c# - Return class derived from generic constraint from a task -

i have created generic task interface

public interface icalcloadertask {        task<t> execute<t>(basetaskparameters taskparams, cancellationtoken cancellationtoken) t : calcloadertaskresult; } 

and class derived calcloadertaskresult

public class copynmstoreresult : calcloadertaskresult {     public string nmstoreresultsfilepath { get; set; } } 

in implementation of icalcloaderclass, cannot figure out way return results class

public async task<t> execute<t>(basetaskparameters taskparams, cancellationtoken cancellationtoken) t : calcloadertaskresult     {         var copyfiletaskparams = (copyfilestaskparameters)taskparams;         var globalnmstoreresultsfilepath = string.format("{0}\\whateverthefilenameis.xls", copyfiletaskparams.globalsharefolderpath);         var localnmstoreresultsfilepath = string.format("{0}\\whateverthefilenameis.xls", copyfiletaskparams.globalsharefolderpath);          await fileutility.copyfileasync(globalnmstoreresultsfilepath, localnmstoreresultsfilepath, cancellationtoken);          var result = new copynmstoreresult         {             nmstoreresultsfilepath = globalnmstoreresultsfilepath         };         return result; //cannot implicitly convert type     } 

how return thisresult? or pattern incorrect? re-using type of pattern different return types derived calcloadertaskresult. calcloadertaskresult abstract

since implementation returns object of same type, formulate interface differently

public interface icalcloadertask<t>     t : calcloadertaskresult {        task<t> execute(basetaskparameters taskparams, cancellationtoken cancellationtoken) } 

i.e. type (interface) generic instead of method.

and let implementation implement concrete type

public class myimplementation : icalcloadertask<copynmstoreresult> {     public async task<copynmstoreresult> execute(basetaskparameters taskparams,                                                  cancellationtoken cancellationtoken)     {         ...         var result = new copynmstoreresult         {             nmstoreresultsfilepath = globalnmstoreresultsfilepath         };         return result;     } } 

because caller cannot choose type of return value anyway.

it technically possible let the generic type parameter open

public class myimplementation<t> : icalcloadertask<t> t ... 

but since return type of method hard-coded, there no advantage in doing so.

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note: possible let method return different types. adding generic type constraint new allows create object new t(). constraint new means type t must have default constructor.

t mymethod<t>()     t : mybase, new {     return new t(); }