Custom IEqualityComparer to calculate Difference between two lists using LINQ Except

Question

I am using this answer Difference between two lists to find a difference between two lists using Linq Except. However, my objects in the lists are of generic type, so I wrote a Comparer for the generic types, then I pass it to Except method. However I receive the following error message:

Error CS1929 'List' does not contain a definition for 'Except' and the best extension method overload 'ParallelEnumerable.Except(ParallelQuery, ParallelQuery, IEqualityComparer)' requires a receiver of type 'ParallelQuery'

My call:

var differenceList = list1.Except(list2, new PropertyComparer<Guid>("ObjectId"));

And my comparer:

public class PropertyComparer<T> : IEqualityComparer<T>
{
    private PropertyInfo _PropertyInfo;
    public PropertyComparer(string propertyName)
    {
        _PropertyInfo = typeof(T).GetProperty(propertyName, 
            BindingFlags.GetProperty | BindingFlags.Instance | BindingFlags.Public);
        if (_PropertyInfo == null)
        {
            throw new ArgumentException(
                string.Format("{0} is not a property of type {1}.", propertyName, typeof(T)));
        }
    }

    public int GetHashCode(T obj)
    {
        object propertyValue = _PropertyInfo.GetValue(obj, null);
        if (obj == null) return 0;
        return propertyValue.GetHashCode();
    }

    public bool Equals(T x, T y)
    {
        object xValue = _PropertyInfo.GetValue(x, null);
        object yValue = _PropertyInfo.GetValue(y, null);

        if (xValue == null)
        {
            return yValue == null;
        }

        return xValue.Equals(yValue);
    }
}

And from the declaration of Except method in System.Linq, which fits my comparer:

public static IEnumerable<TSource> Except<TSource>(
    this IEnumerable<TSource> first, 
    IEnumerable<TSource> second, 
    IEqualityComparer<TSource> comparer
);

What am I doing wrong and how can I get it working?

---

UPDATE

list1 and list2 types:

List<Contact>

Where Contact is

public class Contact : EntityBase
{
    public Guid ContactId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public List<Email> Emails {get; set;}
}

EntityBase is

public class EntityBase : IEntity
{
    public IQueryable<T> GetDBEntities<T>(ApplicationDbContext db) where T : class
    {
        return db.Set<T>();
    }

    public List<T> GetLocalEntities<T>() where T : class
    {
        var localProperties = this.GetType().GetProperties();
        foreach (var localProperty in localProperties)
        {
            var localPropertyValue = localProperty.GetValue(this);
            if (localPropertyValue != null && localPropertyValue.IsGenericList() == true)
            {
                var localPropertyValueType = localPropertyValue.GetType(); // List<object>
                var localPropertyValueTypeDecoupled = localPropertyValueType.GetGenericArguments().Single(); // List<T>
                if (localPropertyValueTypeDecoupled == typeof(T))
                {
                    return (List<T>)localPropertyValue;
                }
            }
        }
        throw new Exception("Entity Types Validation Error");
    }

    public void ProcessEntityReference<T>(ApplicationDbContext db) where T : class
    {
        // T is Email
        var remoteList = this.GetDBEntities<T>(db).ToList();
        var updatedList = GetLocalEntities<T>();
        var toBeAdded = updatedList.Except(remoteList, new PropertyComparer<Guid>("ContactId"));
        var toBeDeleted = new List<object>();
        throw new NotImplementedException();
    }

    public void ProcessEntityReferences(ApplicationDbContext db)
    {
        this.ProcessEntityReference<Email>(db);
    }
}

Answer 1

You should pass the same type used by your list to the Except method, in your example, you are using Guid but it should be of type Contact, also, your Contact class doesn't have a property called "ObjectId", try changing that for "ContactId", the following seems to work fine:

static void Main(string[] args)
{
     var list1 = new List<Contact>();
     list1.Add(new Contact() { ContactId = Guid.Parse("FB58F102-0CE4-4914-ABFF-ABBD3895D719") });
     list1.Add(new Contact() { ContactId = Guid.Parse("5A201238-6036-4385-B848-DEE598A3520C") });

     var list2 = new List<Contact>();
     list2.Add(new Contact() { ContactId = Guid.Parse("FB58F102-0CE4-4914-ABFF-ABBD3895D719") });

     var list3 = list1.Except(list2, new PropertyComparer<Contact>("ContactId"));

     foreach (var item in list3) 
         Console.WriteLine(item.ContactId.ToString());

     Console.ReadLine();
}

public class Contact
{
    public Guid ContactId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class PropertyComparer<T> : IEqualityComparer<T>
{
    private PropertyInfo _PropertyInfo;
    public PropertyComparer(string propertyName)
    {
        _PropertyInfo = typeof(T).GetProperty(propertyName,
            BindingFlags.GetProperty | BindingFlags.Instance | BindingFlags.Public);
        if (_PropertyInfo == null)
        {
            throw new ArgumentException(
                string.Format("{0} is not a property of type {1}.", propertyName, typeof(T)));
        }
    }

    public int GetHashCode(T obj)
    {
        object propertyValue = _PropertyInfo.GetValue(obj, null);
        if (obj == null) return 0;
        return propertyValue.GetHashCode();
    }

    public bool Equals(T x, T y)
    {
        object xValue = _PropertyInfo.GetValue(x, null);
        object yValue = _PropertyInfo.GetValue(y, null);

        if (xValue == null)
        {
            return yValue == null;
        }

        return xValue.Equals(yValue);
    }
}

Output:

5a201238-6036-4385-b848-dee598a3520c