Linq Distinct没有带回正确的结果

Question

I'm trying to select a distinct values from a DataTable using Linq . The DataTable gets populated from an excel sheet which has dynamic column apart from each excel sheet has a column name SERIAL NUMBER which is mandatory.

I have a DataTable for demo purpose which consist of 4 serial number as:

• 12345
• 12345
• 98765
• 98765

When I do

var distinctList = dt.AsEnumerable().Select(a => a).Distinct().ToList();

If I do

var distinctList = dt.AsEnumerable().Select(a => a.Field<string>("SERIAL NUMBER").Distinct().ToList();

Then I get the correct results, however but it only contains the one column from dt and not all the other columns

I get all four records instead of 2. Can someone tell me where I'm going wrong please.

Answer 1

The problem is that Distinct method by default uses the default equality comparer, which for DataRow is comparing by reference . To get the desired result, you can use the Distinct overload that allows you to pass IEqualityComparer<T> , and pass DataRowComparer.Default :

The DataRowComparer<TRow> class is used to compare the values of the DataRow objects and does not compare the object references.

var distinctList = dt.AsEnumerable().Distinct(DataRowComparer.Default).ToList();

For more info, see Comparing DataRows (LINQ to DataSet) .

Answer 2

In ToTable method the first parameter specifies if you want Distinct records, the second specify by which column name we will make distinct.

DataTable returnVals = dt.DefaultView.ToTable(true, "ColumnNameOnWhichYouWantDistinctRecords");

Here there is no need to use linq for this task !

Answer 3

So, you want to group them by Serial Number and retrieve the full DataRow? Assuming that after grouping them we want to retrieve the first item:

var distinctList = dt.AsEnumerable().GroupBy(a => a.Field<string>("SERIAL NUMBER"))
                       .Select(a => a.FirstOrDefault()).Distinct().ToList();

EDIT: As requested

var distinctValues = dt.AsEnumerable().Select(a => a.Field<string>("SERIAL NUMBER")).Distinct().ToList();
var duplicateValues = dt.AsEnumerable().GroupBy(a => a.Field<string>("SERIAL NUMBER")).SelectMany(a => a.Skip(1)).Distinct().ToList();
var duplicatesRemoved = dt.AsEnumerable().Except(duplicateValues);

Answer 4

Using Linq a GroupBy would be better suited, by the sounds of it.

var groups = dt.AsEnumerable().GroupBy(a => a.SerialNumber).Select(_ => new {Key = _.Key, Items = _});

This will then contain groupings based on the Serial Number. With each group of items having the same serial number, but other property values different.

Answer 5

Try this:

List<string> distinctValues = (from row in dt.AsEnumerable() select row.Field<string>("SERIAL NUMBER")).Distinct().ToList();

However to me this also works:

List<string> distinctValues = dt.AsEnumerable().Select(row => row.Field<string>("SERIAL NUMBER")).Distinct().ToList();