c# - Why casting from SomeClass<T> where T : BaseClass to SomeClass<DerivedClass : BaseClass> is not possible? -

i trying create generic method returns instance of irowmapper<t>. here classes:

public abstract class person {     public int id { get; set; }      protected void person() { }      protected void person(int id)     {         id = id;     } }  public class employer : person {     public int employeeid { get; set; }      public void employer() { }      public void employer(int id, int employeeid) : base(id)     {         employeeid = employeeid;     } }  public class employee : person {     public int employerid { get; set; }      public void employee() { }      public void employee(int id, int employerid) : base(id)     {         employerid = employerid;     } }  public static class mapbuilder<tresult> tresult : new() {     // ... }  public interface irowmapper<tresult> {     tresult maprow(idatarecord row); } 

now following:

private irowmapper<t> getrowmapper<t>() t : person, new() {     var rowmapper = mapbuilder<t>.mapnoproperties()                                     .map(c => c.id).tocolumn("id");      if (typeof (t) == typeof (employee))     {         rowmapper =             ((mapbuilder<employee>) rowmapper).map(c => c.employerid)                                                 .tocolumn("employerid");     }     else if (typeof (t) == typeof (employer))     {         rowmapper =             ((mapbuilder<employer>) rowmapper).map(c => c.employeeid)                                                 .tocolumn("employeeid");     }      return rowmapper.build(); } 

but following error:

error 2 cannot convert type 'microsoft.practices.enterpriselibrary.data.imapbuildercontext' 'microsoft.practices.enterpriselibrary.data.mapbuilder'

error 2 cannot convert type 'microsoft.practices.enterpriselibrary.data.imapbuildercontext' 'microsoft.practices.enterpriselibrary.data.mapbuilder'

why cast not possible?

i'm not familiar library, looks return value of each method imapbuildercontext<t> , it's written in typical fluent style.

i think might work you:

private irowmapper<t> getrowmapper<t>() t : person, new() {     var rowmapper = mapbuilder<t>.mapnoproperties()                                  .map(c => c.id).tocolumn("id");      if (typeof (t) == typeof (employee))     {         rowmapper = ((imapbuildercontextmap<employee>)rowmapper)             .map(c => c.employerid).tocolumn("employerid");     }     else if (typeof (t) == typeof (employer))     {         rowmapper = ((imapbuildercontextmap<employer>)rowmapper)             .map(c => c.employeeid).tocolumn("employeeid");     }      return rowmapper.build(); }