Example contracts showing delegate call

Delegation.sol

pragma solidity ^0.5.1;

contract Application {
  uint public n;
  address public sender;

  function setN(uint _n) public {
    n = _n;
    sender = msg.sender;
  }
}

contract Proxy {
  uint public n;
  address public sender;

  function callSetN(address _e, uint _n) public {
    _e.call(abi.encodeWithSignature("setN(uint256)", _n)); 
  }

  function delegatecallSetN(address _e, uint _n) public {
    _e.delegatecall(abi.encodeWithSignature("setN(uint256)", _n));
  }
}


contract TestContract1 {
    function foo(Proxy _d, Application _e, uint _n)  public{
        _d.delegatecallSetN(address(_e), _n);
    }
}

contract TestContract2 {
    function foo(Proxy _d, Application _e, uint _n)  public{
        _d.callSetN(address(_e), _n);
    }
}


contract Token {
    string public name;
    string public symbol;
    uint8 public decimals = 18;
    uint256 public totalSupply;

    function balanceOf(address _owner) public returns (uint256 balance);
    function transfer(address _to, uint256 _value) public returns (bool success);
    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success);
    function approve(address _spender, uint256 _value) public returns (bool success);
    function allowance(address _owner, address _spender) public view returns (uint256 remaining);

    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);
}

contract TokenProxy {
    string public name;
    string public symbol;
    uint8 public decimals = 18;
    uint256 public totalSupply;
    
    function balanceOf(address _token, address _owner) public returns (uint256 balance){
        // application logic using delegatecall
        _token.delegatecall(abi.encodeWithSignature("balanceOf(address)", _owner));
    }
}