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    <p>You need to understand what <code>System.Timers.Timer</code> is doing behind the scenes. It is a wrapper around the <code>System.Threading.Timer</code> class. When the timer starts it creates a new <code>System.Threading.Timer</code> instance and passes it a callback method in the <code>System.Timers.Timer</code> instance. The system references this callback delegate as long as the timer remains enabled.</p> <p>We also know that delegates keep a reference to an instance of the class containing the target method. This is why your <code>System.Timers.Timer</code> instance is not collected and does not stop automatically. Removing the <code>Elapsed</code> event handler will not solve this problem because <em>that</em> delegate only holds a reference to the <code>TrayForm</code> instance. Again, it is the <code>System.Threading.Timer</code> that is keeping a reference to the <code>System.Timers.Timer</code>. The whole reference chain remains rooted because <em>the system</em> has to reference the <code>System.Threading.Timer</code> for it to work.</p> <p>Here is the reference chain:</p> <pre><code>System =&gt; _TimerCallback =&gt; Callback =&gt; System.Threading.Timer =&gt; Callback =&gt; System.Timers.Timer </code></pre> <p>When you track this through with Reflector or ILSpy you can see that the "System" in the reference chain above comes into play via the <code>TimerBase.AddTimerNative</code> method which is marked <code>MethodImplOptions.InternalCall</code> so we cannot see exactly how the reference is rooted below this point. But, it is rooted nonetheless.</p> <p>If you disable the timer via <code>Enabled = false</code> or <code>Stop</code> then it will dispose the underlying <code>System.Threading.Timer</code> which should in turn stop referencing the <code>System.Timers.Timer</code>.</p>
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    1. POWhy will local captured timer not go out of scope?
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    1. USBrian Gideon
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    1. CO-1? If someone sees an incorrect statement in my answer let me know. I have double and triple checked my answer by examining the BCL code via ILSpy, but it is possible that I missed something.
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    2. CONot sure who downvoted, but you're spot on. I took a quick peek at the source for `AddTimerNative` and it indeed creates a gc root for the `_TimerCallBack` delegate.
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      1. USdlev
    3. CO@dlev: Thanks for digging into that. I was wondering how `AddTimerNative` was going to "root" that callback. Very informative.
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