Question

While creating my testing framework I’ve found a strange problem.

I want to create a static class that would allow me to compare objects of the same type by their properties, but with possibility to ignore some of them.

I want to have a simple fluent API for this, so a call like TestEqualityComparer.Equals(first.Ignore(x=>x.Id).Ignore(y=>y.Name), second); will return true if the given objects are equal on every property except Id and Name (they will not be checked for equality).

Here goes my code. Of course it’s a trivial example (with some obvious overloads of methods missing), but I wanted to extract the simplest code possible. The real case scenario’s a bit more complex, so I don’t really want to change the approach.

The method FindProperty is almost a copy-paste from AutoMapper library.

Object wrapper for fluent API:

public class TestEqualityHelper<T>
{
    public List<PropertyInfo> IgnoredProps = new List<PropertyInfo>();
    public T Value;
}

Fluent stuff:

public static class FluentExtension
{
    //Extension method to speak fluently. It finds the property mentioned
    // in 'ignore' parameter and adds it to the list.
    public static TestEqualityHelper<T> Ignore<T>(this T value,
         Expression<Func<T, object>> ignore)
    {
        var eh = new TestEqualityHelper<T> { Value = value };

        //Mind the magic here!
        var member = FindProperty(ignore);
        eh.IgnoredProps.Add((PropertyInfo)member);
        return eh;
    }

    //Extract the MemberInfo from the given lambda
    private static MemberInfo FindProperty(LambdaExpression lambdaExpression)
    {
        Expression expressionToCheck = lambdaExpression;

        var done = false;

        while (!done)
        {
            switch (expressionToCheck.NodeType)
            {
                case ExpressionType.Convert:
                    expressionToCheck 
                        = ((UnaryExpression)expressionToCheck).Operand;
                    break;
                case ExpressionType.Lambda:
                    expressionToCheck
                        = ((LambdaExpression)expressionToCheck).Body;
                    break;
                case ExpressionType.MemberAccess:
                    var memberExpression 
                        = (MemberExpression)expressionToCheck;

                    if (memberExpression.Expression.NodeType 
                          != ExpressionType.Parameter &&
                        memberExpression.Expression.NodeType 
                          != ExpressionType.Convert)
                    {
                        throw new Exception("Something went wrong");
                    }

                    return memberExpression.Member;
                default:
                    done = true;
                    break;
            }
        }

        throw new Exception("Something went wrong");
    }
}

The actual comparer:

public static class TestEqualityComparer
{
    public static bool MyEquals<T>(TestEqualityHelper<T> a, T b)
    {
        return DoMyEquals(a.Value, b, a.IgnoredProps);
    }

    private static bool DoMyEquals<T>(T a, T b,
        IEnumerable<PropertyInfo> ignoredProperties)
    {
        var t = typeof(T);
        IEnumerable<PropertyInfo> props;

        if (ignoredProperties != null && ignoredProperties.Any())
        {
            //THE PROBLEM IS HERE!
            props =
                t.GetProperties(BindingFlags.Instance | BindingFlags.Public)
                    .Except(ignoredProperties);
        }
        else
        {
            props = 
                t.GetProperties(BindingFlags.Instance | BindingFlags.Public);
        }
        return props.All(f => f.GetValue(a, null).Equals(f.GetValue(b, null)));
    }
}

That’s basically it.

And here are two test snippets, the first one works, the second one fails:

//These are the simple objects we'll compare
public class Base
{
    public decimal Id { get; set; }
    public string Name { get; set; }
}
public class Derived : Base
{    }

[TestMethod]
public void ListUsers()
{
   //TRUE
   var f = new Base { Id = 5, Name = "asdas" };
   var s = new Base { Id = 6, Name = "asdas" };
   Assert.IsTrue(TestEqualityComparer.MyEquals(f.Ignore(x => x.Id), s));

   //FALSE
   var f2 = new Derived { Id = 5, Name = "asdas" };
   var s2 = new Derived { Id = 6, Name = "asdas" };
   Assert.IsTrue(TestEqualityComparer.MyEquals(f2.Ignore(x => x.Id), s2));
}

The problem is with the Except method in DoMyEquals.

Properties returned by FindProperty are not equal to those returned by Type.GetProperties. The difference I spot is in PropertyInfo.ReflectedType.

• regardless to the type of my objects, FindProperty tells me that the reflected type is Base.

• properties returned by Type.GetProperties have their ReflectedType set to Base or Derived, depending on the type of actual objects.

I don’t know how to solve it. I could check the type of the parameter in lambda, but in the next step I want to allow constructs like Ignore(x=>x.Some.Deep.Property), so it probably will not do.

Any suggestion on how to compare PropertyInfo‘s or how to retrieve them from lambdas properly would be appreciated.

Answer 1

The reason FindProperty is telling you the reflected Type is Base is because that’s the class the lambda would use for the invocation.

You probably know this 🙂

Instead of GetProperties() from Type, could you use this

static IEnumerable<PropertyInfo> GetMappedProperties(Type type)
{
  return type
    .GetProperties()
    .Select(p => GetMappedProperty(type, p.Name))
    .Where(p => p != null);
}

static PropertyInfo GetMappedProperty(Type type, string name)
{
  if (type == null)
    return null;

  var prop = type.GetProperty(name);

  if (prop.DeclaringType == type)
    return prop;
  else
    return GetMappedProperty(type.BaseType, name);
}

To explain more about why the lambda is actually using the Base method directly, and you see essentially a different PropertyInfo, might be better explained looking at the IL

Consider this code:

static void Foo()
{
  var b = new Base { Id = 4 };
  var d = new Derived { Id = 5 };

  decimal dm = b.Id;
  dm = d.Id;
}

And here is the IL for b.Id

IL_002f: callvirt instance valuetype [mscorlib]System.Decimal ConsoleApplication1.Base::get_Id()

And the IL for d.Id

IL_0036: callvirt instance valuetype [mscorlib]System.Decimal ConsoleApplication1.Base::get_Id()