Question

我希望有一个类，它将具有索引器和字段数，如下例所示：

public abstract class ARecord
{        
    public abstract double this[int index] { get; }
    public abstract int NumberOfFields { get; }
}

public class Record : ARecord
{
    public double Field1{ get; private set; }
    public double Field2{ get; private set; }

    public override int NumberOfFields { get { return 2; } }

    public Record(double[] records)
    {
        if (records.Count() != NumberOfFields) // PROBLEM IS HERE. WHEN CALLING THIS FROM DERIVED CLASS NumberOfFields=3!
            throw new ArgumentException();
        this.Field1= records[0];
        this.Field2 = records[1];
    }

    public override double this[int index]
    {
        get { throw new NotImplementedException(); }
    }     
}

public class ChildRecord : Record
{
    public double Field3 { get; private set; }

    public override int NumberOfFields { get { return 3; } }

    public ChildRecord(double[] records)
        : base(new double[] { records[0], records[1] })
    {
        if (records.Count() != NumberOfFields)
            throw new ArgumentException();
        this.Field3 = records[2];
    }

    public override double this[int index]
    {
        get { throw new NotImplementedException(); }
    }     
}

public static class TestRecord
{
    public static void CreateRecord()
    {
        var record = new ChildRecord(new double[]{1.0,1.5,2.5}); // Not working
    }
}

此示例因NumberOfFields构造函数内ChildRecord内的多态调用Record而崩溃。

据我所知，我可以new使用override来解决此问题，但在这种情况下，我无法在基类中将NumberOfFields声明为abstract （我需要的）。

解决此问题的正确方法是什么？设计有问题吗？

Answer 1

你已经制定了这个方法，它不可能按预期工作。假设它做了（通过某种神奇的方式）以你认为应该的方式工作，并且你能够创建你的ChildRecord：

var record = new ChildRecord(new double[] { 1.0, 1.5, 2.5 });

您期望record.NumberOfFields的价值是多少？两三个？此特定对象不能是ChildRecord NumberOfFields == 3，同时Record NumberOfFields == 2。无论您将NumberOfFields键入为record，ARecord还是Record，都会获得实例化类的ChildRecord实现结果。

换句话说：将ARecord.NumberOfFields暴露给外界是没有意义的，因为没有一个正确的答案 - 它不是一个有意义的概念。

如果您放弃暴露它，您可以进行类似这样的验证（省略索引器和属性）：

public abstract class ARecord
{
    public abstract double this[int index] { get; }
}

public class Record : ARecord
{
    private const int NumberOfFields = 2;

    public Record(double[] records)
    {
        if (records.Count() != NumberOfFields)
            throw new ArgumentException();
        this.Field1 = records[0];
        this.Field2 = records[1];
    }
}

public class ChildRecord : Record
{
    private const int NumberOfFields = 3;

    public ChildRecord(double[] records)
        : base(new double[] { records[0], records[1] })
    {
        if (records.Count() != NumberOfFields)
            throw new ArgumentException();
        this.Field3 = records[2];
    }
}

从基础构造函数C＃调用基类重写方法

1 个答案: