Finding property differences between two C# object

Question

The project I'm working on needs some simple audit logging for when a user changes their email, billing address, etc. The objects we're working with are coming from different sources, one a WCF service, the other a web service.

I've implemented the following method using reflection to find changes to the properties on two different objects. This generates a list of the properties that have differences along with their old and new values.

public static IList GenerateAuditLogMessages(T originalObject, T changedObject)
{
    IList list = new List();
    string className = string.Concat("[", originalObject.GetType().Name, "] ");

    foreach (PropertyInfo property in originalObject.GetType().GetProperties())
    {
        Type comparable =
            property.PropertyType.GetInterface("System.IComparable");

        if (comparable != null)
        {
            string originalPropertyValue =
                property.GetValue(originalObject, null) as string;
            string newPropertyValue =
                property.GetValue(changedObject, null) as string;

            if (originalPropertyValue != newPropertyValue)
            {
                list.Add(string.Concat(className, property.Name,
                    " changed from '", originalPropertyValue,
                    "' to '", newPropertyValue, "'"));
            }
        }
    }

    return list;
}

I'm looking for System.IComparable because "All numeric types (such as Int32 and Double) implement IComparable, as do String, Char, and DateTime." This seemed the best way to find any property that's not a custom class.

Tapping into the PropertyChanged event that's generated by the WCF or web service proxy code sounded good but doesn't give me enough info for my audit logs (old and new values).

Looking for input as to if there is a better way to do this, thanks!

@Aaronaught, here is some example code that is generating a positive match based on doing object.Equals:

Address address1 = new Address();
address1.StateProvince = new StateProvince();

Address address2 = new Address();
address2.StateProvince = new StateProvince();

IList list = Utility.GenerateAuditLogMessages(address1, address2);

"[Address] StateProvince changed from 'MyAccountService.StateProvince' to 'MyAccountService.StateProvince'"

It's two different instances of the StateProvince class, but the values of the properties are the same (all null in this case). We're not overriding the equals method.

Answer 1

I think this method is quite neat, it avoids repetition or adding anything to classes. What more are you looking for?

The only alternative would be to generate a state dictionary for the old and new objects, and write a comparison for them. The code for generating the state dictionary could reuse any serialisation you have for storing this data in the database.

Answer 2

Here a short LINQ version that extends object and returns a list of properties that are not equal:

usage: object.DetailedCompare(objectToCompare);

public static class ObjectExtensions
    {

        public static List<Variance> DetailedCompare<T>(this T val1, T val2)
        {
            var propertyInfo = val1.GetType().GetProperties();
            return propertyInfo.Select(f => new Variance
                {
                    Property = f.Name,
                    ValueA = f.GetValue(val1),
                    ValueB = f.GetValue(val2)
                })
                .Where(v => !v.ValueA.Equals(v.ValueB))
                .ToList();
        }

        public class Variance
        {
            public string Property { get; set; }
            public object ValueA { get; set; }
            public object ValueB { get; set; }
        }

    }