问：一个.NET程序在运行时到底启动了多少个线程？
答：至少3个。
- 启动CLR并运行Main方法的主线程
- 调试器帮助线程
- Finalizer线程
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Main thread: {0}",
Thread.CurrentThread.ManagedThreadId);
Console.ReadKey();
}
}
(ProcessExplorer
http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx)
通常，CLR会根据情况启动更多的特殊线程。
- Finalizer线程：该线程负责运行GC进行垃圾对象回收。
- 并发的GC线程：GC会根据情况启动更多的线程并发进行垃圾回收。
- 服务器GC线程：在服务器GC模式下，CLR可能会为多核机器的每个核创建GC托管堆和回收线程。
- 调试器帮助线程：该线程负责为类似WinDbg等调试器提供帮助。
- AppDomain卸载线程：CLR可能会启动一个工作线程来卸载应用程序域。
- ThreadPool线程：ThreadPool会根据情况创建线程。
Short answer, there are other threads with the main thread that are required to support the application.
This can be tested in Visual Studio.
Create a simple Console application. Put a debug point, and start debugging. Once the application gets to that debug point, open "Threads" windows by
Debug -> Windows -> ThreadsYou will see something like:
Some of the above threads are for debugger, but mostly an app would have main thread, GC (Garbage Collection) and Finalizer thread etc.
参考：
http://www.cnblogs.com/gaochundong/archive/2013/04/18/dotnet_app_default_thread_count.html
http://*.com/questions/38854906/default-threading-model-for-a-net-application