Using default comparer builder option throws StackOverflowException.

[amir734jj]

Using Default throws StackOverflowException exception but using OrderBy(x => x.Weight) works fine.

public class Component : ComparableBase<Component>
{
    static Component()
    {
        DefaultComparer = ComparerBuilder.For<Component>().Default();
    }
    
    public double AccWeight { get; set; }

    public double Weight { get; set; }
}

[StephenCleary]

The Default comparer will forward to the IComparable<T> implementation. What behavior were you expecting?

[amir734jj]

This throws an exception

            var c1 = new Component
            {
                Weight = 0.7,
                AccWeight = 0.2
            };
            
            var c2 = new Component
            {
                Weight = 0.7,
                AccWeight = 0.2
            };

            var result = c1.Equals(c2);

[StephenCleary]

Yes, the Default comparer forwards to IComparable<T>, which is implemented by the DefaultComparer, which is just Default, which forwards to IComparable<T>, ... So of course you're going to get a stack overflow exception.

Again, what behavior were you expecting?

[amir734jj]

I expected it to compare one property at a time and return true.

[amir734jj]

I may be wrong but I don't think using Equals method should never throw StackOverflowException regardless of implementation. It should return either true or false.

[StephenCleary]

@amir734jj: Nito.Comparers does not reflect over all properties of types. That's rarely the desired behavior.

The Default() comparer is behaving appropriately. You would have the same exact problem with the .NET Comparer<T>.Default or EqualityComparer<T>.Default.

[amir734jj]

I understand. I am passionate about this subject because I spend too much time daily writing and maintaining Equals and GetHashCode methods. We don't necessarily need to use reflection every time. We can just create an Expression and compile it once.

   public class Component
    {
        public double AccumulatedWeight { get; set; }

        public double Weight { get; set; }
        
        public Nested NestedRef { get; set; }
    }
    
    public class Nested
    {
        public double NestedProp { get; set; }
    }
    
    class Program
    {
        static void Main(string[] args)
        {   
            var c1 = new Component
            {
                Weight = 0.7,
                AccumulatedWeight = 0.2,
                NestedRef = new Nested
                {
                    NestedProp = 223
                }
            };
            
            var c2 = new Component
            {
                Weight = 0.7,
                AccumulatedWeight = 0.2,
                NestedRef = new Nested
                {
                    NestedProp = 223
                }
            };

            var result = BuildEquals<Component>()(c1, c2);
            
            Console.WriteLine(result);
        }
        
        public static Func<T, T, bool> BuildEquals<T>()
        {
            return ((Expression<Func<T, T, bool>>) BuildEquals(typeof(T))).Compile();
        }

        public static Expression BuildEquals(Type type)
        {
            bool IsComplexType(Type nestedType)
            {
                return !nestedType.Namespace.StartsWith("System.Collections") && nestedType.IsClass;
            }
            
            var arg1 = Expression.Parameter(type);
            var arg2 = Expression.Parameter(type);
            
            var body = type
                .GetProperties()
                .Select(x =>
                {
                    if (IsComplexType(x.PropertyType))
                    {
                        return (Expression) Expression.Invoke(BuildEquals(x.PropertyType),
                            Expression.Property(arg1, x.Name),
                            Expression.Property(arg2, x.Name));
                    }
                    else
                    {
                        return Expression.Equal(Expression.Property(arg1, x.Name), Expression.Property(arg2, x.Name));
                    }
                })
                .Aggregate((x, y) => Expression.And(x, y));

            return Expression.Lambda(body, arg1, arg2);
        } 

[StephenCleary]

You can use a similar approach to BuildEquals along with Nito.Comparers as such:

public static IFullEqualityComparer<T> BuildAllMembersEqualityComparer<T>()
{
    var result = EqualityComparerBuilder.For<T>().Null();
    var type = typeof(T);
    var parameter = Expression.Parameter(type);
    foreach (var prop in type.GetProperties())
    {
        var expression = Expression.Property(parameter, prop.Name);
        dynamic selector = Expression.Lambda(expression, parameter).Compile();
        dynamic childComparer = null;
        if (IsComplexType(prop.PropertyType))
            childComparer = ((MethodInfo)MethodBase.GetCurrentMethod()).MakeGenericMethod(prop.PropertyType).Invoke(null, null);
        result = EqualityComparerExtensions.ThenEquateBy(result, selector, childComparer);
    }
    return result;

    bool IsComplexType(Type nestedType) => !nestedType.Namespace.StartsWith("System.Collections") && nestedType.IsClass;
}

This code takes a similar approach as run-time comparers: starting with Null (everything is equivalent), and adding a ThenEquateBy clause for each member.

Using Nito.Comparers in this way, the same lambda expressions are used to implement both Equals and GetHashCode.