how to use hashtable in Linq

Question

I am trying to use hash table in linq to fetch the key whose value is ABC. What I have done so far:

Hashtable h=new Hashtable ();
h.Add(1 , "ABC");
h.Add(2 , "AgC");
h.Add(3 , "ABC");
h.Add(4 , "AhC");

Expected output: 1, 3 (keys having value "ABC")

ICollection c= h.Keys;

var posi= from a in c
          where h[a]="ABC"
          select a;

But the above query is not working and giving compile time error.

The error is:

could not find an implementation of the query pattern for source type 'System.Collections.ICollection'.

What I am doing wrong? I am new to C#. How to use Hashtable in LINQ?

I would recommend using a `Dictionary`. [More info on `Hashtable` vs `Dictionary` here.](http://stackoverflow.com/questions/301371/why-is-dictionary-preferred-over-hashtable) — Loetn, May 12 '14 at 06:37

score 4 · Accepted Answer · answered May 12 '14 at 06:31

4

You should not use non-generic HashTable to start with. Use generic Dictionary<int, string> instead:

var d = new Dictionary<int, string>();
d.Add(1 , "ABC");
d.Add(2 , "AgC");
d.Add(3 , "ABC");
d.Add(4 , "AhC");

var posi = from a in d
           where a.Value == "ABC"
           select a.Key;

answered May 12 '14 at 06:31

MarcinJuraszek

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Thanks for the answer sir but how can we implement the same using Hashtable? Thanks in advance – user2623213 May 12 '14 at 06:34
And also if I am using group by a.Value I am getting error can't convert to lambda expression. Why? – user2623213 May 12 '14 at 06:39
How could a possibly know without you showing how query with `group by` looks like? – MarcinJuraszek May 12 '14 at 06:41
posi = from a in d Group by a.Value select a.Key; – user2623213 May 12 '14 at 06:42
You should start with LINQ tutorials. You have to specify not only the key, but also what values you want to group. Should be `group a by a.Value into g` – MarcinJuraszek May 12 '14 at 06:43
@user2623213 A Dictionary IS a Hashtable. It's just a generic version. – Dennis_E May 12 '14 at 08:11

score 2 · Answer 2 · edited May 23 '17 at 12:21

Use a Dictionary<int, string> instead of a Hashtable (see here for why) then do the following:

var dist = new Dictionary<int, string>();
dist.Add(1 , "ABC");
dist.Add(2 , "AgC");
dist.Add(3 , "ABC");
dist.Add(4 , "AhC");

var keys = dist.Where(d=> d.Value == "ABC").Select(d=> d.Key);

But If you want Hastable, Then please take a look this link

link 1

link 2

But My opinion is, Please use Dictionary .

Because Dictionary is a generic collections, And It's are a lot faster as there's no boxing/unboxing.

score 2 · Answer 3 · answered May 12 '14 at 06:44

2

If you really want to use that HashTable instead of the Dictionary you can do the following:

var posi = from a in c.Cast<int>()
           where h[a] == "ABC"
           select a;

answered May 12 '14 at 06:44

Stormenet

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score 0 · Answer 4 · answered May 12 '14 at 06:35

You just need to do this to get position:

int indexOf ( object s, Hashtable h )
{
    foreach ( DictionaryEntry e in h )
        if ( e.Value.Equals(s) )
            return (int)e.Key;
    return -1;
}

and call it like this:

var posi = indexOf("ABC", yourHashTable);

but it is true that using Dictionary would be much easier.

how to use hashtable in Linq

4 Answers4