[英]LINQ: is there a way to supply a predicate with more than one parameter to where clause
想知道是否有办法执行以下操作:我基本上想要为具有多个参数的where子句提供谓词,如下所示:
public bool Predicate (string a, object obj)
{
// blah blah
}
public void Test()
{
var obj = "Object";
var items = new string[]{"a", "b", "c"};
var result = items.Where(Predicate); // here I want to somehow supply obj to Predicate as the second argument
}
var result = items.Where(i => Predicate(i, obj));
您想要的操作称为“部分评估”; 它在逻辑上与将两参数函数“卷曲”成两个单参数函数有关。
static class Extensions
{
static Func<A, R> PartiallyEvaluateRight<A, B, R>(this Func<A, B, R> f, B b)
{
return a => f(a, b);
}
}
...
Func<int, int, bool> isGreater = (x, y) => x > y;
Func<int, bool> isGreaterThanTwo = isGreater.PartiallyEvaluateRight(2);
现在你可以在where
子句中使用isGreaterThanTwo
。
如果您想提供第一个参数,那么您可以轻松编写PartiallyEvaluateLeft
。
合理?
currying操作(部分适用于左边)通常写成:
static class Extensions
{
static Func<A, Func<B, R>> Curry<A, B, R>(this Func<A, B, R> f)
{
return a => b => f(a, b);
}
}
现在你可以做一个工厂:
Func<int, int, bool> greaterThan = (x, y) => x > y;
Func<int, Func<int, bool>> factory = greaterThan.Curry();
Func<int, bool> withTwo = factory(2); // makes y => 2 > y
这一切都清楚了吗?
你期待这样的事吗?
public bool Predicate (string a, object obj)
{
// blah blah
}
public void Test()
{
var obj = "Object";
var items = new string[]{"a", "b", "c"};
var result = items.Where(x => Predicate(x, obj)); // here I want to somehow supply obj to Predicate as the second argument
}
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.