CALL vs CALLCODE vs DELEGATECALL in Solidity

Contract_calls.sol

pragma solidity ^0.5.0;

contract C1 {
  uint public num;
  address public sender;

  function callSetNum(address c2, uint _num) public {
    (bool res,) = c2.call(abi.encodePacked(bytes4(keccak256("setNum(uint256)")), _num));
    if(!res) revert(); // C2's num is set 
  }
  
  function c2setNum(address _c2, uint _num) public {
      C2 c2 = C2(_c2);
      c2.setNum(_num);
  }
  
//   Function callcode is now disallowed (in favor of delegatecall). 
//      It is still possible to use it via inline assembly.
//      https://solidity.readthedocs.io/en/v0.5.0/050-breaking-changes.html
//   function callcodeSetNum(address c2, uint _num) public {
//     bool res;
//     In version 5.0 and upward the next line will case:
//        TypeError: "callcode" has been deprecated in favour of "delegatecall"
//     (res,) = c2.callcode(abi.encodePacked(bytes4(keccak256("setNum(uint256)")), _num));
//     if(!res) revert(); // C1's num is set
//   }

  function delegatecallSetNum(address c2, uint _num) public {
    (bool res,) = c2.delegatecall(abi.encodePacked(bytes4(keccak256("setNum(uint256)")), _num));
    if(!res) revert(); // C1's num is set 
  }
}

contract C2 {
  uint public num;
  address public sender;

  function setNum(uint _num) public {
    num = _num;
    sender = msg.sender;
    // msg.sender is C1 if invoked by C1.callcodeSetNum
    // msg.sender is C3 if invoked by C3.f1()

  }
}

contract C3 {
    function f1(C1 c1, C2 c2, uint _num) public {
        c1.delegatecallSetNum((address)(c2), _num);
    }
}

Please very please

can someone explain me why the function transferDai doesnot work in the following code? you ll save my life

pragma solidity ^0.5.9;

// Adding only the ERC-20 function we need
interface DaiToken {
function balanceOf(address guy) external view returns (uint);
}

contract DaiFaucet {

address dai;
DaiToken daitoken;

constructor() public{
    dai = 0x89d24A6b4CcB1B6fAA2625fE562bDD9a23260359;
    daitoken = DaiToken(dai);
}

function balance() public view returns (uint) {
    return daitoken.balanceOf(msg.sender); // I have dai here
}

function balanceBis() public view returns (uint) {
	return daitoken.balanceOf(0x820Af9cea6CF890F36CAe589853190C622d39058); // a custom target address of mine
}

function transferdai() public  {
    uint value = 1; 
    address target = 0x820Af9cea6CF890F36CAe589853190C622d39058;
    (bool res,) = dai.delegatecall(abi.encodePacked(bytes4(keccak256("transfer(address,uint256)")), target, value));
     if(!res) revert();
    
}

}