减去c+中的两个DateTime对象+/cli

上面的代码获取编译器错误：

System::DateTime ^now = System::DateTime::Now;
System::DateTime ^now2 = System::DateTime::Now;
System::TimeSpan ^span = now->Subtract(now2);

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now和now2是c++/cli句柄，但该方法需要实际对象。从c++/cli调用方法的正确方法是什么？

DateTime

和

TimeSpan

是值类型

test.cpp(104) : error C2664: 'System::TimeSpan System::DateTime::Subtract(System::DateTime)' : cannot convert parameter 1 from 'System::DateTime ^' to 'System::DateTime'  
1>        No user-defined-conversion operator available, or  
1>        No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called  

---

DateTime

和

TimeSpan

是值类型

test.cpp(104) : error C2664: 'System::TimeSpan System::DateTime::Subtract(System::DateTime)' : cannot convert parameter 1 from 'System::DateTime ^' to 'System::DateTime'  
1>        No user-defined-conversion operator available, or  
1>        No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called  

---

DateTime:：Now是值类型，而Subtract返回值类型。在完全理解引用类型和值类型之间的差异之前，不要编写任何C++/CLI代码。任何关于.NET的书都会对此进行解释。DateTime:：Now是一种值类型，而Subtract返回一种值类型。在完全理解引用类型和值类型之间的区别之前，不要编写任何C++/CLI代码。任何一本关于.NET的半正经的书都会解释它。我想更正确的是：

System:：TimeSpan=now2.Subtract（现在）我猜更正确：
System:：TimeSpan=now2.Subtract（现在）
System::Diagnostics::Stopwatch^ stopwatch = gcnew System::Diagnostics::Stopwatch();
stopwatch->Start();
// code
stopwatch->Stop();
// stopwatch->Elapsed