使用字符串值为数字操作数创建自定义谓词 [英] Creating a Custom Predicate using a String Value for the Numeric Operand
问题描述
我试图将数字操作数表达式的字符串值("GreaterThan"，"Equals"等)传递给参数.我创建了下面的代码，该代码可以工作，但它笨拙".我不喜欢if块，而且我认为有一种方法可以使用自定义LINQ比较谓词.我试着按照
I am trying to pass in a string value ("GreaterThan", "Equals", etc.) for a numeric operand expression to a parameter. I have created the code below which works, but it's "clunky". I don't like the if blocks and I think there's a way to do this with a custom LINQ comparison predicate. I tried to follow the reply posted in this post, but I can't seem to follow it. Any ideas on how to clean up my method?
这里的代码显示了我想如何将字符串"GreaterThan"传递给函数
Here's code showing how I want to pass a string value of "GreaterThan" to a function
var myValues = new Dictionary<string, int> {{"Foo", 1}, {"Bar", 6}};
var failed = DoAnyValuesFail(myValues, "GreaterThan", 4);
这是我写的笨拙"的示例方法:
Here's the sample method that I wrote that's "clunky":
public bool DoAnyValuesFail(Dictionary<string, int> dictionary, string expression, int failureValue)
{
var failureValues = new List<KeyValuePair<string, int>>();
if (expression == "GreaterThan")
failureValues = dictionary.Where(x => x.Value > failureValue).ToList();
if (expression == "LessThan")
failureValues = dictionary.Where(x => x.Value < failureValue).ToList();
if (expression == "Equals")
failureValues = dictionary.Where(x => x.Value == failureValue).ToList();
return failureValues.Any();
}
---更新-最终版本---
我认为，以下答复中的部分困惑在于，我对功能，谓词和委托人的术语不了解.对于那个很抱歉.无论如何，我确实想澄清一件事，那就是"GreaterThan"，"LessThan"和"Equals"的值来自配置文件，因此它们必须是在运行时进行调整的魔术字符串".
I think part of the confusion in the responses below is that I'm not that up to speed on my terminology of fuctions, predicates and delegates. Sorry about that. Regardless, I did want to clarify one thing and that is that the values of "GreaterThan", "LessThan" and "Equals" come from a configuration file, so they need to be "Magic Strings" that adjust at run time.
因此，基于Matthew Haugen和Enigmativity的反馈，我提出了以下代码，我认为这些代码最适合我的需求.如果您认为这是错误的或需要调整的，我欢迎您提出任何建议.
Therefore, based on the feedback from Matthew Haugen and Enigmativity, I have come up with the following code that I think works best for my needs. I'm open to any suggestions if you think it's wrong or needs adjusted.
// These values actually come from a configuration file... shown here as hard coded just for illustration purposes
var failureValue = 2;
var numericQualifier = "<";
// This comes from my external data source
var myValues = new Dictionary<string, int> { { "Foo", 1 }, { "Bar", 6 } };
// This is the delegate (am I using that term correctly?) called Compare which is setup as an extension method
var failureValues = myValues.Where(x => numericQualifier.Compare()(x.Value, failureValue)).ToList();
if (failureValues.Any())
Console.WriteLine("The following values failed: {0}", string.Join(", ", failureValues));
这就是我的Compare扩展方法:
public static class MyExtensions
{
public static Func<int, int, bool> Compare(this string expression)
{
switch (expression)
{
case "GreaterThan":
case ">":
return (v, f) => v > f;
case "LessThan":
case "<":
return (v, f) => v < f;
case "Equals":
case "=":
return (v, f) => v == f;
default:
throw new ArgumentException(string.Format("The expression of '{0}' is invalid. Valid values are 'GreaterThan', 'LessThan' or 'Equals' or their respective symbols (>,<,=)", expression));
}
}
}
推荐答案
鉴于您需要将表达式与字符串进行匹配，我倾向于这样做:
Given your requirement to match the expression against a string I would be inclined to do this:
private Dictionary<string, Func<int, int, bool>> _predicates =
new Dictionary<string, Func<int, int, bool>>
{
{ "GreaterThan", (v, f) => v > f },
{ "LessThan", (v, f) => v < f },
{ "Equals", (v, f) => v == f },
};
public bool DoAnyValuesFail(
Dictionary<string, int> dictionary,
string expression,
int failureValue)
{
return _predicates.ContainsKey(expression)
? dictionary.Any(kvp => _predicates[expression](kvp.Value, failureValue))
: false;
}
但是，正如其他人所说，我认为这是一个更好的选择:
However, as others have said I think this is a better option:
public bool DoAnyValuesFail(
Dictionary<string, int> dictionary,
Func<int, bool> predicate)
{
return dictionary.Any(kvp => predicate(kvp.Value));
}
然后像这样简单地调用它:
And then simply call it like so:
var failed = DoAnyValuesFail(myValues, x => x > 4);
但是，将它变得更加简单仅一步之遥:
But then you are only one step away from making it even simpler:
var failed = myValues.Any(x => x.Value > 4);
不需要DoAnyValuesFail方法-意味着代码更简单，潜在的错误更少，并且没有魔术"字符串.
No DoAnyValuesFail method required - meaning simpler code, less potential bugs, and no "magic" strings.
此代码比您的原始代码更清晰，更简洁.
This code is much clearer and actually more terse than your original line.
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