Can you create a simple 'EqualityComparer' using a lambda expression

Question

Short question:

Is there a simple way in LINQ to objects to get a distinct list of objects from a list based on a key property on the objects.

Answer 1

This idea is being debated here, and while I'm hoping the .NET Core team adopt a method to generate IEqualityComparer<T>s from lambda, I'd suggest you to please vote and comment on that idea, and use the following:

Usage:

IEqualityComparer<Contact> comp1 = EqualityComparerImpl<Contact>.Create(c => c.Name);
var comp2 = EqualityComparerImpl<Contact>.Create(c => c.Name, c => c.Age);

class Contact { public Name { get; set; } public Age { get; set; } }

Code:

public class EqualityComparerImpl<T> : IEqualityComparer<T>
{
  public static EqualityComparerImpl<T> Create(
    params Expression<Func<T, object>>[] properties) =>
    new EqualityComparerImpl<T>(properties);

  PropertyInfo[] _properties;
  EqualityComparerImpl(Expression<Func<T, object>>[] properties)
  {
    if (properties == null)
      throw new ArgumentNullException(nameof(properties));

    if (properties.Length == 0)
      throw new ArgumentOutOfRangeException(nameof(properties));

    var length = properties.Length;
    var extractions = new PropertyInfo[length];
    for (int i = 0; i < length; i++)
    {
      var property = properties[i];
      extractions[i] = ExtractProperty(property);
    }
    _properties = extractions;
  }

  public bool Equals(T x, T y)
  {
    if (ReferenceEquals(x, y))
      //covers both are null
      return true;
    if (x == null || y == null)
      return false;
    var len = _properties.Length;
    for (int i = 0; i < _properties.Length; i++)
    {
      var property = _properties[i];
      if (!Equals(property.GetValue(x), property.GetValue(y)))
        return false;
    }
    return true;
  }

  public int GetHashCode(T obj)
  {
    if (obj == null)
      return 0;

    var hashes = _properties
        .Select(pi => pi.GetValue(obj)?.GetHashCode() ?? 0).ToArray();
    return Combine(hashes);
  }

  static int Combine(int[] hashes)
  {
    int result = 0;
    foreach (var hash in hashes)
    {
      uint rol5 = ((uint)result << 5) | ((uint)result >> 27);
      result = ((int)rol5 + result) ^ hash;
    }
    return result;
  }

  static PropertyInfo ExtractProperty(Expression<Func<T, object>> property)
  {
    if (property.NodeType != ExpressionType.Lambda)
      throwEx();

    var body = property.Body;
    if (body.NodeType == ExpressionType.Convert)
      if (body is UnaryExpression unary)
        body = unary.Operand;
      else
        throwEx();

    if (!(body is MemberExpression member))
      throwEx();

    if (!(member.Member is PropertyInfo pi))
      throwEx();

    return pi;

    void throwEx() =>
      throw new NotSupportedException($"The expression '{property}' isn't supported.");
  }
}

Answer 2

You could group by the key value and then select the top item from each group. Would that work for you?

Answer 3

This is the best i can come up with for the problem in hand. Still curious whether theres a nice way to create a EqualityComparer on the fly though.

Galleries.SelectMany(x => x.Images).ToLookup(x => x.id).Select(x => x.First());

Create lookup table and take 'top' from each one

Note: this is the same as @charlie suggested but using ILookup - which i think is what a group must be anyway.

Answer 4

implement IEquatable on GalleryImage because it is generated

A different approach would be to generate GalleryImage as a partial class, and then have another file with the inheritance and IEquatable, Equals, GetHash implementation.

Answer 5

Building on Charlie Flowers' answer, you can create your own extension method to do what you want which internally uses grouping:

    public static IEnumerable<T> Distinct<T, U>(
        this IEnumerable<T> seq, Func<T, U> getKey)
    {
        return
            from item in seq
            group item by getKey(item) into gp
            select gp.First();
    }

You could also create a generic class deriving from EqualityComparer, but it sounds like you'd like to avoid this:

    public class KeyEqualityComparer<T,U> : IEqualityComparer<T>
    {
        private Func<T,U> GetKey { get; set; }

        public KeyEqualityComparer(Func<T,U> getKey) {
            GetKey = getKey;
        }

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

        public int GetHashCode(T obj)
        {
            return GetKey(obj).GetHashCode();
        }
    }

Answer 6

What about a throw away IEqualityComparer generic class?

public class ThrowAwayEqualityComparer<T> : IEqualityComparer<T>
{
  Func<T, T, bool> comparer;

  public ThrowAwayEqualityComparer(Func<T, T, bool> comparer)   
  {
    this.comparer = comparer;
  }

  public bool Equals(T a, T b)
  {
    return comparer(a, b);
  }

  public int GetHashCode(T a)
  {
    return a.GetHashCode();
  }
}

So now you can use Distinct with a custom comparer.

var distinctImages = allImages.Distinct(
   new ThrowAwayEqualityComparer<GalleryImage>((a, b) => a.Key == b.Key));

You might be able to get away with the <GalleryImage>, but I'm not sure if the compiler could infer the type (don't have access to it right now.)

And in an additional extension method:

public static class IEnumerableExtensions
{
  public static IEnumerable<TValue> Distinct<TValue>(this IEnumerable<TValue> @this, Func<TValue, TValue, bool> comparer)
  {
    return @this.Distinct(new ThrowAwayEqualityComparer<TValue>(comparer);
  }

  private class ThrowAwayEqualityComparer...
}