generics and inheritance: List<T>

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jonpb - 28 Mar 2008 00:58 GMT

Hi,

The following code:

public class SubPoint2D : Point2D
{
public int Code;
}

void ProcessPoints(List<Point2D> pts)
{
return;
}

private void Test()
{
List<SubPoint2D> subPts2D = new List<SubPoint2D>();

ProcessPoints(subPts2D);
}

results in a compiler error: cannot convert List<SubPoint2D> to
List<Point2D>.

Is there a way to tell the compiler that SubPoint2D actually is a
Point2D and therefore the two List<T> types are equivalent as far as
Point2D? I tried:
ProcessPoints(subPts2D as List<Point2D>);
but that didn't work either.

Thanks,
John

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Jon Skeet [C# MVP] - 28 Mar 2008 01:13 GMT

> The following code:
>
[quoted text clipped - 21 lines]
> Point2D and therefore the two List<T> types are equivalent as far as
> Point2D?

They're not equivalent at all.

Consider what happens when you try to add a Point2D to each of them:
with a List<Point2D> it's fine. With a List<SubPoint2D> it should go
bang.

Basically generics in C# don't support covariance. It's a pain, but it
also gives more safety. It's likely that C# 4 will have more support
for this, at the interface and delegate level, but that's all.

What you *can* do is make ProcessPoints a generic method:

void ProcessPoints(List<T> pts) where T : Point2D

That may help you, depending on what you need to do in ProcessPoints.

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Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
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jonpb - 28 Mar 2008 17:06 GMT

>> The following code:
>>
[quoted text clipped - 37 lines]
>
> That may help you, depending on what you need to do in ProcessPoints.

Thanks very much Jon. The generic method works in this particular case,
as all it does is draw the shape there is no manipulation of the lists
or changing of values, but your point is well taken.

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generics and inheritance: List<T>

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