时间:2010-01-30 02:32:45

标签: c# .net linq predicate

我想做这样的事情:

List<SomeClass> list1 = ...
List<SomeClass> list2 = ...
Predicate<SomeClass> condition = ...

...

list2.RemoveAll (!condition);

...

list2.AddRange (list1.FindAll (condition));

但是,这会导致编译器错误,因为!无法应用于Predicate<SomeClass>。有没有办法做到这一点?

2 个答案:

答案 0 :(得分:32)

您可以使用lambda表达式来定义一个匿名委托,它是否定谓词结果的结果:

list.RemoveAll(x => !condition(x));    

另一种选择:

static Predicate<T> Negate<T>(Predicate<T> predicate) {
     return x => !predicate(x);
}

用法:

// list is List<T> some T
// predicate is Predicate<T> some T
list.RemoveAll(Negate(predicate));

list.RemoveAll(!condition)不起作用的原因是代理上没有定义!运算符。这就是您必须按照condition定义新委托的原因,如上所示。

答案 1 :(得分:9)

这实际上是可行的,但可能与您习惯的形式略有不同。在.NET中,lambda表达式可以解释为委托 OR expression trees。在表达式树上执行NOT操作相对简单。

以下是使用代码作为起点的示例:

namespace Sample
{
    using System;
    using System.Collections.Generic;
    using System.Linq.Expressions;

    internal class ExpressionSample
    {
        private static Expression<TDelegate> Negate<TDelegate>(Expression<TDelegate> expression)
        {
            return Expression.Lambda<TDelegate>(Expression.Not(expression.Body), expression.Parameters);
        }

        private static void Main()
        {
            // Match any string of length 2 or more characters
            Expression<Predicate<string>> expression = (s) => s.Length > 1;

            // Logical negation, i.e. match string of length 1 or fewer characters
            Expression<Predicate<string>> negatedExpression = ExpressionSample.Negate(expression);

            // Compile expressions to predicates
            Predicate<string> predicate = expression.Compile();
            Predicate<string> negativePredicate = negatedExpression.Compile();

            List<string> list1 = new List<string> { string.Empty, "an item", "x", "another item" };
            List<string> list2 = new List<string> { "yet another item", "still another item", "y", string.Empty };

            list2.RemoveAll(negativePredicate);
            list2.AddRange(list1.FindAll(predicate));

            list2.ForEach((s) => Console.WriteLine(s));
        }
    }
}