C# didnt call drived class function in polymorphism when pass parameter

Question

I have 2 drived calss and base class

class Base{
    int a = 1000;
}
class Drivedone : Base{
    public void GetInt(){ return a + 1; }
}

class Drivedtwo : base {
    public void GetInt(){ return a + 2; }
}

void testFunction(Base argBase){
    argBase.GetInt(); // this is Error
}

main(){
    Base one = new Drivedone();
    Base two = new Drivedtwo();
    testFunction(one);//want print 1001
    testFunction(two);//want print 1002
}

that was error. because GetInt didnt defined in Base class.

So i try this way.

class Base{
    int a = 1000;
    public void GetInt(){ return 0; }
}
class Drivedone : Base{
    public new void GetInt(){ return a + 1; }
}

class Drivedtwo : base {
    public new void GetInt(){ return a + 2; }
}

void testFunction(Base argBase){
    argBase.GetInt(); // this is Error
}

main(){
    Base one = new Drivedone();
    Base two = new Drivedtwo();
    testFunction(one);//want print 1001
    testFunction(two);//want print 1002
}

this way i tried this way. define base function and with new keyword function in base class.

but this time. GetInt function always return 0;

how do i that??? please help me