c# - Slow LINQ query for .ToArray() -

i using following query

foreach (var calldetailsfornode_rearrange in calldetailsfornodes_rearrange) {     var test = r1 in dtrowfornode.asenumerable()                join r2 in dtfilerowfornode.asenumerable()                on r1.field<int>("lng_upload_id") equals r2.field<int>("lng_upload_id")                ((r1.field<string>("txt_called_number") == calldetailsfornode_rearrange.caller2.tostring()) || r1.field<string>("txt_calling_number") == calldetailsfornode_rearrange.caller2.tostring())                select r2.field<string>("txt_file_name");      var d = test.distinct(); } 

upto here query run in no time. added

string[] str =d.toarray(); strfilename = string.join(",", str); 

it takes 4-5 seconds run. makes slow on adding .toarray() ?

upto here query run in no time.

up here, hasn't done anything, except build deferred-execution model represents pending query. doesn't start iterating until call movenext() on iterator, i.e. via foreach, in case via .toarray().

so: takes time because doing work.

consider:

static ienumerable<int> getdata() {     console.writeline("a");     yield return 0;     console.writeline("b");     yield return 1;     console.writeline("c");     yield return 2;     console.writeline("d"); } static void main() {     console.writeline("start");     var data = getdata();     console.writeline("got data");     foreach (var item in data)         console.writeline(item);     console.writeline("end"); } 

this outputs:

start got data 0 b 1 c 2 d end 

note how work doesn't happen @ once - both deferred (a comes after got data) , spooling (we don't a,...,d,0,...2).

---

related: how distinct() works, comments:

public static ienumerable<t> distinct<t>(this ienumerable<t> source) {     var seen = new hashset<t>();     foreach(var item in source) {         if(seen.add(item)) yield return item;     } } 

...

and new join operation:

public static string join(this ienumerable<string> source, string separator) {     using(var iter = source.getenumerator()) {         if(!iter.movenext()) return "";         var sb = new stringbuilder(iter.current);         while(iter.movenext())             sb.append(separator).append(iter.current);         return sb.tostring();     } } 

and use:

string s = d.join(",");