1. How would you call a function on a third party contract B, on behalf of the sender, through your contract A, guaranteeing no reverts?

题目意思就是如何通过合约A代表发送者调用合约B的函数，答案很简单咯，就是delegatecall。

contract B{
    uint value;
    function test(){
        value = 10;
    }
}

contract A{
    function _delegate(address _B){
        (bool suc,) = _B.delegatecall(abi.encodeWithSignature("test"));
        require(suc, "delegate call failed");
    }
}

2. What’s the main difference between a transparent proxy and a universal proxy?

题目意思是透明合约和代理合约的主要区别是啥？社区里这篇文章很详细的介绍了这俩代理模式的。可以瞅一瞅。

contract TransparentProxy{
    uint public value;
    address public implementation;
    address public owner;
    constructor(address _v1){
        implementation = _v1;
        owner = msg.sender;
    }
    modifier onlyOwner(){
        require(msg.sender == owner, "only owner");
        _;
    }
    function _upgrade(address _newImplementation) public onlyOwner{
        implementation = _newImplementation;
    }

    function _delegatecall() public {
        (bool suc,) = implementation.delegatecall(msg.data);
        require(suc, "delegatecall failed");
    }
}

contract BoxV1{
    uint public value;
    function set(uint _value) public{
        value = _value;
    }
}

contract UniversalProxy{
    uint public value;
    address public implementation;
    constructor(address _v1){
        implementation = _v1;

    }   
    function _delegatecall() public {
        (bool suc,) = implementation.delegatecall(msg.data);
        require(suc, "delegatecall failed");
    }
}

abstract contract Box{
    uint public value;
    address public implementation;
    address public owner ;
    modifier onlyOwner(){
        require(msg.sender == owner, "only owner");
        _;
    }
    function _upgrade(address _newimplementation) public onlyOwner {
        implementation = _newimplementation;
    }
}
contract BoxV1 is Box{
    function set(uint _value) public {
        value = _value;
    }
}

参考答案：

通用代理优势：通过在实现合约上定义所有函数，它可以依靠Solidity编译器检查任何函数选择器冲突。此外，通用代理的大小要小得多，从而使部署更便宜。在每次调用中，从存储中需要读取的内容更少，降低了开销。 通用代理劣势：如果一次合约忘记写upgrade函数，将会永远无法升级了。

3. How could you destroy the implementation of, and effectively brick, a universal proxy? Assumptions:

• You re not the owner of the proxy
• No one owns the implementation
• The implementation uses delegatecall to guarantee that the next implementation is not sterile

题目大概意思是，你怎么能破坏通用代理的实现合约，并有效地将其变成砖头（废弃）？条件如下：

• 你不是代理合约的owner
• 没人是implementation合约的owner
• implementation合约使用了delegatecall去保证每个implementation合约总是可升级的（可以看上面的通用代理劣势）

参考答案：因为implementation合约里有调用delegatecall，去调用别的合约，那只要在该合约中加一个selfdestruct函数，即可完成对通用代理的毁灭。所以在implementation合约里最好不要使用selfdestruct和delegatecall函数。详细可以查看这篇文档