c# - Why is IEnumerable(of T) not accepted as extension method receiver -

complete question before code:

why ienumerable<t> where t : itest not accepted receiver of extension method expects this ienumerable<itest>?

and code:

i have 3 types:

public interface itest { } public class element : itest { } public class elementinfo : itest { } 

and 2 extension methods:

public static class extensions {     public static ienumerable<elementinfo> method<t>(         ienumerable<t> collection)          t : itest     { →        return collection.toinfoobjects();     }      public static ienumerable<elementinfo> toinfoobjects(         ienumerable<itest> collection)     {         return collection.select(item => new elementinfo());     } } 

the compiler error (on marked line):

cs1929 : 'ienumerable<t>' not contain definition 'toinfoobjects' , best extension method overload 'extensions.toinfoobjects(ienumerable<itest>)' requires receiver of type 'ienumerable<itest>'

why so? receiver of toinfoobjects extension method ienumerable<t> , generic type constraint, t must implement itest.

why receiver not accepted? guess covariance of ienumerable<t> not sure.

if change toinfoobjects receive ienumerable<t> t : itest, ok.

consider this:

public struct valueelement : itest { } 

and this:

ienumerable<valueelement> collection = ... collection.method(); //ok, valueelement implement itest, required. collection.toinfoobjects() //error, ienumerable<valueelement> not ienumerable<itest>                            //variance not work value types. 

so not every type allowed method allowed toinfoobjects. if add class constraint t in method, code compile.