Pass a lambda expression in place of IComparer or

问题:

I happened to have seen some code where this guy passed a lambda expression to a ArrayList.Sort(IComparer here) or a IEnumerable.SequenceEqual(IEnumerable list, IEqualityComparer here) where an IComparer or an IEqualityComparer was expected.

I can't be sure if I saw it though, or I am just dreaming. And I can't seem to find an extension on any of these collections that accepts a Func<> or a delegate in their method signatures.

Is there such an overload/extension method? Or, if not, is it possible to muck around like this and pass an algorithm (read delegate) where a single-method interface is expected?

Update Thanks, everyone. That's what I thought. I must've been dreaming. I know how to write a conversion. I just wasn't sure if I'd seen something like that or just thought I'd seen it.

Yet another update Look, here, I found one such instance. I wasn't dreaming after all. Look at what this guy is doing here. What gives?

And here's another update: Ok, I get it. The guy's using the Comparison<T> overload. Nice. Nice, but totally prone to mislead you. Nice, though. Thanks.

回答1:

I'm not much sure what useful it really is, as I think for most cases in the Base Library expecting an IComparer there's an overload that expects a Comparison... but just for the record:

in .Net 4.5 they've added a method to obtain an IComparer from a Comparison: Comparer.Create

so you can pass your lambda to it and obtain an IComparer.

回答2:

I was also googling the web for a solution, but i didn't found any satisfying one. So i've created a generic EqualityComparerFactory:

public static class EqualityComparerFactory<T>
{
    private class MyComparer : IEqualityComparer<T>
    {
        private readonly Func<T, int> _getHashCodeFunc;
        private readonly Func<T, T, bool> _equalsFunc;

        public MyComparer(Func<T, int> getHashCodeFunc, Func<T, T, bool> equalsFunc)
        {
            _getHashCodeFunc = getHashCodeFunc;
            _equalsFunc = equalsFunc;
        }

        public bool Equals(T x, T y)
        {
            return _equalsFunc(x, y);
        }

        public int GetHashCode(T obj)
        {
            return _getHashCodeFunc(obj);
        }
    }

    public static IEqualityComparer<T> CreateComparer(Func<T, int> getHashCodeFunc, Func<T, T, bool> equalsFunc)
    {
        if (getHashCodeFunc == null)
            throw new ArgumentNullException("getHashCodeFunc");
        if (equalsFunc == null)
            throw new ArgumentNullException("equalsFunc");

        return new MyComparer(getHashCodeFunc, equalsFunc);
    }
}

The idea is, that the CreateComparer method takes two arguments: a delegate to GetHashCode(T) and a delegate to Equals(T,T)

Example:

class Person
{
    public int Id { get; set; }
    public string LastName { get; set; }
    public string FirstName { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        var list1 = new List<Person>(new[]{
            new Person { Id = 1, FirstName = "Walter", LastName = "White" },
            new Person { Id = 2, FirstName = "Jesse", LastName = "Pinkman" },
            new Person { Id = 3, FirstName = "Skyler", LastName = "White" },
            new Person { Id = 4, FirstName = "Hank", LastName = "Schrader" },
        });

        var list2 = new List<Person>(new[]{
            new Person { Id = 1, FirstName = "Walter", LastName = "White" },
            new Person { Id = 4, FirstName = "Hank", LastName = "Schrader" },
        });


        // We're comparing based on the Id property
        var comparer = EqualityComparerFactory<Person>.CreateComparer(a => a.Id.GetHashCode(), (a, b) => a.Id==b.Id);
        var intersection = list1.Intersect(list2, comparer).ToList();
    }
}

回答3:

You can provide a lambda for a Array.Sort method, as it requires a method that accepts two objects of type T and returns an integer. As such, you could provide a lambda of the following definition (a, b) => a.CompareTo(b). An example to do a descending sort of an integer array:

int[] array = { 1, 8, 19, 4 };

// descending sort 
Array.Sort(array, (a, b) => -1 * a.CompareTo(b));

回答4:

public class Comparer2<T, TKey> : IComparer<T>, IEqualityComparer<T>
{
    private readonly Expression<Func<T, TKey>> _KeyExpr;
    private readonly Func<T, TKey> _CompiledFunc
    // Constructor
    public Comparer2(Expression<Func<T, TKey>> getKey)
    {
        _KeyExpr = getKey;
        _CompiledFunc = _KeyExpr.Compile();
    } 

    public int Compare(T obj1, T obj2)
    {
        return Comparer<TKey>.Default.Compare(_CompiledFunc(obj1), _CompiledFunc(obj2));
    }

    public bool Equals(T obj1, T obj2)
    { 
        return EqualityComparer<TKey>.Default.Equals(_CompiledFunc(obj1), _CompiledFunc(obj2));
    }

    public int GetHashCode(T obj)
    {
         return EqualityComparer<TKey>.Default.GetHashCode(_CompiledFunc(obj));
    }
}

use it like this

ArrayList.Sort(new Comparer2<Product, string>(p => p.Name));

回答5:

You can't pass it directly however you could do so by defining a LambdaComparer class that excepts a Func<T,T,int> and then uses that in it's CompareTo.

It is not quite as concise but you could make it shorter through some creative extension methods on Func.

回答6:

I vote for the dreaming theory.

You can't pass a function where an object is expected: derivatives of System.Delegate (which is what lambdas are) don't implement those interfaces.

What you probably saw is a use of the of the Converter<TInput, TOutput> delegate, which can be modeled by a lambda. Array.ConvertAll uses an instance of this delegate.

回答7:

These methods don't have overloads that accept a delegate instead of an interface, but:

• You can normally return a simpler sort key through the delegate you pass to Enumerable.OrderBy
• Likewise, you could call Enumerable.Select before calling Enumerable.SequenceEqual
• It should be straightforward to write a wrapper that implements IEqualityComparer<T> in terms of Func<T, T, bool>
• F# lets you implement this sort of interface in terms of a lambda :)

回答8:

In case if you need this function for use with lambda and possibly two different element types:

static class IEnumerableExtensions
{
    public static bool SequenceEqual<T1, T2>(this IEnumerable<T1> first, IEnumerable<T2> second, Func<T1, T2, bool> comparer)
    {
        if (first == null)
            throw new NullReferenceException("first");

        if (second == null)
            throw new NullReferenceException("second");

        using (IEnumerator<T1> e1 = first.GetEnumerator())
        using (IEnumerator<T2> e2 = second.GetEnumerator())
        {
            while (e1.MoveNext())
            {
                if (!(e2.MoveNext() && comparer(e1.Current, e2.Current)))
                    return false;
            }

            if (e2.MoveNext())
                return false;
        }

        return true;
    }
}