How to combine / merge two StringCollection into one in C#

Question

How to combine / merge two StringCollection in C#

var collection1 = new StringCollection () { "AA", "BB", "CC" };
var collection2 = new StringCollection () { "DD", "EE", "FF" };
var resultCollection = collection1 + collection2 ; // TODO

StringCollection is an obsolete type, you shouldn't be using it *at all*. — Servy, Oct 14 '20 at 18:09
I would agree that just about any generic collection like List would be better, and I had never heard of StringCollection before, but if it's an obsolete type why would it still be supported in .NET 5.0RC1 https://learn.microsoft.com/en-us/dotnet/api/system.collections.specialized.stringcollection?view=netcore-3.1 Is it being kept around just to support legacy code? Maybe Mark is working on legacy code and needs to continue using StringCollection. — TJ Rockefeller, Oct 14 '20 at 19:17
@TJRockefeller If you work on the .NET world, you should know that all code is made retrocompatible all the way to .NET 1.0. So it is still going to exist as long as Microsoft doesn't reverse that decision and finally obsolete so much **** :) — Camilo Terevinto, Oct 14 '20 at 19:31

score 2 · Answer 1 · answered Oct 14 '20 at 16:17

You can copy all to an array like this

    var collection1 = new StringCollection() { "AA", "BB", "CC" };
    var collection2 = new StringCollection() { "DD", "EE", "FF" };

    var array = new string[collection2.Count + collection1.Count];

    collection1.CopyTo(array, 0);
    collection2.CopyTo(array, collection1.Count);

score 1 · Accepted Answer · answered Oct 14 '20 at 16:40

1

If you still want a string collection you can just use AddRange

var collection1 = new StringCollection () { "AA", "BB", "CC" };
var collection2 = new StringCollection () { "DD", "EE", "FF" };
var resultCollection = new StringCollection();
resultCollection.AddRange(collection1.Cast<string>.ToArray());
resultCollection.AddRange(collection2.Cast<string>.ToArray());

Seems odd that StringCollection doesn't have any direct support for adding other StringCollections. If efficiency is a concern, Beingnin's answer is probably more efficient than the answer here, and if you still need it in a StringCollection you can take the array that is generated and use AddRange to add that array of strings to a new StringCollection

answered Oct 14 '20 at 16:40

TJ Rockefeller

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"is probably more efficient" -> it is definitively more efficient. It's one loop over each collection versus two loops in your answer. Also, why not just call `AddRange` on one of the existing collections? – Camilo Terevinto Oct 14 '20 at 17:41
@CamiloTerevinto it's not clear what the intent is for the string collections here. If you call `AddRange` on an existing collection it will modify that collection instead of giving you a new collection. The question does not suggest that it would be okay to mutate one of the original collections, so this combines the two collections into a new collection without changing the original collections. – TJ Rockefeller Oct 14 '20 at 19:04

ssilas777 · Answer 3 · 2020-10-15T03:50:54.727

0

you can cast as array and use Union, please note this will also remove duplicates

var resultCollection = collection1.Cast<string>().Union(collection2.Cast<string>())

edited Oct 15 '20 at 03:50

answered Oct 14 '20 at 16:12

ssilas777

9,672
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1

I think the collection variables need to be cast. – LarsTech Oct 14 '20 at 16:15
No there is no 'Union' extension for it. – Rahul Uttarkar Oct 14 '20 at 16:16
2

@MarkMacneilBikeio `var resultCollection = collection1.Cast().Union(collection2.Cast());` – LarsTech Oct 14 '20 at 16:17
@LarsTech Thanks for noticing it, updated my answer – ssilas777 Oct 14 '20 at 16:20
Working fine now after casting. But why do we need cast explicitly to string again when its not necessary (Microsoft needs to avoid extra casting) – Rahul Uttarkar Oct 14 '20 at 16:28
1

You need to cast because `StringCollection` implements `IEnumerable` but not `IEnumerable`. – madreflection Oct 14 '20 at 16:28
@madreflection Oh Okay – Rahul Uttarkar Oct 14 '20 at 16:29

Enoch Andrade · Answer 4 · 2020-10-14T18:06:53.010

-2

you can occupy List instead of StringCollection ...

        var collection1 = new List<string>() { "AA", "BB", "CC" };
        var collection2 = new List<string>() { "DD", "EE", "FF" };
        var resultCollection = collection1.Concat(collection2).ToList();

edited Oct 14 '20 at 18:06

answered Oct 14 '20 at 16:20

Enoch Andrade

1
1

why use "object"? – LarsTech Oct 14 '20 at 16:23

How to combine / merge two StringCollection into one in C#

4 Answers4