linq query group by in a list of strings

Question

I am very new to linq query with c#. I have a list of strings and I would like to get a new list or delete all double strings.

List<string> zipcodes = new List<string>();
zipcodes.Add("1234");
zipcodes.Add("1234");
zipcodes.Add("1234");
zipcodes.Add("4321");
zipcodes.Add("4321");

List<string> groupbyzipcodes =
(from zip in zipcodes
group zip by zip into newgroup
select newgroup.ToList());

cannot implicitly convert type 'System.collection.Generic.IEnumarable<System.Collections.Generic.List<string>' to 'System.collections.Generic.List<string>. an explicit conversion exists (are you missing a cast?)

score 13 · Accepted Answer · answered Jul 30 '15 at 08:15

13

You can also use the distinct keyword in LINQ:

var dedupedlist = zipcodes.Distinct().ToList();

For more see https://msdn.microsoft.com/en-us/library/vstudio/bb348436(v=vs.100).aspx

answered Jul 30 '15 at 08:15

William Moore

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nice one! but where would this be used ? – kimiahdri Sep 11 '22 at 17:58

score 5 · Answer 2 · edited May 23 '17 at 12:06

5

this is the most efficient way to remove duplicate values

var groupbyzipcodes = new HashSet<string>(zipcodes);

I already discussed that issue (HashSet vs Distinct()) here

edited May 23 '17 at 12:06

Community

1
1

answered Jul 30 '15 at 08:14

fubo

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An interesting read: https://stackoverflow.com/questions/6298679/whats-better-for-creating-distinct-data-structures-hashset-or-linqs-distinct – William Moore Jul 30 '15 at 08:16
2

Interesting, I would have thought that distinct would have been implemented in the most efficient way. Personally I feel that distinct is more readable, but it depends on the performance requirements of the application. – William Moore Jul 30 '15 at 08:20

score 2 · Answer 3 · answered Jul 30 '15 at 08:15

2

There is a dedicated Linq method for this purpose:

var newList = zipCodes.Distinct().ToList();

https://msdn.microsoft.com/en-us/library/vstudio/bb348436(v=vs.100).aspx

answered Jul 30 '15 at 08:15

w.b

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score 2 · Answer 4 · answered Jul 30 '15 at 08:16

2

If you only have a list of strings you can use distinct()

zipcodes.Distinct();

It will give you the uniq values of the list

Rgds

answered Jul 30 '15 at 08:16

Marcus Höglund

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Jamiec · Answer 5 · 2015-07-30T08:32:12.927

2

As others have answered - there exists Distinct to get a distinct list from an IEnumerable<T>.

Your specific problem was that you have a ToList() inside the brackets, and it should be outside.

List<string> groupbyzipcodes =
(from zip in zipcodes
group zip by zip into newgroup
select newgroup.ToList())

should have been

List<string> groupbyzipcodes =
(from zip in zipcodes
group zip by zip into newgroup
select newgroup.Key).ToList();

edited Jul 30 '15 at 08:32

answered Jul 30 '15 at 08:20

Jamiec

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Try running this code and you'll get `Cannot implicitly convert type 'System.Collections.Generic.List>' to 'System.Collections.Generic.List'`. You need to do `select newgroup.Key` to get this working. – Chris Jul 30 '15 at 08:28
@Chris - thanks, good spot. – Jamiec Jul 30 '15 at 08:32

linq query group by in a list of strings

5 Answers5