Constructor Chaining

[escher4096]

I have a class A that is inheriting from class B. Class B has a
constructor that I would like to use in Class A, so I chain to it.
However class B's constructor often misbehaves and it causes a mess
and throws a meaningless error. I would like to wrap the call to class
B's constructor in a try/catch so that I can at least throw something
of use. I can't see how I would do this with chaining however.

Everything that class B calls in its constructor has been marked
internal or private so I can't just decompile it and copy and paste
its constructor code out without copying several other functions (all
of which also call private and internal methods - you can see this
would get out of control).

In C# you chain by doing:

public MyClassName() :[this or base](arguments)
{
}

what I ned is something more like:

public MyClassName()
{

}

[escher4096]

On Sep 11, 2:59*pm, escher4096 <[e-mail]:

I have a class A that is inheriting from class B. Class B has a
constructor that I would like to use in Class A, so I chain to it.
However class B's constructor often misbehaves and it causes a mess
and throws a meaningless error. I would like to wrap the call to class
B's constructor in a try/catch so that I can at least throw something
of use. I can't see how I would do this with chaining however.

Everything that class B calls in its constructor has been marked
internal or private so I can't just decompile it and copy and paste
its constructor code out without copying several other functions (all
of which also call private and internal methods - you can see this
would get out of control).

In C# you chain by doing:

public MyClassName() :[this or base](arguments)
{

}

what I ned is something more like:

public MyClassName()
{

}

oops miscued and posted by accident :)

what I need is something more like:

public MyClassName()
{
try
{
[this or base](arguments);
}
catch(blah blah blah)
{
throw new meaningful exception....
}
}

Is it possible to do this in C#?

Thanks

-Cam

[escher4096]

On Sep 11, 2:59*pm, escher4096 <[e-mail]:

I have a class A that is inheriting from class B. Class B has a
constructor that I would like to use in Class A, so I chain to it.
However class B's constructor often misbehaves and it causes a mess
and throws a meaningless error. I would like to wrap the call to class
B's constructor in a try/catch so that I can at least throw something
of use. I can't see how I would do this with chaining however.

Everything that class B calls in its constructor has been marked
internal or private so I can't just decompile it and copy and paste
its constructor code out without copying several other functions (all
of which also call private and internal methods - you can see this
would get out of control).

In C# you chain by doing:

public MyClassName() :[this or base](arguments)
{

}

what I ned is something more like:

public MyClassName()
{

}

oops miscued and posted too soon :)

what I need is something more like:

public MyClassName()
{
try
{
base(arguments);
}
catch(blah blah blah)
{
thgrow new meaningful exception
}
}

Is this possible in C#?

Thanks

-Cam

[Peter Duniho]

On Thu, 11 Sep 2008 13:59:03 -0700, escher4096 <ca*****@ogmios.cawrote:

I have a class A that is inheriting from class B. Class B has a
constructor that I would like to use in Class A, so I chain to it.
However class B's constructor often misbehaves and it causes a mess
and throws a meaningless error. I would like to wrap the call to class
B's constructor in a try/catch so that I can at least throw something
of use. I can't see how I would do this with chaining however.

You can't do it.

If the constructor throws an exception, then the object isn't properly
constructed and thus is unusable. I never thought about it before, but in
hindsight this may be why C# puts the call to a base constructor outside
the method body, rather than inside the way Java does it, to make it very
clear that you can't legitimately catch exceptions thrown by base
constructors.

If your class would be usable even though the base class isn't properly
constructed, then your own class probably shouldn't be inheriting that
class anyway. You may be better off compositing the class into your own,
and providing whatever fallback implementation is appropriate when you
can't create an instance of class B.

Pete

[raylopez99]

On Sep 11, 2:10*pm, "Peter Duniho" <[e-mail]>
wrote:

>
If your class would be usable even though the base class isn't properly *
constructed, then your own class probably shouldn't be inheriting that *
class anyway. *You may be better off compositing the class into your own, *
and providing whatever fallback implementation is appropriate when you *
can't create an instance of class B.

Yes, I agree with Pete here. I say make the misbehaving base class
"virtual" so that it doesn't do much, and let the derived class do the
real work.

RL

[Jon Skeet [C# MVP]]

On Sep 11, 9:59*pm, escher4096 <[e-mail]:

I have a class A that is inheriting from class B. Class B has a
constructor that I would like to use in Class A, so I chain to it.
However class B's constructor often misbehaves and it causes a mess
and throws a meaningless error. I would like to wrap the call to class
B's constructor in a try/catch so that I can at least throw something
of use. I can't see how I would do this with chaining however.

<snip>

As others have said, you can't do this. What you *can* do is make your
constructor private and expose static methods to create instances.
They can do error handling however you want.

Jon

[G.S.]

On Sep 12, 2:02*am, "Jon Skeet [C# MVP]" <[e-mail]:

On Sep 11, 9:59*pm, escher4096 <[e-mail]:

I have a class A that is inheriting from class B. Class B has a
constructor that I would like to use in Class A, so I chain to it.
However class B's constructor often misbehaves and it causes a mess
and throws a meaningless error. I would like to wrap the call to class
B's constructor in a try/catch so that I can at least throw something
of use. I can't see how I would do this with chaining however.

<snip>

As others have said, you can't do this. What you *can* do is make your
constructor private and expose static methods to create instances.
They can do error handling however you want.

Jon

Or wrap A's construction in try/catch and handle, because as Peter
said, you don't want the A object whose base B construction failed
anyway.

[Jon Skeet [C# MVP]]

On Sep 12, 4:35*pm, "G.S." <[e-mail]:

Or wrap A's construction in try/catch and handle, because as Peter
said, you don't want the A object whose base B construction failed
anyway.

But if the purpose is to rewrap the exception in a more meaningful
one, you don't want to have to do that at *every* call site. By
wrapping it within a single static method, class A is correctly taking
responsibility for things.

Jon

[G.S.]

On Sep 12, 11:48*am, "Jon Skeet [C# MVP]" <[e-mail]:

On Sep 12, 4:35*pm, "G.S." <[e-mail]:

Or wrap A's construction in try/catch and handle, because as Peter
said, you don't want the A object whose base B construction failed
anyway.

But if the purpose is to rewrap the exception in a more meaningful
one, you don't want to have to do that at *every* call site. By
wrapping it within a single static method, class A is correctly taking
responsibility for things.

Jon

yes, yes, you're right, of course but I guess I was thinking that if
its rethrown, it's gotta be handled