Write a solidity program for multilevel inheritance.

In multilevel inheritance a contract inherits from a contract that itself inherits from another (A → B → C); in Solidity this is written with the is keyword.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

// Level 1: base contract
contract A {
    uint internal a;

    function setA(uint _a) public {
        a = _a;
    }

    function getA() public view returns (uint) {
        return a;
    }
}

// Level 2: B inherits A
contract B is A {
    uint internal b;

    function setB(uint _b) public {
        b = _b;
    }

    function getB() public view returns (uint) {
        return b;
    }
}

// Level 3: C inherits B (and therefore A)
contract C is B {
    function sum() public view returns (uint) {
        return a + b;   // uses variables from A and B
    }
}

How it works

• B is A gives B all public and internal members of A.
• C is B gives C the members of both B and A, so C can read a and b directly (they are internal, which allows access in derived contracts but not from outside).
• Only contract C needs to be deployed; its bytecode contains the code of all three levels.

Testing in Remix: deploy C, call setA(10) and setB(20), then sum() returns 30; getA() and getB() return 10 and 20.

To change inherited behaviour, mark the base function virtual and the derived one override.