Unexpected result when comparing values retrieved with PropertyInfo.GetValue()

Question

I've got some code which I use to loop through the properties of certain objects and compare the property values, which looks somewhat like this:

public static bool AreObjectPropertyValuesEqual(object a, object b)
{

 if (a.GetType() != b.GetType())
  throw new ArgumentException("The objects must be of the same type.");

 Type type = a.GetType();

 foreach (PropertyInfo propInfo in type.GetProperties())
 {
  if (propInfo.GetValue(a, null) != propInfo.GetValue(b, null))
  {
   return false;
  }
 }
 return true;
}

Now for the weird behaviour. I've created a class called PurchaseOrder with a couple of properties, all of which are simple datatypes (strings, ints, etc.). I create one instance in my Unit-Test code and another one is created by my DataModel, fetching the data from a database (MySql, I'm using the MySqlConnector).

Eventhough the debugger shows me, that the property values are identical, the comparison in the code above fails.

That is: my object A created in the UnitTest has an Amount property value of 10. The object B retrieved from my Repository has an Amount property value of 10. Comparison fails! If I change the code to

if (propInfo.GetValue(a, null).ToString() != propInfo.GetValue(b, null).ToString())
{
 ...
}

everything works as I would expect. The comparison doesn't fail as well if I create PurchaseOrder instances directly in the UnitTest.

I'd be very thankful for any anwers. Have a good day!

Answer 1

The reason for the failure is that the equality test that gets applied above is a reference equality test. Since the two objects returned by propInfo.GetValue(foo, null), though equal by their own definitions, are separate objects, their references are different, and thus the equality fails.

Answer 2

PropertyInfo.GetValue returns an object, and your unit test is doing an == reference comparison. Try this instead:

if (!propInfo.GetValue(a, null).Equals(propInfo.GetValue(b, null)))

You may want to replace null with something more sensible...

Alternatively, you could try:

if ((int?)propInfo.GetValue(a, null) != (int?)propInfo.GetValue(b, null))

(or whatever simple type you have if it's not an int) which forces the value type == behaviour.