继承和转换错误（通用接口）

Question

如何重组我的代码以消除在指示点发生的运行时错误？

DataSeries<SimpleDataPoint> 需要能够以某种方式转换回 IDataSeries<IDataPoint>

我尝试过使用两个接口的继承，如下所示：

public class DataSeries<TDataPoint> : IDataSeries<TDataPoint>, IDataSeries<IDataPoint> 但收到编译器错误：

'DataSeries<TDataPoint>' 不能同时实现

'IDataSeries<TDataPoint>' 和

'IDataSeries<IDataPoint>' 因为它们可能会针对某些类型参数替换统一

使用协变似乎不是一种选择，因为我无法使接口协变或逆变。

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication1 {

class Program {

    [STAThread]
    static void Main(string[] args) {

        var source = new object();

        // compiles fine, but ...
        // runtime error here - cannot cast
        var ds = (IDataSeries<IDataPoint>)new DataSeries<SimpleDataPoint>(source);

        Console.ReadKey();
    }
}

public interface IDataPoint {
    int Index { get; set; }
    double Value { get; set; }
    DateTime TimeStampLocal { get; set; }
    IDataPoint Clone();
}

public sealed class SimpleDataPoint : IDataPoint {
    public int Index { get; set; }
    public double Value { get; set; }
    public DateTime TimeStampLocal { get; set; }
    public IDataPoint Clone() {
        return new SimpleDataPoint {
            Index = Index,
            Value = Value,
            TimeStampLocal = TimeStampLocal,
        };
    }
}

public interface IDataSeries<TDataPoint> where TDataPoint : class, IDataPoint {
    object Source { get; }
    int Count { get; }
    double GetValue(int index);
    DateTime GetTimeStampLocal(int index);
    TDataPoint GetDataPoint(int index);
    TDataPoint GetLastDataPoint();
    void Add(TDataPoint dataPoint);
    IDataSeries<TDataPoint> Branch(object source);
}

public class DataSeries<TDataPoint> : IDataSeries<TDataPoint> where TDataPoint : class, IDataPoint {

    readonly List<TDataPoint> _data = new List<TDataPoint>();

    public object Source {
        get;
        private set;
    }

    public DataSeries(object source) {
        Source = source;
    }

    public int Count {
        get { return _data.Count; }
    }
    public TDataPoint GetDataPoint(int index) {
        return _data[index];
    }
    public TDataPoint GetLastDataPoint() {
        return _data[_data.Count - 1];
    }
    public DateTime GetTimeStampLocal(int index) {
        return _data[index].TimeStampLocal;
    }
    public double GetValue(int index) {
        return _data[index].Value;
    }
    public void Add(TDataPoint dataPoint) {
        _data.Add(dataPoint);
    }

    public IDataSeries<TDataPoint> Branch(object source) {
        throw new NotImplementedException();
    }
}
}

Answer 1

问题是new DataSeries<SimpleDataPoint> 不是 IDataSeries<IDataPoint>，因为调用IDataSeries<IDataPoint>.Value = new AnotherDataPoint() 和IDataPoint value = IDataSeries<IDataPointBase>.Value 可能会失败。也就是说，运行时不能保证你正在做的事情是类型安全的，所以它会抛出一个异常来告诉你。只有当您的接口被标记为协变或逆变时，运行时才能保证这些操作中的一个是安全的。它被标记为两者都不是，因此它不是类型安全的，因此无法完成。

如果你打算绕过类型安全，你可以创建一个不安全的代理：

public class DataSeries<TDataPoint> : IDataSeries<TDataPoint>
    where TDataPoint : class, IDataPoint
{
    // ...

    public IDataSeries<IDataPoint> GetUnsafeProxy ()
    {
        return new UnsafeProxy(this);
    }

    private class UnsafeProxy : IDataSeries<IDataPoint>
    {
        private readonly DataSeries<TDataPoint> _owner;

        public UnsafeProxy (DataSeries<TDataPoint> owner)
        {
            _owner = owner;
        }

        public object Source
        {
            get { return _owner.Source; }
        }

        public int Count
        {
            get { return _owner.Count; }
        }

        public double GetValue (int index)
        {
            return _owner.GetValue(index);
        }

        public DateTime GetTimeStampLocal (int index)
        {
            return _owner.GetTimeStampLocal(index);
        }

        public IDataPoint GetDataPoint (int index)
        {
            return _owner.GetDataPoint(index);
        }

        public IDataPoint GetLastDataPoint ()
        {
            return _owner.GetLastDataPoint();
        }

        public void Add (IDataPoint dataPoint)
        {
            _owner.Add((TDataPoint)dataPoint);
        }

        public IDataSeries<IDataPoint> Branch (object source)
        {
            return (IDataSeries<IDataPoint>)_owner.Branch(source);
        }
    }

你可以像这样使用这个代理：

IDataSeries<IDataPoint> ds = new DataSeries<SimpleDataPoint>(source).GetUnsafeProxy();

请注意，最后两个方法使用类型转换，因此调用它们是不安全的，它们可以在类型不兼容的情况下抛出。如果不仅要将DataSeries 转换为基本类型，还想转换为其他类型，则必须向不安全代理添加更多类型转换并失去更多类型安全性。选择权在你。

Answer 2

所以我的问题让我想到了代码异味，以及诸如“我真正想要实现的目标是什么？”

好吧，这就是我想实现的目标：我只想将DataSeries<TDataPoint> 转换为IReadOnlyDataSeries<IDataPoint>，仅当我将它作为输入传递给处理来自IReadonlyDataSeries<IDataPoint> 对象的只读数据的类时.

以下是所做的重要更改：

// here's the covariant, read-only part of the interface declaration
public interface IReadOnlyDataSeries<out TDataPoint> where TDataPoint : class, IDataPoint {
    object Source { get; }
    int Count { get; }
    double GetValue(int index);
    DateTime GetTimeStampLocal(int index);
    TDataPoint GetDataPoint(int index);
    TDataPoint GetLastDataPoint();
}

// add a few bits to the read-write fully-typed interface, breaking covariance,
// but being able to implicitly cast to the covariant readonly version when needed
public interface IDataSeries<TDataPoint> : IReadOnlyDataSeries<TDataPoint> where TDataPoint : class, IDataPoint {
    void Add(TDataPoint dataPoint);
    IDataSeries<TDataPoint> Branch(object source);
}

这是修改后的代码的完整版本：

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication1 {

class Program {

    [STAThread]
    static void Main(string[] args) {

        var source = new object();

        // implicit conversion works great!!
        // therefore I can achieve the goal of passing the fully-typed read-write dataseries
        // into objects that just want simple read-only data
        var inputSeries = new DataSeries<SimpleDataPoint>(source);

        // passing inputSeries into the constructor involves an implicit
        // conversion to IReadOnlyDataSeries<IDataPoint>
        var processor = new DataProcessor(inputSeries);

        Console.ReadKey();
    }

    public class DataProcessor {

        IReadOnlyDataSeries<IDataPoint> InputSeries;
        DataSeries<SimpleDataPoint> OutputSeries;

        public DataProcessor(IReadOnlyDataSeries<IDataPoint> inputSeries) {
            InputSeries = inputSeries;
            OutputSeries = new DataSeries<SimpleDataPoint>(this);
        }
    }
}

public interface IDataPoint {
    int Index { get; set; }
    double Value { get; set; }
    DateTime TimeStampLocal { get; set; }
    IDataPoint Clone();
}

public sealed class SimpleDataPoint : IDataPoint {
    public int Index { get; set; }
    public double Value { get; set; }
    public DateTime TimeStampLocal { get; set; }
    public IDataPoint Clone() {
        return new SimpleDataPoint {
            Index = Index,
            Value = Value,
            TimeStampLocal = TimeStampLocal,
        };
    }
}

// here's the covariant, read-only part of the interface declaration
public interface IReadOnlyDataSeries<out TDataPoint> where TDataPoint : class, IDataPoint {
    object Source { get; }
    int Count { get; }
    double GetValue(int index);
    DateTime GetTimeStampLocal(int index);
    TDataPoint GetDataPoint(int index);
    TDataPoint GetLastDataPoint();
}

// add a few bits to the read-write fully-typed interface, breaking covariance,
// but being able to implicitly cast to the covariant readonly version when needed
public interface IDataSeries<TDataPoint> : IReadOnlyDataSeries<TDataPoint> where TDataPoint : class, IDataPoint {
    void Add(TDataPoint dataPoint);
    IDataSeries<TDataPoint> Branch(object source);
}

public class DataSeries<TDataPoint> : IDataSeries<TDataPoint> where TDataPoint : class, IDataPoint {

    readonly List<TDataPoint> _data = new List<TDataPoint>();

    public object Source {
        get;
        private set;
    }

    public DataSeries(object source) {
        Source = source;
    }

    public int Count {
        get { return _data.Count; }
    }
    public TDataPoint GetDataPoint(int index) {
        return _data[index];
    }
    public TDataPoint GetLastDataPoint() {
        return _data[_data.Count - 1];
    }
    public DateTime GetTimeStampLocal(int index) {
        return _data[index].TimeStampLocal;
    }
    public double GetValue(int index) {
        return _data[index].Value;
    }
    public void Add(TDataPoint dataPoint) {
        _data.Add(dataPoint);
    }

    public IDataSeries<TDataPoint> Branch(object source) {
        throw new NotImplementedException();
    }

}
}

Answer 3

原始代码的这个最小轮廓表明可以通过在IDataSeries 接口声明中使TDataPoint 协变来解决该问题：

using System;

namespace ConsoleApplication1
{
    class Program
    {
        [STAThread]
        static void Main(string[] args)
        {
            var ds = (IDataSeries<IDataPoint>)new DataSeries<SimpleDataPoint>();

            Console.ReadKey();
        }
    }

    public interface IDataPoint { }

    public sealed class SimpleDataPoint : IDataPoint { }

    public interface IDataSeries<out TDataPoint> where TDataPoint : class, IDataPoint { }

    public class DataSeries<TDataPoint> : IDataSeries<TDataPoint> where TDataPoint : class, IDataPoint { }
}