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<String> with the fields name where the object are as following?

e.g.

obj1.Name = "aaa";
obj1.LastName = "bbb";
obj1.Address = "xcs";
obj2.Name = "aaa";
obj2.LastName = "ccc";
obj2.Address = "jk";

objective:

list<<String>String> containing 2 Strings LastName and Address

I see reflection as the way to go but with 35 fields I am afraid it's too heavy. Any other idea, like linq?

The cost of reflection is relative. How often are you doing this? There are ways to do this much faster, but if you aren't doing it in a tight loop, reflection should be fine (and is simple). — Marc Gravell
– Marc Gravell, Commented Jun 17, 2010 at 9:41
(for example, it would be possible to write this via ILGenerator or Expression - but is the complexity warranted? let me know...) — Marc Gravell
– Marc Gravell, Commented Jun 17, 2010 at 9:50
Marc, I am doing it inside WCF but it will be used for a couple of users, so I decided for reflection. Thanks you very much for you efforts. a — ajj
– ajj, Commented Jun 18, 2010 at 7:25

Marc Gravell · Accepted Answer · 2010-06-17 10:45:14Z

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); }

    }
}

This is awesome. Would it be difficult to make this utility recursive (i.e.: walking the object graph) ? :-)

Hans Olsson · Accepted Answer · 2010-06-17 09:37:58Z

1

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).

edited Jun 17, 2010 at 9:37

answered Jun 17, 2010 at 9:32

Hans Olsson

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