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 IEqualityComparers from lambda, I'd suggest you to please vote and comment on that idea, and use the following:

Usage:

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

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

Code:

public class EqualityComparerImpl : IEqualityComparer
{
  public static EqualityComparerImpl Create(
    params Expression>[] properties) =>
    new EqualityComparerImpl(properties);

  PropertyInfo[] _properties;
  EqualityComparerImpl(Expression>[] 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> 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.");
  }
}