C#/2.0: Tricky issue with generics.
- From: "Thomas Tomiczek" <t.tomiczek@xxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 12 Apr 2005 18:39:27 +0200
Ok, working my way through a complex library conversion to .NET 2.0 and C#
2.0 with generics I am totally stuck on one thing -if anyone sees the issue
I would be more than glad.
The situation is that we have a tree of "Node" objects. A Node is a
visualizable item, like a component. THe menues of all or ouf applications,
for example, are node hierarchies that an adapter transposes into a menu
model.
Now, Nodes have an internal DataObject, which can hold the actual data -
often a business object. This was so far a Object, now it will be T.
So, the definition of Node looks like this now:
public class Node<T> :
SmartObject,
INamed
where
T : class
{
The T is supposed to indicate the type of the DataObject, which can be any
class. There is a class NodeList that is basically a glorified IList with a
lot of additional functionality. It looks like this:
public class NodeList<T> :
ObjectBindingList<T>
where
T : Node<Object>
{
The idea is that a NodeList can hold any Node and should be strong-typable
for this. Now, here the issues come. A Node itself has a Collection
"Children". This is similar to the collection if children in the component
hierarchy. Nodes are not restricted to subtypes - and the constructor of the
NodeList can take a Node that is the parent. If there is a parent in a
NodeList, there is a lot of stuff going on when you add/remove thigs. So,
there is aconstructor like this:
public NodeList(Node<Object> Parent)
As you can see, ANY Node can be the parent.
And here the issues start. In the constructor of the Node I create the
ChildNodes:
_ChildNodes = new NodeList<Node<Object>> (this);
And the issue is: this blows. And I have no clue why. The error messages
are:
Error 2 The best overloaded method match for
'Foundation.Nodes.NodeList<Foundation.Nodes.Node<object>>.NodeList(Foundation.Nodes.Node<object>)'
has some invalid arguments
C:\WORK\ThonaConsulting\EntityBroker\03.0\Foundation\Nodes\Node.cs 29 18
Foundation
Error 3 Argument '1': cannot convert from 'Foundation.Nodes.Node<T>' to
'Foundation.Nodes.Node<object>'
C:\WORK\ThonaConsulting\EntityBroker\03.0\Foundation\Nodes\Node.cs 29 46
Foundation
Now, this I simply do not understand. The argument "this" is a Node<T> - and
is not convertible to Node<object>? Why not? T is supposed to be a class,
and object is the base-class of all classes. Also, I cano not even cast
this:
_ChildNodes = new NodeList<Node<Object>> ((Node<T>)this);
produces the exactly SAME error. I simply do not understand why. As I said,
the case should be totally valid in my optinion.
Anyone a clue? This starts to look like a compiler error to me, but I still
fear I just overlook something. I mean, this is created WITHIN A node, so
thi sshould be totally valid, or?
Thomas Tomiczek
.
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