I have two contracts Vase and Box

contract Vase {
    uint256 public vase1;
    event VaseValueChanged(uint256 newValue);

    function createVase(uint256 newValue) public {
        vase1 = newValue;
        emit VaseValueChanged(newValue);

    // Reads the last stored value
    function retrieveVase() external view returns (uint256) {
        return vase1;

contract Box {
    uint256 private value;

    // Stores a new value in the contract
    function store(uint256 newValue, uint256 breadth) public {
        value = newValue;


Both are upgradable smartcontracts and I am using the openzeppelin library and using its CLI for upgrading these contracts.

How can I call retrieveVase() method of Vase contract from Box contract. I don't want to inherit Vase in Box contract.

  • I'm not quite sure how this is related to upgradable contracts? Or is this simply a question about "how can I reference another contract in my contract"? Sep 23, 2020 at 7:52

1 Answer 1


You can use one of the 2 following options:

  1. Via interface contract:


pragma solidity ^0.5.11;

contract A {
    uint256 public testVariable = 5;
    function getTestVariable() public view returns(uint256) {
        return testVariable;


pragma solidity ^0.5.11;

contract B {
    A a_contract_instance;
    constructor(address _a_contract_address) public {
        a_contract_instance = A(_a_contract_address);
    function callToContractA() public view returns(uint256) {
        return a_contract_instance.getTestVariable();

interface A {
    function getTestVariable() external view returns(uint256);
  1. Via .call() function:
pragma solidity ^0.5.0;

contract ExistingContract  {
    uint256 test_val;
    address test_addr;
    function getA() public view returns (uint256, address) {
        return (test_val, test_addr);
    function setA(uint256 _val) public {
        test_val = _val;
        test_addr = msg.sender;

contract Caller {
    function externalCallGetA(address _addr) public returns(uint256) {
        (bool success, bytes memory result) = address(_addr).call(abi.encodeWithSignature("getA()"));
        return abi.decode(result, (uint256));
    function externalCallSetA(address _addr, uint256 _var) public{
        (bool success,) = address(_addr).call(abi.encodeWithSignature("setA(uint256)", _var));

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