Re: building a c# project - C# Discussion Boards

6-Sep-06 9:33

How can I cast an inherithed generic class?

class c1{}
class c2 : c1{}
interface I1<t>{ void meth(T t);}

class g1<t> : I1<t> where T : c1{}
class g2<t> : g1<t> where T : c2{}

static main(....)
{
I1<c1> oI1 = null;
g2<c2> og2 = new g2<c2>();
oI1 = (I1<c1>) og2;
********************* I get this error :
Unable to cast object of type 'g2`1[c2]' to type 'I1`1[c1]'.
*********************

Why? If c2 is inherited from c1 and g2 is inherited from g1 and I1, this should not work?

Thanks

Nader Elshehabi6-Sep-06 10:17

6-Sep-06 10:17

Hello

Well, your code compiled well with me after some modifications. Here it is:

class c1{}
class c2 : c1{}
interface I1{ }

class g1<t> : I1 where t: c1{}
class g2<t> : g1<c1> where t: c2{}

I1 oI1 = null;
g2<c2> og2 = new g2<c2>();

public void MyMethod()
{
oI1 = (I1)og2;
}

You must have made another mistake somewhere!!

Regards

daveroberge6-Sep-06 10:46

6-Sep-06 10:46

Thanks for your quick answer.

You're right, I made a mistake. I didn't write enough detail and it was confusing. I'm sorry.

Here's all my example :

class c1{}
class c2 : c1{}
interface I1<t> {void Add(t a);}
class g1<t> : I1<t> where t : c1{
public virtual void Add(t a){...}}
class g2<t> : g1<c1> where t : c2 { }

I1<c1> oI1 = null;
g2<c2> og2 = new g2<c2>();
oI1 = (I1<c1>)og2;

*******At this point, everything is good****
However if I do : og2.Add(...) my parameter can be type c1!!! And I don't want because in my g2 class, the object used must be of type c1.
If a change this line
class g2<t> : g1<c1> where t : c2 { }' for
class g2<t> : g1<c2> where t : c2 { }', everyghing is good at the compile and I have to pass a class c2 in the Add method. A class c1 is not allowed and that's good. Unfortunately, casting doesn't work.

Thanks again

Nader Elshehabi6-Sep-06 11:30

6-Sep-06 11:30

Hello

daveroberge wrote:
it was confusing

Was??! H$#l I'm lost in all those Gs an Cs?!! WTF | :WTF:

Anyway, we will work this out bit by bit.

daveroberge wrote:
if I do : og2.Add(...) my parameter can be type c1!!! And I don't want

C2 is inherited from C1!! So C1 could fit in the place of C2 sometimes.

Now I don't get three things:
1- You want to cast what to what?
2- Where is the call that makes the error?
3- One time you say your parameter can be C1 then you say C1 is not allowed. I'm totally lost!!

Please repost your code with the error -don't modify your cast-, and comment on the line with the error. The most important thing is to check the "Ignore HTML tags..." below the post editting textbox so that the generic code tags would appear in your code.

PS. If I didn't reply for a while that means I fell asleep:->. Then I'll first thing in the morning!! It's 12:26AM here in Cairo and I should have been in bed about an hour ago!! Sleepy | :zzz:

Regards

daveroberge6-Sep-06 14:04

6-Sep-06 14:04

Hope your night was good Big Grin | :-D

this is a better code :

interface I1<t> { void Add(t a); }

class c1{
public string sVal = "test";}

class c2 : c1{
public string sVal2 = "test2";}

class g1<t> : I1<t> where t : c1{
public virtual void Add(t a){a.sVal = "svalmodified";}}

class g2<t> : g1<c2> where t : c2 {
public override void Add(c2 a){
a.sVal2 = "testmodified2";
base.Add(a);}}

private I1<c1> GetG1(){
g1<c1> oG1 = new g1<c1>();
oG1.Add(new c1());
return oG1;}

private I1<c1> GetG2(){
g2<c2> oG2 = new g2<c2>();
oG2.Add(new c2());
return (I1<c1>)oG2;} ********* see below the error ********

private void TestMethod(){
I1<c1> oTest1 = GetG1();
I1<c1> oTest2 = GetG2();}

****** error ******
Unable to cast object of type 'WindowsApplication2.g2`1[WindowsApplication2.c2]' to type 'WindowsApplication2.I1`1[WindowsApplication2.c1]'

I hope it's less confusing... Wink | ;)

Nader Elshehabi7-Sep-06 1:59

7-Sep-06 1:59

Hello

Gotcha!! Now your problem is clear. Here it is: -Watch carefully for generic types-

1- First you declare an object of type I1<c1> you name it oTest2
2- Then you call GetG2()
3- In the method you declare another object of type g2<c2> with the name oG2
Are following me??

Now the problem is that the definition of class g2 is that you inherit it from class g1<c2> -notice the c2 here!!-. So in the base class g1 which has the interface I1 the generic type is c2!! So by default the generic type of I1 -which is the interface of g1 and has the same generic type- is c2, not c1!! That's why when you try to return oG2 and cast it to interface I1<c1> it doesn't work!! It implements interface I1<c2> not I1<c1>. I hope now you see your problem now.

Suggested solutions -among many other solutions of course-.
1- Return type I1<c2> instead
private I1<c2> GetG2()
{
g2<c2> oG2 = new g2<c2>();
oG2.Add(new c2());
return (I1<c2>)oG2;
}

I1<c2> oTest2 = GetG2();
This will preserve your inheritance structure.

OR
Change your inheritance to implement I1<c1> interface in G2

class g2<t> : g1<c2>, I1<c1> where t : c2
{
public override void Add(c2 a)
{
a.sVal2 = "testmodified2";
base.Add(a);
}

public void Add(c1 a)
{
//Do something
}
}

I hope this helps!!

Regards

daveroberge7-Sep-06 3:07

7-Sep-06 3:07

Yes it helps me a lot for my understanding.

I thought that because c2 is inherited from c1, I1<c2> and I1<c1> was the same thing. So I1<c1> is the base of I1<c2> and in this case, it could be possible to cast from a I1<c2> to I1<c1>.

The problem with your solution #1 is that I can't put both return value from GetG1() & GetG2() in the same objet or an array of the same type (Ex: I1<c1>[]). The solution #2 is better but you're gonna be able to add an object of type c1.

Is there a pattern to do what I want? I mean, you have a factory accepting a base class X and you want to inherite a new factory but in this one, you only want to accept an inherited class from X.

Thanks for your help!!!

Nader Elshehabi7-Sep-06 9:04