GetKnownVersion

private static long? GetKnownVersion(ICommand cmd)
{
    const BindingFlags allInstance = BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance;
    var knownVersionAttributeType = typeof (KnownVersionAttribute);

    var commandType = cmd.GetType();
    var knownVersionProperties =
        commandType.GetProperties(allInstance).Where(p => p.IsDefined(knownVersionAttributeType, false));

    if (knownVersionProperties.IsEmpty())
    {
        return null;
    }
    else if (knownVersionProperties.Count() > 1)
    {
        throw new CommandMappingException(string.Format("Multiple {0} attributes found on command {1}.",
                                                        knownVersionAttributeType.FullName, commandType.FullName));
    }

    var knownVersionProperty = knownVersionProperties.Single();
    if (!typeof (long).IsAssignableFrom(knownVersionProperty.PropertyType))
    {
        throw new CommandMappingException(
            string.Format("Known version property {0} on command {1} is not of a valid type." +
                            "The type must be assignable to {2} and it is not.", knownVersionProperty.Name,
                            commandType.FullName, typeof (long).FullName));
    }

    long knownVersion = (long)knownVersionProperty.GetValue(cmd, null);
    return knownVersion;
}