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__gc and __nogc:

• when native C++ code is recompiled with /clr, the classes don't automatically become managed and are marked __nogc;If your class does meet the requirements of the CLR, however, you can make your class managed by marking it with the __gc modifier to indicate that it is a garbage-collected class, or the __value modifier to indicate that it is a CTS value type.
• For .net 3.5, __gc is old syntax. To use it, should use /clr:oldsyntax switch;
• '__gc' can only be applied to a class, struct, interface, array or pointer;
  
• __gc class is very similar with ref class; __value is similar with value class;
• It is neccesarry to choose /clr swithc in .net 3.5 that ref class and value class are compiled .

#paragma managed(unmanaged):

• Enable function-level control for compiling functions as managed or unmanaged.

• They tell compiler the following code is managed or unmanaged;

gcroot《T *〉 and gcroot《T ^〉:

• To use it, include "vcclr.h";
• gcroot《T ^〉 is used with ref class or value class;
• gcroot《T *〉 is used with __gc or __nogc;
• Create handler/pointer of a mananged class object in unmanaged class, can use them;