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Finding property differences between two C# objects

Asked 2010-03-05T15:46:21.707
59

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

c# .net reflection auditing
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1 Answer

29

IComparable is for ordering comparisons. Either use IEquatable instead, or just use the static System.Object.Equals method. The latter has the benefit of also working if the object is not a primitive type but still defines its own equality comparison by overriding Equals.

object originalValue = property.GetValue(originalObject, null);
object newValue = property.GetValue(changedObject, null);
if (!object.Equals(originalValue, newValue))
{
    string originalText = (originalValue != null) ?
        originalValue.ToString() : "[NULL]";
    string newText = (newText != null) ?
        newValue.ToString() : "[NULL]";
    // etc.
}

This obviously isn't perfect, but if you're only doing it with classes that you control, then you can make sure it always works for your particular needs.

There are other methods to compare objects (such as checksums, serialization, etc.) but this is probably the most reliable if the classes don't consistently implement IPropertyChanged and you want to actually know the differences.

Update for new example code:

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

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

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

The reason that using object.Equals in your audit method results in a "hit" is because the instances are actually not equal!

Sure, the StateProvince may be empty in both cases, but address1 and address2 still have non-null values for the StateProvince property and each instance is different. Therefore, address1 and address2 have different properties.

Let's flip this around, take this code as an example:

Address address1 = new Address(
answered 2010-03-05T15:56:24.373

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