扩展方法，在通用List< T>上的SumIf. [英] Extension method, SumIf on generic List<T>

问题描述

我需要为List(T)写一个通用的扩展方法，该方法有条件地考虑T的每个字符串属性，然后在满足条件的情况下求和T的相应十进制属性.到目前为止，我的努力:

I need to write an generic extension method for List(T) that conditionally considers each string property of T, then sums a corresponding decimal property of T if a condition is met. My effort thus far:

// foreach(p in Persons) { if(p.Name == "mort"){sum p.Amount;} }

public static double SumIf<T>(this T o, List<T> ListItems, 
          string targetStr, ?strVals?, ?dblVals?)
{
    double sum = 0;
    foreach(T item in ListItems)
    {
        if(item.?strVal? == targetStr){ sum += item.?dblVal? ; }
    }
    return sum;
}

感谢您的指导， 凡人

推荐答案

听起来您想要一种提取string属性和double属性的方法(假设您帖子中的十进制"是拼写错误，而不是"double" ")-Func在此处合适:

It sounds like you want a way of extracting the string property and double property (assuming that the "decimal" in your post was a typo rather than the "double" in your code) - Func is appropriate here:

public static double SumIf<T>(this IEnumerable<T> source, 
          string targetText,
          Func<T, string> textSelector,
          Func<T, double> valueSelector)
{
    double sum = 0;
    foreach (T item in source)
    {
        if (textSelector(item) == targetText)
        {
            sum += valueSelector(item);
        }
    }
    return sum;
}

(请注意，我已经删除了未使用的初始参数，并使其成为列表本身的扩展方法.对我来说，不使用该值似乎有点难闻……我还更改了参数类型到IEnumerable<T>，因为您实际上不需要将其作为列表.)

(Note that I've removed the unused initial parameter, and made it an extension method on the list itself. Not using the value feels like a bit of a smell to me... I've also changed the parameter type to IEnumerable<T> as you don't need it to be a list really.)

请注意，这实际上等效于:

Note that this actually mostly equivalent to:

public static double SumIf<T>(this IEnumerable<T> source, 
          string targetText,
          Func<T, string> textSelector,
          Func<T, double> valueSelector)
{
    return source.Where(x => textSelector(x) == targetText)
                 .Sum(valueSelector);
}

我个人可能会使用通用谓词函数，而不是字符串和文本选择器:

I'd personally probably go for a general predicate function instead of a string and a text selector:

public static double SumIf<T>(this IEnumerable<T> source, 
          Func<T, bool> predicate,
          Func<T, double> valueSelector)
{
    return source.Where(predicate)
                 .Sum(valueSelector);
}

那你会用

double sum = list.SumIf(x => x.Name == "mort", x => x.Amount);

...对我来说似乎和:

... which seems just as good to me as:

double sum = list.SumIf("mort", x => x.Name, x => x.Amount);

...但更加灵活.

如评论中所述，您真的需要吗?您是否在足够的地方使用它以使简单的位置/总和"呼叫难以忍受?哎呀，您可以使用条件运算符将其转换为Sum调用:

As noted in comments, do you really need this at all? Are you using it in sufficient places to make the simple Where/Sum calls unbearable? Heck, you can turn it into just a Sum call using the conditional operator:

double sum = list.Sum(x => x.Name == "mort" ? x => x.Amount : 0d);

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