c# - Why an unreferenced object is not collected? -

this program prints "true" console.

allocate object, make weakreference of that, go out of block scope, , check weakreference.isalive.

public static void main (string[] args) {     test (); }  static void test () {     weakreference wref = null;      { // block scope         var obj = new object ();         wref = new weakreference (obj);     }      // obj out of scope     // console.writeline (obj);      gc.collect ();      console.writeline (wref.isalive); // => true } 

why obj not collected, though obj out of scope?

the program compiled mono 3.12.0.

edit:

sorry, inappropriate example.

the following program print true. block scope seems not related. tried not debug mode.

    public static void main (string[] args)     {         test ();     }      static void test ()     {         weakreference wref = null;          var obj = new object ();         wref = new weakreference (obj);         obj = null;          gc.collect ();          console.writeline (wref.isalive); // => true     } 

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$ mcs -debug- program.cs $ mono program.exe 

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you expect gc 100% deterministic, when it's not.

a few things might have happened:

• the jit optimized null assignment away.
• a pointer instance can left on in temporary stack location, or in register (you called function, passing variable, put variable in register on architectures).