c# - Issues with the "where" in generic inheritance -

in following code problem on line entities = dospecialstuff(entities); because i'm "missing" in part dospecialstuff() methode

error
the type 't' cannot used type parameter 't' in generic type or method
'myworkclass<t>.dospecialstuff<t>(ienumerable<t> entities)'. there no implicit reference conversion 't' 'ispezial'.)

code

here ineritance structure

public class baseclass { }  public interface ispezial  {      int myspecial{get;set;}  }  public class class1 : baseclass { }  public class class2 : baseclass, ispezial {     public int myspecial{ get;set;} } 

here data provider

public class myrepository {     public static ienumerable<tresult> load<tresult>() tresult : baseclass, new()     {         list<tresult> myenum = new list<tresult>();          if (typeof(ispezial).isassignablefrom(typeof(tresult)))         {             myenum.add((new class2() { myspecial = 0 }) tresult);             myenum.add((new class2() { myspecial = 1 }) tresult);             myenum.add((new class2() { myspecial = 2 }) tresult);         }         else         {             myenum.add((new class1() tresult));             myenum.add((new class1() tresult));             myenum.add((new class1() tresult));         }          return myenum;     } } 

and there class stuff , provides errors boss

public class myworkclass<t> t : baseclass, new() {     public void donormalstuff()     {         var entities = myrepository.load<t>();         if (typeof(ispezial).isassignablefrom(typeof(t)))         {             entities = dospecialstuff(entities);         }     }      public ienumerable<t> dospecialstuff<t>(ienumerable<t> entities) t : ispezial     {         var list = new list<t>();          return list.where(special => special.myspecial==2);     } } 

some further question

how can avoid new() in where's?

and how can change add part myrepository myenum.add(new class1()); instead of myenum.add((new class1() tresult)); without changing return ienumerable<tresult>?

since there no implicit conversion between t (baseclass or child of baseclass), it's not enough check if ispezial assignable (which do, , it's that), need explicitly cast variable when passing parameter, , cast back. it's not best class design, though.

 entities = dospecialstuff(entities.cast<ispezial>()).cast<t>(); 

2nd question: since explicitly call class2() , class1() constructors, you're not relying on polymorphism/generic typing of c#, why new() constraint there (if wrote new t(), example). long keep code that, can remove constraint.

3rd: can make use of dynamics keyword, make list of dynamic type, , cast in last moment:

list<dynamic> myenum = new list<dynamic>(); //your code myenum.add((new class2() { myspecial = 0 })); //it's ok write want //your code return myenum.cast<tresult>();