Update the queue message with the settings for the MessageUpdateFields.Visibility flag, regardless of the value of the message content. eg
message.SetMessageContent(""); queue.UpdateMessage(message, visibilityTimeout, MessageUpdateFields.Visibility);
this will not change the content of the message to an empty line, and the content will remain untouched and will only update the visibility timeout.
To refresh the content, as well as the timeout for visibility,
queue.UpdateMessage(message, visibilityTimeout, MessageUpdateFields.Content | MessageUpdateFields.Visibility);
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