我的智能合约如何将 WETH 换成 ETH？

Question

My smart contract receives WETH. I want to create a function anyone can run to convert the WETH into ETH.

That's the code I have

function swapWeth() public nonReentrant {
   uint balanceWETH = IWETH(WETH).balanceOf(address(this));

   if (balanceWETH > 0) {
      IERC20(WETH).approve(address(this), balanceWETH);
      IWETH(WETH).withdraw(balanceWETH);
   }
}

When I call the function I always get the following message and my transaction never goes through.

We were not able to estimate gas. There might be an error in the contract and this transaction may fail.

How do I debug this?

I just want to call the withdraw function of the WETH smart contract to swap my contracts's WETH into ETH. The user initiating the transaction just pays for the gas fee.

I just want to convert my contract's WETH into ETH.

Any help would be appreciated.

Thanks

Edit : I changed the code to this and it still fails. I'm on Goerli.Does someone has a working example?

  function withdrawETH() public {
    address WETH9 = 0xB4FBF271143F4FBf7B91A5ded31805e42b2208d6; // Goerli
    uint balanceWETH = IERC20(WETH9).balanceOf(address(this));
    IWETH9(WETH9).withdraw(balanceWETH);
  }

Contract address: https://goerli.etherscan.io/address/0xd9e84819483410b08ec24320eb88acfc4ee6e8c8

Edit : See Petr's response and comments bellow. The problem was with my 'receive' function

Answer 1

IERC20(WETH).approve(address(this), balanceWETH);

This line gives approval

• from your contract address - that's the address that calls the approve() function
• to your contract address - first param, value address(this)

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The approval is given by the address that executes the approve() function.

If you want to get approval from the user, they need to invoke the approve() function directly on the WETH contract in a separate transaction - not through your contract.