上面没有提到的一个重要区别是System.Timers.Timer会无声地吞咽异常，而System.Threading.Timer不会。

例如:

var timer = new System.Timers.Timer { AutoReset = false };
timer.Elapsed += (sender, args) =>
{
    var z = 0;
    var i = 1 / z;
};
timer.Start();

vs

var timer = new System.Threading.Timer(x =>
{
    var z = 0;
    var i = 1 / z;
}, null, 0, Timeout.Infinite);

这两个类在功能上是等效的，除了System.Timers.Timer有一个选项可以通过设置SynchronizingObject来调用ISynchronizeInvoke来调用所有的计时器过期回调。否则，两个计时器都调用线程池线程的过期回调。

当你将System.Timers.Timer拖到Windows窗体设计面上时，Visual Studio将SynchronizingObject设置为窗体对象，这将导致在UI线程上调用所有过期回调。

上面没有提到的一个重要区别是System.Timers.Timer会无声地吞咽异常，而System.Threading.Timer不会。

例如:

var timer = new System.Timers.Timer { AutoReset = false };
timer.Elapsed += (sender, args) =>
{
    var z = 0;
    var i = 1 / z;
};
timer.Start();

vs

var timer = new System.Threading.Timer(x =>
{
    var z = 0;
    var i = 1 / z;
}, null, 0, Timeout.Infinite);

System.Threading.Timer是一个简单、轻量级的定时器，使用回调方法，由线程池线程提供服务。不建议与Windows窗体一起使用，因为它的回调不会发生在用户界面线程上。System.Windows.Forms.Timer是Windows窗体的更好选择。对于基于服务器的定时器功能，可以考虑使用System.Timers。定时器，它引发事件并具有其他特性。

源

这篇文章给出了一个相当全面的解释:

“比较.NET框架类库中的计时器类”-也可以作为.chm文件获得

具体的区别似乎是System.Timers.Timer是面向多线程应用程序的，因此通过其SynchronizationObject属性是线程安全的，而System.Threading.Timer却不是开箱即用的线程安全的。

我不相信这两者之间有什么区别，因为它涉及到你的间隔可以有多小。

我从MSDN上找到了一个简短的对比

The .NET Framework Class Library includes four classes named Timer, each of which offers different functionality: System.Timers.Timer, which fires an event and executes the code in one or more event sinks at regular intervals. The class is intended for use as a server-based or service component in a multithreaded environment; it has no user interface and is not visible at runtime. System.Threading.Timer, which executes a single callback method on a thread pool thread at regular intervals. The callback method is defined when the timer is instantiated and cannot be changed. Like the System.Timers.Timer class, this class is intended for use as a server-based or service component in a multithreaded environment; it has no user interface and is not visible at runtime. System.Windows.Forms.Timer, a Windows Forms component that fires an event and executes the code in one or more event sinks at regular intervals. The component has no user interface and is designed for use in a single-threaded environment. System.Web.UI.Timer, an ASP.NET component that performs asynchronous or synchronous web page postbacks at a regular interval.