Pass a lambda expression in place of IComparer or IEqualityComparer or any single-method interface?

Question

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, IEqualityCom

Answer 1

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));

Answer 2

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;
    }
}

Answer 3

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 :)

Answer 4

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.

Answer 5

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));

Answer 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.