using anonymous objects will do the trick:

var data = TESTDB.Where(i => i.ALPHA == 1).Select(i => new {i.A, i.B, i.C}).Distinct();

To retain the model:

List<Book> books = db.Book.Select(i => new Book {Author = i.Author, Title = i.Title}).Distinct().ToList();

LINQ to DataSet, distinct by multiple columns

Well, you can do the projection first:

var qry = db.Customers.Select(cust => new {cust.ID, cust.Name, cust.Region})
                    .Distinct();

Or in query syntax:

var qry = (from cust in db.Customers
          select new {cust.ID, cust.Name, cust.Region}).Distinct();

That do?

LINQ to SQL select distinct from multiple colums

The trouble you're having is that VB.NET treats the objects returned from a Linq query differently than C# does, which is why a lot of the answers here are from baffled C# developers. VB.NET returns mutable objects from Linq queries. C# returns immutable objects. So, in C# equality is already handled for you, but in VB.NET you have to specify which fields are considered equal using the Key keyword. You can see this easily in LinqPad yourself:

Dim items As New List(Of KeyValuePair(Of Integer, String))()
items.Add(New KeyValuePair(Of Integer, String)(1, "David"))
items.Add(New KeyValuePair(Of Integer, String)(2, "James"))
items.Add(New KeyValuePair(Of Integer, String)(3, "Smith"))
items.Add(New KeyValuePair(Of Integer, String)(2, "James"))
items.Add(New KeyValuePair(Of Integer, String)(5, "Joe"))

items.Dump()

Dim uhOhResult = (from a in items select New With {a.Key, a.Value}).Distinct()
usOhResult.Dump()
Dim distinctResult = (from a in items select New With {Key a.Key, Key a.Value}).Distinct()
distinctResult.Dump()

In your example, put the Key keyword in to define which fields participate in the equality check, and distinct will work properly.

Dim customer = (From cus In db.Customers Select Key cus.CustomerId, Key cus.CustomerName).Distinct()

See here: Linq Group on Multiple Fields - VB.NET, Anonymous, Key
and here: Distinct in LINQ with anonymous types (in VB.NET)

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