Comparing 2 objects and retrieve a list of fields with different values

Question

Given a class with 35 fields and 2 objects with a certain number of different fields value. Is there an clever way to get a list with the fields name where the ob

Answer 1

OK; this is insanely more effort than I would normally go to, but this might be a useful utility method. It creates IL on the fly (cached) to do the work, handling value-type vs ref-type objects, inbuilt IL equality, equality operators (==), and EqualityComparer<T> for the rest:

using System.Collections.Generic;
using System.Reflection;
using System.Reflection.Emit;

namespace ConsoleApplication2
{
    using System;
    class Program
    {
        static void Main()
        {
            WriteDeltas(new Foo {X = 123, Y = DateTime.Today, Z = null},
                        new Foo {X = 124, Y = DateTime.Today, Z = null});
            WriteDeltas(new Foo { X = 123, Y = DateTime.Today, Z = null },
                        new Foo { X = 123, Y = DateTime.Now, Z = new Dummy()});
            WriteDeltas(new Bar { X = 123, Y = DateTime.Today, Z = null },
                        new Bar { X = 124, Y = DateTime.Today, Z = null });
            WriteDeltas(new Bar { X = 123, Y = DateTime.Today, Z = null },
                        new Bar { X = 123, Y = DateTime.Now, Z = new Dummy() });
        }
        static void WriteDeltas<T>(T x, T y)
        {
            Console.WriteLine("----");
            foreach(string delta in PropertyComparer<T>.GetDeltas(x,y))
            {
                Console.WriteLine(delta);
            }

        }

    }
    class Dummy {}
    class Foo
    {
        public int X { get; set; }
        public DateTime Y { get; set; }
        public Dummy Z { get; set; }
    }
    struct Bar
    {
        public int X { get; set; }
        public DateTime Y { get; set; }
        public Dummy Z { get; set; }
    }

    public static class PropertyComparer<T>
    {
        private static readonly Func<T, T, List<string>> getDeltas;
        static PropertyComparer()
        {
            var dyn = new DynamicMethod(":getDeltas", typeof (List<string>), new[] {typeof (T), typeof (T)},typeof(T));
            var il = dyn.GetILGenerator();
            il.Emit(OpCodes.Newobj, typeof (List<string>).GetConstructor(Type.EmptyTypes));
            bool isValueType = typeof (T).IsValueType;
            OpCode callType = isValueType ? OpCodes.Call : OpCodes.Callvirt;
            var add = typeof(List<string>).GetMethod("Add");
            foreach (var prop in typeof(T).GetProperties())
            {
                if (!prop.CanRead) continue;
                Label next = il.DefineLabel();
                switch (Type.GetTypeCode(prop.PropertyType))
                {
                    case TypeCode.Boolean:
                    case TypeCode.Byte:
                    case TypeCode.Char:
                    case TypeCode.Double:
                    case TypeCode.Int16:
                    case TypeCode.Int32:
                    case TypeCode.Int64:
                    case TypeCode.SByte:
                    case TypeCode.Single:
                    case TypeCode.UInt16:
                    case TypeCode.UInt32:
                    case TypeCode.UInt64:
                        if(isValueType) {il.Emit(OpCodes.Ldarga_S, (byte)0);} else {il.Emit(OpCodes.Ldarg_0);}
                        il.EmitCall(callType, prop.GetGetMethod(), null);
                        if (isValueType) { il.Emit(OpCodes.Ldarga_S, (byte)1); } else { il.Emit(OpCodes.Ldarg_1); }
                        il.EmitCall(callType, prop.GetGetMethod(), null);
                        il.Emit(OpCodes.Ceq);
                        break;
                    default:
                        var pp = new Type[] {prop.PropertyType, prop.PropertyType};
                        var eq = prop.PropertyType.GetMethod("op_Equality", BindingFlags.Public | BindingFlags.Static, null, pp, null);
                        if (eq != null)
                        {
                            if (isValueType) { il.Emit(OpCodes.Ldarga_S, (byte)0); } else { il.Emit(OpCodes.Ldarg_0); }
                            il.EmitCall(callType, prop.GetGetMethod(), null);
                            if (isValueType) { il.Emit(OpCodes.Ldarga_S, (byte)1); } else { il.Emit(OpCodes.Ldarg_1); }
                            il.EmitCall(callType, prop.GetGetMethod(), null);
                            il.EmitCall(OpCodes.Call, eq, null);

                        }
                        else
                        {
                            il.EmitCall(OpCodes.Call, typeof(EqualityComparer<>).MakeGenericType(prop.PropertyType).GetProperty("Default").GetGetMethod(), null);
                            if (isValueType) { il.Emit(OpCodes.Ldarga_S, (byte)0); } else { il.Emit(OpCodes.Ldarg_0); }
                            il.EmitCall(callType, prop.GetGetMethod(), null);
                            if (isValueType) { il.Emit(OpCodes.Ldarga_S, (byte)1); } else { il.Emit(OpCodes.Ldarg_1); }
                            il.EmitCall(callType, prop.GetGetMethod(), null);
                            il.EmitCall(OpCodes.Callvirt, typeof(EqualityComparer<>).MakeGenericType(prop.PropertyType).GetMethod("Equals", pp), null);
                        }
                        break;
                }
                il.Emit(OpCodes.Brtrue_S, next); // equal
                il.Emit(OpCodes.Dup);
                il.Emit(OpCodes.Ldstr, prop.Name);
                il.EmitCall(OpCodes.Callvirt, add, null);
                il.MarkLabel(next);
            }
            il.Emit(OpCodes.Ret);
            getDeltas = (Func<T, T, List<string>>)dyn.CreateDelegate(typeof (Func<T, T, List<string>>));
        }
        public static List<string> GetDeltas(T x, T y) { return getDeltas(x, y); }

    }
}

Answer 2

Reflection is the way to go with this and I don't think 35 fields is a problem.

(After confusing myself utterly I come back to thinking that I understand the question and reflection would be fine for this).