I need to stop executing constructor

[terminalXXX]

using namespace System;
ref class A
{
public:
    A(){Console::WriteLine(L"A constructor");}
    virtual void func(){foo();}
    void foo()
    {
        Console::WriteLine(L"A from foo");
    }
};
ref class A_Mock:public A
{
public:
    virtual void func() override 
    {
        Console::WriteLine(L"A_MOCK");
    }
};

int main(array<System::String ^> ^args)
{
    Console::WriteLine(L"Hello World");
    A^b=gcnew A_Mock();
    b->func();
    return 0;
} 

Supposing that class A is the development code; and so for unit-testing purpose I create a mock for it. But I don't know how to override A's constructor such that it will not get executed in my unit-test code.
Thank you.