禁用和启用计时器会导致第二个实例？

Question

I have a Start and Stop button on a form that start and stop a repeating SQL query which sends data to a pubnub channel. When I fire up the form and click start, I see what I expect on the subscribed clients. However, if I click stop then start again, I now get duplicate data. A third time gives me triplicate data, etc. What is causing this? Here are the start and stop methods:

private void btnQuery1Start_Click(object sender, EventArgs e)
{

    lblQuery1Status.Text = "Status: Running";
    btnQuery1Start.Enabled = false;
    txtQuery1Interval.Enabled = false;
    btnQuery1Stop.Enabled = true;
    query1Timer.Elapsed += new ElapsedEventHandler(doQuery1);
    query1Timer.Interval = Convert.ToInt32(txtQuery1Interval.Text) * 1000;
    query1Timer.Enabled = true;

}
private void btnQuery1Stop_Click(object sender, EventArgs e)
{
    btnQuery1Start.Enabled = true;
    btnQuery1Stop.Enabled = false;
    txtQuery1Interval.Enabled = true;
    query1Timer.Enabled = false;
    lblQuery1Status.Text = "Status: Stopped";
}

I can post doQuery1 if necessary, but it's using an OdbcConnection and data reader to get a single integer result then it's serializing it with Newtonsoft.Json and sending it using Pubnub.publish() . I'm hoping though that this is something obvious I'm just missing in the btnQuery1Start_Click() method above.

Answer 1

No, you have a single timer - but you're adding an event handler to it every time you click start:

query1Timer.Elapsed += new ElapsedEventHandler(doQuery1);

Just move that line into wherever you construct the timer, so it only gets added once, and it'll be fine.

(I'd personally rewrite it as query1Timer.Elapsed += doQuery1; , but that's your call...)

Answer 2

每次按Start时，都会将事件处理程序添加到计时器中，但是在停止时，请勿将其删除。

Answer 3

Every time you click start you are executing the line:

query1Timer.Elapsed += new ElapsedEventHandler(doQuery1);

This adds a new event handler each time it is called causing your doQuery1 method to be executed multiple times