我找到了解决方案.如果子消息中的任何值未设置，则 HasValue 返回 false.然后我必须创建一个具有所有默认值的新 Imessage.因此，此代码可用于打印所有消息和子消息中的所有字段名称.

I found a solition. HasValue return false, if any values inside submessage are not set. Then I must create a new Imessage which have all default values. So then this code works for printing all field names in all messages and submessages.

private void PrintAllReportableFieldsinMessage(Google.Protobuf.IMessage message)
{
    foreach (var fieldDescriptor in message.Descriptor.Fields.InFieldNumberOrder())
    {
        if (fieldDescriptor.FieldType == Google.Protobuf.Reflection.FieldType.Message)
        {
            if (fieldDescriptor.Accessor.HasValue(message))
            {
                IMessage submessage = fieldDescriptor.Accessor.GetValue(message) as IMessage;
                PrintAllReportableFieldsinMessage(submessage);
            }
            else {
                IMessage cleanSubmessage = fieldDescriptor.MessageType.Parser.ParseFrom(ByteString.Empty);
                PrintAllReportableFieldsinMessage(cleanSubmessage);
            }
        }
        else
        {
            Google.Protobuf.Reflection.FieldOptions options = fieldDescriptor.GetOptions();
            if (options != null && options.GetExtension(HelloworldExtensions.Reportable))
            {
                var fieldValue = fieldDescriptor.Accessor.GetValue(message);
                var fieldName = fieldDescriptor.Name;
                Dispatcher.Invoke(() =>
                {
                    lReadableResult.Content += fieldName + ":" + fieldValue + "|";
                });
            }
        }
    }
}