c# - Error Applying Entity relation design with Abstract Classes when assigning a List containing a more derived class -


i'm defining 2 abstract classes: ac1, , ac2. ac1 has property (variable) list of type ac2 (list<ac2>) (that's way of defining 1 many relation).

then, defined r1 comes ac1 (r1 : ac1) , r2 : ac2. problem comes when i'm writing r1's constructor. i'm passing parameter list of type r2 (list<r2>) , try assign property (variable) defined in ac1 (list<ac2>), fails cannot implicitly convert r2 ac2 (even when r2 comes ac2).

code example:

abstract class ac2 {     //several properties , methods here }  abstract class ac1 {     list<ac2> dammvariable {get; set;} //problematic variable (see class r1)     //other stuff here }  class r2 : ac2 {     //some properties exclusive r2 here     public r2(){} }  class r1 : ac1 {     //some properties exclusive r1 here     public r1(list<r2> r2s)     {         this.dammvariable = r2s; //i found error right here     } }

i'll have other classes coming abstract classes, each time create class (x2, example) comes ac2 i'll need x1 class has list<x2>.

am failing in design or implementation?

i'll appreciate help.

there's couple issues here.

  • firstly, you'll need change protection level of dammvariable @ least protected in order access in subclasses.

  • but you'll faced type safety problem:

cs0029: cannot implicitly convert type system.collections.generic.list<r2> system.collections.generic.list<ac2>

this because list<t> invariant, not able assign list<r2> list<ac2> despite inheritance relationship between r2 , ac2.

assuming once assigned, never need add / remove elements dammvariable, change type list type allows covariance, e.g.

abstract class ac1 {     protected ienumerable<ac2> dammvariable {get; set;} )

more on covariance / contravariance in this question


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