Cast Action<T> to Action<object> [modified]

[J4amieC]

Why not just to the following, from what I could see would give youthe exact same result, with the benefit of actually compiling!

public interface IBase
{
Func OnComplete { get; set; }
}

public class Base : IBase
{

public Base(Func onComplete)
{
    OnComplete = onComplete;
}

public Func OnComplete { get; set; }

}
public class GenericBase : IBase
{
public GenericBase(Func onComplete)
{
OnComplete = onComplete;
}

public Func OnComplete { get; set; }

}

[BobJanova]

Generic inheritance is a bit complicated. Here's the reason that you can't cast 'down' the generic type parameter inheritance tree. Imagine this test class:

class BadMistake {
void CompleteHandler(Dictionary<string,int> complexParameter){
// some stuff
}

GenericBase<Dictionary<string,int>> instance;

public void CauseDeath(){
instance = new GenericBase<Dictionary<string,int>>(CompleteHandler);
((Base)instance).OnComplete(16);
}
}

Now, CompleteHandler is expecting to be passed a Dictionary, and you can't compile something which would cause that not to be the case. But using the base class, you can pass an int (or anything else) to it. Method parameter compatibility for generics works backwards. This example will compile if the type parameter in the base is a subclass of T, for example try:

public class Base
{
public Func<T> OnComplete {get;set;} where T:Panel
public Base(Func<T> onComplete) where T:Panel
{
OnComplete = onComplete;
}
}
public class GenericBase<T> : Base
{
public GenericBase(Func<T> onComplete) where T:UserControl : base(onComplete)
{
OnComplete = onComplete;
}
}

(if I got the syntax on the constructor there right). Also, there isn't really a reason to make a non generic base type and inherit from it in this way. You can just use a GenericBase<object> if you want one that can deal with anything.

[Adriaan Davel]

I understand that I should avoid boxing & un-boxing (and I do want to, and I do it everywhere), in this case it's not an option. Bit of a complex scenario I'll have to explain (and the code is part of a 5 000 file solution), but I'm quite irritated that this is disallowed, I thought polymorphism / covariance / contravariance is exactly about this...

____________________________________________________________ Be brave little warrior, be VERY brave

[Lost User]

It is, for objects - aka reference-types, not for value-types. C# is really strict on the difference. The posted alternative with the interface does look like it (could) solve your problem. Alternatively, you could decide only to pass objects, and to wrap your ints in a MyInt class.

I are Troll :suss:

[Rob Philpott]

Because it just can't. If asked questions such as this I like to mutter something about Covariance and Contravariance and wonder off. Usually works in the office environment anyway.

Regards, Rob Philpott.

[RobCroll]

It's a design issue and it makes sense when you think about it. If you want to use a collection of type object, then use ArrayList. The generic List is a strongly typed version of ArrayList, so if strong typing is not required, use ArrayList.

"You get that on the big jobs."

[PIEBALDconsult]

In .net, everything is an object. :-D

[PIEBALDconsult]

Rob Philpott wrote:

Covariance and Contravariance

I thought those were added in .net 4.

[Lost User]

That's one of those generalizations that I loathe. Yes, structs are objects too. ..but that doesn't make 'em interchangeable :)

I are Troll :suss:

[Rob Philpott]

You see, I'd have wondered off by the point anyone questions it... This is what I'm thinking of: http://blogs.msdn.com/b/ericlippert/archive/2007/10/19/covariance-and-contravariance-in-c-part-three-member-group-conversion-variance.aspx[^]

Regards, Rob Philpott.

[J4amieC]

Eddy Vluggen wrote:

but that doesn't make 'em interchangeable

Yes it does, assuming the container being used to intercahnge them types the variable as object.

[Lost User]

J4amieC wrote:

Yes it does, assuming the container being used to intercahnge them types the variable as object.

The keyword here is "assuming", and your assumption is a leaky abstraction :) Reference-types are objects, value-types can be boxed in an reference-type. And no, a namespace isn't an object, it's not even a type. Once you state that everything is an object, people will assume that anything can be used as a base to derive from (since OO is partly about inheriting from existing objects).

I are Troll :suss:

[J4amieC]

Eddy Vluggen wrote:

Once you state that everything is an object, people will assume that anything can be used as a base to derive from

Any non-sealed Type can be used as a base to derive from. What's your point?

[Lost User]

J4amieC wrote:

Any non-sealed Type can be used as a base to derive from.

You can circumvent the impossibility of inheriting a structure by pointing out that it's a sealed class - but that doesn't change the validity of my statement. I took the example of a namespace to prevent the hairsplitting discussion that structs are merely mutilated classes under the hood :)

J4amieC wrote:

What's your point?

That the text "everything is an object" is incorrect. I stated that literally, didn't I? :laugh:

I are Troll :suss:

[PIEBALDconsult]

(Whoops, sorry to have started that. :-O )