C#选择查询不修改外部变量 [英] C# Select query not modifying external variable

问题描述

我有一个包含n个元素的类，以及一个返回元素的平方根的属性:

I have a class with n elements, and a property that returns the root sum square of the elements:

public double Length 
{ 
    get 
    { 
        double sum = 0.0; 
        Elements.Select(t => sum += t * t);  
        return Math.Sqrt(sum); 
    } 
}

但是，这不起作用-不管元素的值如何，总和仍为0.0.
为什么这行不通?

However, that doesn't work - no matter the value of the elements, sum remains 0.0.
Why doesn't this work?

注意:我已经以另一种方式实现了它，但是我想了解为什么上面的代码不起作用

Note: I have already implemented it another way, but am looking to understand why the above code does not work

推荐答案

LINQ使用延迟执行 –选择方法不会立即对所有元素执行lambda，但是返回 IEnumerable< T> ，该代码在执行时会在枚举的每个元素.

LINQ uses deferred execution – the Select Method doesn't execute the lambda for all elements immediately, but returns an IEnumerable<T> which, when executed, performes the lambda on each element as it's enumerated.

还要注意，LINQ用于查询，而不是用于为每个元素执行代码块.您应该编写代码，使lambda中没有语句，只有没有副作用的表达式.尝试计算总和时，可以使用求和方法:

Also note that LINQ is for querying, not for executing a block of code for each element. You should write your code such that there is no statement in the lambda, only a expression that is free of side-effects. You can use the Sum Method when you're trying to calculate a sum:

public double Length 
{ 
    get 
    { 
        double sum = elements.Select(t => t * t).Sum();
        return Math.Sqrt(sum); 
    } 
}

或

public double Length 
{ 
    get 
    { 
        double sum = elements.Sum(t => t * t);
        return Math.Sqrt(sum); 
    } 
}

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