How to get only one message from MQTT, when messages receiving in C#?

Question

I developed simple IOT device using nodemcu module with 03 sensors. When device is connected, device sent sensor's data to mqtt broker as well. After I getting that data to my .net back end using M2QTT libraries. According to my code, client_MqttMsgPublishReceived method is works as loop. But I need to get that data message, only one time (As I requesting).

I used libraries as:

using uPLibrary.Networking.M2Mqtt;  
using uPLibrary.Networking.M2Mqtt.Messages;

My Code :

private void button2_Click(object sender, EventArgs e)
{
    var client = new MqttClient(IPAddress.Parse("172.30.30.21"));
    client.MqttMsgPublishReceived += client_MqttMsgPublishReceived;
    var clientId = Guid.NewGuid().ToString();
    client.Connect(clientId);

    client.Subscribe(new string[] { "topic" }, new byte[] { MqttMsgBase.QOS_LEVEL_AT_LEAST_ONCE });          
}
static void client_MqttMsgPublishReceived(object sender, MqttMsgPublishEventArgs e)
{
    string result = Encoding.UTF8.GetString(e.Message);
}