Add some notes regarding timer resolution on Windows (#8044)

* Add some notes regarding timer resolution on Windows

Closes #7706

Author: kouvel

snippets/csharp/System.Timers/Timer/Interval/resolution1.cs

@@ -2322,6 +2322,9 @@ End Sub
  After the specified time delay, the task is completed in the <xref:System.Threading.Tasks.TaskStatus.RanToCompletion> state.  
   
  This method depends on the system clock. This means that the time delay will approximately equal the resolution of the system clock if the `millisecondsDelay` argument is less than the resolution of the system clock, which is approximately 15 milliseconds on Windows systems.  
+
+> [!NOTE]
+>  The system clock that is used is the same clock used by [GetTickCount](/windows/win32/api/sysinfoapi/nf-sysinfoapi-gettickcount), which is not affected by changes made with [timeBeginPeriod](/windows/win32/api/timeapi/nf-timeapi-timebeginperiod) and [timeEndPeriod](/windows/win32/api/timeapi/nf-timeapi-timeendperiod).
   
    
   
@@ -2387,6 +2390,9 @@ End Sub
  For usage scenarios and additional examples, see the documentation for the <xref:System.Threading.Tasks.Task.Delay%28System.Int32%29> overload.  
   
  This method depends on the system clock. This means that the time delay will approximately equal the resolution of the system clock if the `delay` argument is less than the resolution of the system clock, which is approximately 15 milliseconds on Windows systems.  
+
+> [!NOTE]
+>  The system clock that is used is the same clock used by [GetTickCount](/windows/win32/api/sysinfoapi/nf-sysinfoapi-gettickcount), which is not affected by changes made with [timeBeginPeriod](/windows/win32/api/timeapi/nf-timeapi-timebeginperiod) and [timeEndPeriod](/windows/win32/api/timeapi/nf-timeapi-timeendperiod).
   
    
   
@@ -2459,6 +2465,9 @@ End Sub
  For usage scenarios and additional examples, see the documentation for the <xref:System.Threading.Tasks.Task.Delay%28System.Int32%29> overload.  
   
  This method depends on the system clock. This means that the time delay will approximately equal the resolution of the system clock if the `millisecondsDelay` argument is less than the resolution of the system clock, which is approximately 15 milliseconds on Windows systems.  
+
+> [!NOTE]
+>  The system clock that is used is the same clock used by [GetTickCount](/windows/win32/api/sysinfoapi/nf-sysinfoapi-gettickcount), which is not affected by changes made with [timeBeginPeriod](/windows/win32/api/timeapi/nf-timeapi-timebeginperiod) and [timeEndPeriod](/windows/win32/api/timeapi/nf-timeapi-timeendperiod).
   
    
   
@@ -2528,6 +2537,9 @@ End Sub
  For usage scenarios and additional examples, see the documentation for the <xref:System.Threading.Tasks.Task.Delay%28System.Int32%29> overload.  
   
  This method depends on the system clock. This means that the time delay will approximately equal the resolution of the system clock if the `delay` argument is less than the resolution of the system clock, which is approximately 15 milliseconds on Windows systems.  
+
+> [!NOTE]
+>  The system clock that is used is the same clock used by [GetTickCount](/windows/win32/api/sysinfoapi/nf-sysinfoapi-gettickcount), which is not affected by changes made with [timeBeginPeriod](/windows/win32/api/timeapi/nf-timeapi-timebeginperiod) and [timeEndPeriod](/windows/win32/api/timeapi/nf-timeapi-timeendperiod).
   
    
   

snippets/visualbasic/VS_Snippets_CLR_System/system.timers.timer.interval/vb/resolution1.vb

@@ -102,6 +102,9 @@ Sub TimerCallback(state As Object)
   
 > [!NOTE]
 >  As long as you are using a <xref:System.Threading.Timer>, you must keep a reference to it. As with any managed object, a <xref:System.Threading.Timer> is subject to garbage collection when there are no references to it. The fact that a <xref:System.Threading.Timer> is still active does not prevent it from being collected.  
+
+> [!NOTE]
+>  The system clock that is used is the same clock used by [GetTickCount](/windows/win32/api/sysinfoapi/nf-sysinfoapi-gettickcount), which is not affected by changes made with [timeBeginPeriod](/windows/win32/api/timeapi/nf-timeapi-timebeginperiod) and [timeEndPeriod](/windows/win32/api/timeapi/nf-timeapi-timeendperiod).
   
  When a timer is no longer needed, use the <xref:System.Threading.Timer.Dispose%2A> method to free the resources held by the timer. Note that callbacks can occur after the <xref:System.Threading.Timer.Dispose> method overload has been called, because the timer queues callbacks for execution by thread pool threads. You can use the <xref:System.Threading.Timer.Dispose%28System.Threading.WaitHandle%29> method overload to wait until all callbacks have completed.  
   
@@ -271,6 +274,9 @@ Sub TimerCallback(state As Object)
  If `dueTime` is zero (0), `callback` is invoked immediately. If `dueTime` is <xref:System.Threading.Timeout.Infinite?displayProperty=nameWithType>, `callback` is not invoked; the timer is disabled, but can be re-enabled by calling the <xref:System.Threading.Timer.Change%2A> method.  
   
  Because the <xref:System.Threading.Timer> class has the same resolution as the system clock, which is approximately 15 milliseconds on Windows 7 and Windows 8 systems, the `callback` delegate executes at intervals defined by the resolution of the system clock if `period` is less than the resolution of the system clock. If `period` is zero (0) or <xref:System.Threading.Timeout.Infinite?displayProperty=nameWithType> and `dueTime` is not <xref:System.Threading.Timeout.Infinite?displayProperty=nameWithType>, `callback` is invoked once; the periodic behavior of the timer is disabled, but can be re-enabled using the <xref:System.Threading.Timer.Change%2A> method.  
+
+> [!NOTE]
+>  The system clock that is used is the same clock used by [GetTickCount](/windows/win32/api/sysinfoapi/nf-sysinfoapi-gettickcount), which is not affected by changes made with [timeBeginPeriod](/windows/win32/api/timeapi/nf-timeapi-timebeginperiod) and [timeEndPeriod](/windows/win32/api/timeapi/nf-timeapi-timeendperiod).
   
  The method specified for `callback` should be reentrant, because it is called on <xref:System.Threading.ThreadPool> threads. The method can be executed simultaneously on two thread pool threads if the timer interval is less than the time required to execute the method, or if all thread pool threads are in use and the method is queued multiple times.  
   
@@ -350,6 +356,9 @@ Sub TimerCallback(state As Object)
  If `dueTime` is zero (0), `callback` is invoked immediately. If `dueTime` is <xref:System.Threading.Timeout.Infinite?displayProperty=nameWithType>, `callback` is not invoked; the timer is disabled, but can be re-enabled by calling the <xref:System.Threading.Timer.Change%2A> method.  
   
  Because the <xref:System.Threading.Timer> class has the same resolution as the system clock, which is approximately 15 milliseconds on Windows 7 and Windows 8 systems, the `callback` delegate executes at intervals defined by the resolution of the system clock if `period` is less than the resolution of the system clock. If `period` is zero (0) or <xref:System.Threading.Timeout.Infinite?displayProperty=nameWithType> and `dueTime` is not <xref:System.Threading.Timeout.Infinite?displayProperty=nameWithType>, `callback` is invoked once; the periodic behavior of the timer is disabled, but can be re-enabled using the <xref:System.Threading.Timer.Change%2A> method.  
+
+> [!NOTE]
+>  The system clock that is used is the same clock used by [GetTickCount](/windows/win32/api/sysinfoapi/nf-sysinfoapi-gettickcount), which is not affected by changes made with [timeBeginPeriod](/windows/win32/api/timeapi/nf-timeapi-timebeginperiod) and [timeEndPeriod](/windows/win32/api/timeapi/nf-timeapi-timeendperiod).
   
  The method specified for `callback` should be reentrant, because it is called on <xref:System.Threading.ThreadPool> threads. The method can be executed simultaneously on two thread pool threads if the timer interval is less than the time required to execute the method, or if all thread pool threads are in use and the method is queued multiple times.  
   
@@ -421,6 +430,9 @@ Sub TimerCallback(state As Object)
  If `dueTime` is zero (0), `callback` is invoked immediately. If `dueTime` is negative one (-1) milliseconds, `callback` is not invoked; the timer is disabled, but can be re-enabled by calling the <xref:System.Threading.Timer.Change%2A> method.  
   
  Because the <xref:System.Threading.Timer> class has the same resolution as the system clock, which is approximately 15 milliseconds on Windows 7 and Windows 8 systems, the `callback` delegate executes at intervals defined by the resolution of the system clock if `period` is less than the resolution of the system clock. If `period` is zero (0) or negative one (-1) milliseconds and `dueTime` is positive, `callback` is invoked once; the periodic behavior of the timer is disabled, but can be re-enabled using the <xref:System.Threading.Timer.Change%2A> method.  
+
+> [!NOTE]
+>  The system clock that is used is the same clock used by [GetTickCount](/windows/win32/api/sysinfoapi/nf-sysinfoapi-gettickcount), which is not affected by changes made with [timeBeginPeriod](/windows/win32/api/timeapi/nf-timeapi-timebeginperiod) and [timeEndPeriod](/windows/win32/api/timeapi/nf-timeapi-timeendperiod).
   
  The method specified for `callback` should be reentrant, because it is called on <xref:System.Threading.ThreadPool> threads. The method can be executed simultaneously on two thread pool threads if the timer interval is less than the time required to execute the method, or if all thread pool threads are in use and the method is queued multiple times.  
   
@@ -504,6 +516,9 @@ Sub TimerCallback(state As Object)
  If `dueTime` is zero (0), `callback` is invoked immediately. If `dueTime` is <xref:System.Threading.Timeout.Infinite?displayProperty=nameWithType>, `callback` is not invoked; the timer is disabled, but can be re-enabled by calling the <xref:System.Threading.Timer.Change%2A> method.  
   
  Because the <xref:System.Threading.Timer> class has the same resolution as the system clock, which is approximately 15 milliseconds on Windows 7 and Windows 8 systems, the `callback` delegate executes at intervals defined by the resolution of the system clock if `period` is less than the resolution of the system clock. If `period` is zero (0) or <xref:System.Threading.Timeout.Infinite?displayProperty=nameWithType> and `dueTime` is not <xref:System.Threading.Timeout.Infinite?displayProperty=nameWithType>, `callback` is invoked once; the periodic behavior of the timer is disabled, but can be re-enabled using the <xref:System.Threading.Timer.Change%2A> method.  
+
+> [!NOTE]
+>  The system clock that is used is the same clock used by [GetTickCount](/windows/win32/api/sysinfoapi/nf-sysinfoapi-gettickcount), which is not affected by changes made with [timeBeginPeriod](/windows/win32/api/timeapi/nf-timeapi-timebeginperiod) and [timeEndPeriod](/windows/win32/api/timeapi/nf-timeapi-timeendperiod).
   
  The method specified for `callback` should be reentrant, because it is called on <xref:System.Threading.ThreadPool> threads. The method can be executed simultaneously on two thread pool threads if the timer interval is less than the time required to execute the method, or if all thread pool threads are in use and the method is queued multiple times.  
   

xml/System.Threading.Tasks/Task.xml

@@ -69,6 +69,9 @@
   
 > [!IMPORTANT]
 >  The System.Timers.Timer class has the same resolution as the system clock. This means that the <xref:System.Timers.Timer.Elapsed> event will fire at an interval defined by the resolution of the system clock if the <xref:System.Timers.Timer.Interval%2A> property is less than the resolution of the system clock. For more information, see the <xref:System.Timers.Timer.Interval%2A> property.  
+
+> [!NOTE]
+>  The system clock that is used is the same clock used by [GetTickCount](/windows/win32/api/sysinfoapi/nf-sysinfoapi-gettickcount), which is not affected by changes made with [timeBeginPeriod](/windows/win32/api/timeapi/nf-timeapi-timebeginperiod) and [timeEndPeriod](/windows/win32/api/timeapi/nf-timeapi-timeendperiod).
   
  When <xref:System.Timers.Timer.AutoReset%2A> is set to `false`, a <xref:System.Timers.Timer?displayProperty=nameWithType> object raises the <xref:System.Timers.Timer.Elapsed> event only once, after the first <xref:System.Timers.Timer.Interval%2A> has elapsed. To keep raising the <xref:System.Timers.Timer.Elapsed> event regularly at the interval defined by the <xref:System.Timers.Timer.Interval%2A>, set <xref:System.Timers.Timer.AutoReset%2A> to `true`, which is the default value.  
   
@@ -753,16 +756,14 @@
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- You use the <xref:System.Timers.Timer.Interval%2A> property to determine the frequency at which the <xref:System.Timers.Timer.Elapsed> event is fired. Because the <xref:System.Timers.Timer> class depends on the system clock, it has the same resolution as the system clock. This means that the <xref:System.Timers.Timer.Elapsed> event will fire at an interval defined by the resolution of the system clock if the <xref:System.Timers.Timer.Interval%2A> property is less than the resolution of the system clock. The following example sets the <xref:System.Timers.Timer.Interval%2A> property to 5 milliseconds. When run on a Windows 7 system whose system clock has a resolution of approximately 15 milliseconds, the event fires approximately every 15 milliseconds rather than every 5 milliseconds.  
+ You use the <xref:System.Timers.Timer.Interval%2A> property to determine the frequency at which the <xref:System.Timers.Timer.Elapsed> event is fired. Because the <xref:System.Timers.Timer> class depends on the system clock, it has the same resolution as the system clock. This means that the <xref:System.Timers.Timer.Elapsed> event will fire at an interval defined by the resolution of the system clock if the <xref:System.Timers.Timer.Interval%2A> property is less than the resolution of the system clock. The following example sets the <xref:System.Timers.Timer.Interval%2A> property to 5 milliseconds. When run on a Windows system whose system clock has a resolution of approximately 15 milliseconds, the event fires approximately every 15 milliseconds rather than every 5 milliseconds.  
+
+> [!NOTE]
+>  The system clock that is used is the same clock used by [GetTickCount](/windows/win32/api/sysinfoapi/nf-sysinfoapi-gettickcount), which is not affected by changes made with [timeBeginPeriod](/windows/win32/api/timeapi/nf-timeapi-timebeginperiod) and [timeEndPeriod](/windows/win32/api/timeapi/nf-timeapi-timeendperiod).
   
  :::code language="csharp" source="~/snippets/csharp/System.Timers/Timer/Interval/interval2.cs" id="Snippet1":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.timers.timer.interval/vb/interval2.vb" id="Snippet1":::  
   
- You can use the following code to determine the resolution of the system clock on the current system:  
-  
- :::code language="csharp" source="~/snippets/csharp/System.Timers/Timer/Interval/resolution1.cs" id="Snippet2":::
- :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.timers.timer.interval/vb/resolution1.vb" id="Snippet2":::  
-  
  If your app requires greater resolution than that offered by the <xref:System.Timers.Timer> class or the system clock, use the high-resolution multimedia timers; see [How to: Use the High-Resolution Timer](https://msdn.microsoft.com/library/aa964692.aspx).  
   
  If the interval is set after the <xref:System.Timers.Timer> has started, the count is reset. For example, if you set the interval to 5 seconds and then set the <xref:System.Timers.Timer.Enabled%2A> property to `true`, the count starts at the time <xref:System.Timers.Timer.Enabled%2A> is set. If you reset the interval to 10 seconds when count is 3 seconds, the <xref:System.Timers.Timer.Elapsed> event is raised for the first time 13 seconds after <xref:System.Timers.Timer.Enabled%2A> was set to `true`.