协变集合不起作用

Question

对不起，如果问题是多余的，但我找不到适合我的特殊情况的解决方案。 请考虑这段代码：

public interface IPoint {}
public class GazePoint : IPoint {}
public Point AvgPoint(IEnumerable<IPoint> locations) {}

List<GazePoint> gazePoints = new List<GazePoint>();
//...
// this doesn't work:
Point avg = AvgPoint(gazePoints);

您能否解释一下为什么它不起作用（我假设 C# 4.0 已经解决了这个问题）以及如何更改 AvgPoint() 方法的签名以使接收 IPoint 的不同实现成为可能。 （我不想将gazePoints集合转换为另一种类型的集合，因为它处于一个大循环中，并且我担心性能。

[更新]：我将 GazePoint 定义为结构，这就是问题的根源。不过，我不知道为什么 struct 在这里不起作用。

Answer 1

我不确定您遇到的具体问题是什么，但它对我来说是这样的：

首先，一些实际的类实现：

public interface IPoint
{
    int X { get; set; }
    int Y { get; set; }
}

public class Point : IPoint
{
    public int X { get; set; }
    public int Y { get; set; }

    public Point()
    {
    }

    public Point(int x, int y)
    {
        X = x;
        Y = y;
    }
}

public class GazePoint : IPoint
{
    public int X { get; set; }
    public int Y { get; set; }

    public GazePoint()
    {
    }

    public GazePoint(int x, int y)
    {
        X = x;
        Y = y;
    }
}

然后是一个实际的 AvgPoint 方法实现：

public static Point AvgPoint(IEnumerable<IPoint> locations)
{
    if (locations == null || !locations.Any()) return new Point(0, 0);

    return new Point((int) locations.Average(l => l.X), 
        (int) locations.Average(l => l.Y));
}

最后是一些测试：

public static void Main()
{
    var points = new List<Point>
    {
        new Point(1, 2),
        new Point(3, 4)
    };

    var gazePoints = new List<GazePoint>
    {
        new GazePoint(1, 2),
        new GazePoint(3, 4)
    };

    Point avgPoint = AvgPoint(points);
    Point avgGazePoint = AvgPoint(gazePoints);

    Console.WriteLine("Average Point = {0}, {1}", avgPoint.X, avgPoint.Y);
    Console.WriteLine("Average GazePoint = {0}, {1}", avgGazePoint.X, avgGazePoint.Y);
}

如果您的目标是让该方法返回传入的相同类型的平均值，您可以将其设为通用，如下所示：

public static T AvgPoint<T>(IEnumerable<T> locations) where T : IPoint, new()
{
    if (locations == null || !locations.Any()) return new T {X = 0, Y = 0};

    return new T {X = (int) locations.Average(l => l.X), 
        Y = (int) locations.Average(l => l.Y)};
}