C# 将带有两个参数的委托作为参数函数传递
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Passing a delegate with two parameters as a parameter function
提问by Stefano Ricciardi
I have a sequence of functions that look very similar but for a single line, like the following two (but I have many more of them):
我有一系列看起来非常相似但只有一行的函数，如下两个（但我有更多）：
private static int HowManyHoursInTheFirstYear(IList<T> samples)
{
DateTime firstDate = samples[0].Date;
int count = 0;
while (count < samples.Count &&
samples[count].Date.Year == firstDate.Year)
{
count++;
}
return count;
}
private static int HowManyDaysInTheFirstMonth(IList<T> samples)
{
DateTime firstDate = samples[0].Date;
int count = 0;
while (count < samples.Count &&
samples[count].Date.Month == firstDate.Month) // <--- only change!
count++;
}
return count;
}
I was thinking about using delegates to remove this repetition in code in some elegant way, that would have allowed me to invoke something like:
我正在考虑使用委托以某种优雅的方式删除代码中的这种重复，这将允许我调用以下内容：
HowManyDaysInTheFirstPeriod(
samples,
delegate(DateTime d1, DateTime d2) { return d1.Month == d2.Month; });
thereby declaring a delegate like the following:
从而声明一个如下所示的委托：
delegate bool DateComparer(DateTime first, DateTime second);
and where HowManyDaysInTheFirstPeriod whould be something like the following:
以及 HowManyDaysInTheFirstPeriod 应该如下所示：
private static int HowManySamplesInFirstPeriod
IList<T> samples,
DateComparer comparer)
{
DateTime firstDate = samples[0].Date;
int count = 0;
while (count < samples.Count && comparer())
{
count++;
}
}
Unfortunately, the compiler complains that comparer needs two parameters.
不幸的是，编译器抱怨 comparer 需要两个参数。
I am relatively new to C# and hit a road-block here. How would you solve this?
我对 C# 比较陌生，在这里遇到了障碍。你会如何解决这个问题？
采纳答案by John Feminella
You're almost there! The comparerdelegate parameter is just like any other function: You still need to pass the appropriate arguments to invoke it.In your case, that's going to mean this change:
您快到了！该comparer委托参数就像任何其他的功能：您仍然需要通过适当的参数来调用它。在您的情况下，这将意味着此更改：
while (count < samples.Count && comparer(samples[count].Date, firstDate))
{
count++;
}
(Also, note that samplesshould probably be samples.Count, as I have written above.)
（另外，请注意，samples可能应该是samples.Count，正如我上面写的那样。）
回答by Jon Skeet
You need to pass the comparer the two dates in question. It's probably juts as simple as:
您需要将有问题的两个日期传递给比较器。这可能很简单：
private static int HowManySamplesInFirstPeriod
IList<T> samples,
DateComparer comparer)
{
DateTime firstDate = samples[0].Date;
int count = 0;
while (count < samples.Count
&& comparer(samples[count].Date, firstDate))
{
count++;
}
}
Or you may want to pass them the other way round (i.e. firstDateand then samples[count].Date).
或者您可能想以相反的方式传递它们（即firstDate然后samples[count].Date）。
回答by Kent Boogaart
You need to pass the dates being compared to the delegate. So:
您需要传递与委托进行比较的日期。所以：
comparer(samples[count].Date, firstDate)
回答by jalf
You can do it a bit simpler. Simply provide the function a delegate which extracts whatever should be compared from a DateTime:
你可以做得更简单一点。只需为函数提供一个委托，该委托从 DateTime 中提取应该比较的内容：
private static int HowManySamplesInFirstPeriod<T>
IList<T> samples,
Func<DateTime, int> f // a function which takes a DateTime, and returns some number
{
DateTime firstDate = samples[0].Date;
int count = 0;
while (count < samples && f(samples[count].Date) == f(firstDate))
{
count++;
}
}
and it can then be called as such:
然后可以这样调用它：
HowManySamplesInFirstPeriod(samples, (dt) => dt.Year); // to get the year
HowManySamplesInFirstPeriod(samples, (dt) => dt.Month); // to get the month
The (dt) => dt.yearsyntax may be new to you, but it's a cleaner way of writing "an anonymous delegate which takes an object dt of some generic type, and returns dt.year".
You could write an old-fashioned delegate instead, but this is nicer. :)
该(dt) => dt.year语法可能是新的给你，但它写的更清洁的方式“匿名委托这需要一些通用类型的对象，DT，并返回dt.year”。你可以写一个老式的委托，但这样更好。:)
We can make it a bit more general than that though, by adding another generic type parameter:
不过，我们可以通过添加另一个泛型类型参数使其更通用：
private static int HowManySamplesInFirstPeriod<T, U>
IList<T> samples,
Func<DateTime, U> f // Let's generalize it a bit, since the function may return something other than int (some of the DateTime members return doubles, as I recall)
As usual though, LINQ provides a nicer still alternative:
像往常一样，LINQ 提供了一个更好的选择：
private static int HowManySamplesInFirstPeriod<T>
IList<T> samples,
Func<DateTime, int> f)
{
var firstVal = f(samples.First().Date);
return samples.Count(dt => f(dt.Date) = firstVal)
}
回答by sipwiz
I think jalf's answer needs to be modified slightly to fit the original usage:
我认为需要稍微修改 jalf 的答案以适应原始用法：
private static int HowManyHoursInTheFirstYear(IList<DateTime> samples, Func<DateTime, DateTime, bool> comparer)
{
DateTime firstDate = samples[0].Date;
int count = 0;
while (count < samples.Count && comparer(samples[count], firstDate) ) {
count++;
}
return count;
}
Call using:
调用使用：
HowManyDaysInTheFirstPeriod(samples, (d1, d2) = > { return d1.Month == d2.Month; });
HowManyDaysInTheFirstPeriod(samples, (d1, d2) = > { return d1.Year == d2.Year; });
