在 lambda 表达式中将 Func<T, IConvertible> 的结果转换为 int

Question

我正在抽象一些代码，但遇到了将 Func 的结果转换为整数进行比较的问题。

我有一个函数：

public static IConvertible GetIllnessRateByParity(IEnumerable<ILLNESS_OBJECT> pItems, int pParity, Func<ILLNESS_OBJECT, IConvertible> pSelector)
    {
        var filtered = pItems.Where(a => a.Parity == pParity);
        return Math.Round(filtered.Count(a => Convert.ToInt32(pSelector(a)) == 1) / (double)GetBirthCountByParity(pItems, pParity), 2);
    }

public class ILLNESS_OBJECT
{
    public int ID { get; set; }
    public DateTime Day { get; set; }
    public int Parity { get; set; }
    public int Afterbirth { get; set; }
    public int OvaryInfection { get; set; }
    etc...
}

计算一组数据中特定疾病的发病率。要分析的疾病是通过选择器输入的，该选择器从 ILLNESS_OBJECT 对象中选择一个 IConvertible 值。

我这样调用方法：

GetIllnessRateByParity(data, 1, a => a.AfterBirth)

data 是一个具有非空值的 IEnumerable。

该方法在 Convert.ToInt32(pSelector(a)) 部分引发 InvalidCastException ，但没有明确说明此转换失败的原因，有人可以向我解释一下吗？我的印象是 pSelector(a) 返回一个与 int 兼容的 IConvertible 值，对吧？任何帮助将不胜感激！

编辑：异常消息： CONFIDENTIAL.CLASS.dll 中出现“System.InvalidCastException”类型的异常，但未在用户代码中处理

消息：指定的转换无效。

Answer 1

查看this

using System;
using System.Collections.Generic;
using System.Linq;

public class Program
{
    public static void Main()
    {
        var data = new List<ILLNESS_OBJECT>{
                new ILLNESS_OBJECT{ ID = 1, Parity = 1, Day = DateTime.Now, SomeString = "1", Long = 2L},
                new ILLNESS_OBJECT{ ID = 2, Parity = 2, Day = DateTime.Now, SomeString = "1", Long = 2L},
                new ILLNESS_OBJECT{ ID = 3, Parity = 1, Day = DateTime.Now, SomeString = "2", Long = 1L},
                new ILLNESS_OBJECT{ ID = 4, Parity = 1, Day = DateTime.Now, SomeString = "3", Long = 3L},
        };
        Console.WriteLine(GetIllnessRateByParity(data, 1, x => x.ID));
        Console.WriteLine(GetIllnessRateByParity(data, 1, x => int.Parse(x.SomeString)));
        Console.WriteLine(GetIllnessRateByParity(data, 1, x => (DateTime.Now - x.Day).Days + 1));
        Console.WriteLine(GetIllnessRateByParity(data, 1, x => (int)x.Long));
    }

    public static IConvertible GetIllnessRateByParity(IEnumerable<ILLNESS_OBJECT> pItems, int pParity, Func<ILLNESS_OBJECT, int> pSelector)
    {
        var filtered = pItems.Where(a => a.Parity == pParity);
        return Math.Round(filtered.Count(a => pSelector(a) == 1)/(double)1, 2);
    }

}

public class ILLNESS_OBJECT
{
    public int ID { get; set; }
    public DateTime Day { get; set; }
    public int Parity { get; set; }
    public int Afterbirth { get; set; }
    public string SomeString { get; set; }
    public long Long { get; set; }
}

Answer 2

最后一个参数叫做pSelector，但它实际上并不是一个选择器。

它只是返回一个值（int、float...），而实际的选择是由labmda在Count方法中完成的：

filtered.Count(a => Convert.ToInt32(pSelector(a)) == 1)

所以 pSelector 将返回某种数据类型的值，然后您需要将其转换为 int32。

但您只需要将其与 1 进行比较。鉴于您问题中的信息，您实际上并不需要 int32，您需要一个 bool。

那么，如果你将 pSelector 重构为一个选择器，通过在选择器内将比较移动到 1 来实现：

public static double GetIllnessRateByParity(IEnumerable<ILLNESS_OBJECT> pItems, int pParity, Func<ILLNESS_OBJECT, bool> pSelector)
{
    var filtered = pItems.Where(a => a.Parity == pParity);
    return Math.Round(filtered.Count(pSelector) / (double)GetBirthCountByParity(pItems, pParity), 2);
}
GetIllnessRateByParity(data, 1, a => a.AfterBirth==1)