Extend

public static T Extend<T>(this T target, T source) where T : class
{
    var properties =
        target.GetType().GetProperties().Where(pi => pi.CanRead && pi.CanWrite);

    foreach (var propertyInfo in properties)
    {
        var targetValue = propertyInfo.GetValue(target, null);
        var defaultValue = propertyInfo.PropertyType.GetDefault();

        if (targetValue != null && !targetValue.Equals(defaultValue)) continue;

        var sourceValue = propertyInfo.GetValue(source, null);
        propertyInfo.SetValue(target, sourceValue, null);
    }

    return target;
}

public static object GetDefault(this Type type)
{
    // If no Type was supplied, if the Type was a reference type, or if the Type was a System.Void, return null
    if (type == null || !type.IsValueType || type == typeof(void))
        return null;

    // If the supplied Type has generic parameters, its default value cannot be determined
    if (type.ContainsGenericParameters)
    {
        throw new ArgumentException(
            "{" + MethodBase.GetCurrentMethod() + "} Error:\n\nThe supplied value type <" + type +
            "> contains generic parameters, so the default value cannot be retrieved");
    }

    // If the Type is a primitive type, or if it is another publicly-visible value type (i.e. struct), return a 
    //  default instance of the value type
    if (type.IsPrimitive || !type.IsNotPublic)
    {
        try
        {
            return Activator.CreateInstance(type);
        }
        catch (Exception e)
        {
            throw new ArgumentException(
                "{" + MethodBase.GetCurrentMethod() +
                "} Error:\n\nThe Activator.CreateInstance method could not " +
                "create a default instance of the supplied value type <" + type +
                "> (Inner Exception message: \"" + e.Message + "\")", e);
        }
    }

    // Fail with exception
    throw new ArgumentException("{" + MethodBase.GetCurrentMethod() + "} Error:\n\nThe supplied value type <" +
                                type +
                                "> is not a publicly-visible type, so the default value cannot be retrieved");
}

foo = foo.extend(foo2);