How to change the thickness of the material in loop?

Geometry, Fields and Transportation
1

I write a DetectorMessenger.cc,which can change the thickness of ZnS in .mac as

 void DetectorMessenger::SetNewValue(G4UIcommand *command, G4String val) {
        if (command == fSetThicknessCmd)
        {
            fdet->SetThickness(fSetThicknessCmd->GetNewDoubleValue(val));
        }
    }

In DetectorConstruction,function SetThickness is

 void DetectorConstruction::SetThickness(G4double val)
    {
        if (val<=0){fThickness=50*um;}
        else{fThickness = val ;}
        G4RunManager::GetRunManager()->GeometryHasBeenModified();
    }

fThickness is

 G4Box* solid_ZnS =new G4Box("ZnS",  0.5*env_sizeXY,0.5*env_sizeXY,fThickness/2);

Is that setting right?
Can this change the thickness of the material in loop?