Nethereum: any pancakeswap contract function call results in "execution_reverted", all transactions fail with same error

Question

I'm completely new to web3, and I do realise that there is small portion of information on the web3 for developers, especially when it comes to debugging and issue solving.

My main goal is just to succeed in calling a swap function "swapExactTokensForTokens" on PancakeSwapV2 Testnet on Binance Testnet (chain 97).

I was getting "execution_reverted" for all my transactions even tho passed in parameters shown in bscscan were correct. I though that perhaps it is an issue with parameters, so I tried other method "getAmountsOut" and still got error:

Then I tried calling "EstimateGasAsync" and still got same error "execution reverted: eth_estimateGas". I have absolutely 0 ideas left what the actual issue could be since it gives absolute 0 information on why it fails and the only way I can try figuring it out is by doing shotgun testing which is stupid...

I feel like there is an issue with some of contract addresses/abi or I'm incorrectly instantiating "Web3". But I know that an issue is basically in first lines of code and not in functions or core logic itself.

Code you can use in playground (only private key needs to be added):

    using Nethereum.Contracts;
    using Nethereum.Hex.HexTypes;
    using Nethereum.RPC.Eth.DTOs;
    using Nethereum.Util;
    using Nethereum.Web3;
    using Nethereum.Web3.Accounts;
    using System.Numerics;
    using System.Threading.Tasks;
    
    class PancakeSwapExample
    {
        // ready-to-run code
        static async Task Main(string[] args)
        {
            var pancakeSwapRouterAddressTestnet = "0xdc4904b5f716Ff30d8495e35dC99c109bb5eCf81";
            var tBNBToken = "0x24942357E3A688ACE7C39710aA6d3C91BA1FAC39";
            var CAKEToken = "0xFa60D973F7642B748046464e165A65B7323b0DEE";
            var binanceTestnetNode = "https://data-seed-prebsc-1-s1.binance.org:8545/";
            var binanceChainId = 97;
            var amount = 0.01M; // tBNB
            var abi = @"[{""inputs"":[{""internalType"":""address"",""name"":""_factory"",""type"":""address""},{""internalType"":""address"",""name"":""_WETH"",""type"":""address""}],""stateMutability"":""nonpayable"",""type"":""constructor""},{""inputs"":[],""name"":""WETH"",""outputs"":[{""internalType"":""address"",""name"":"""",""type"":""address""}],""stateMutability"":""view"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""tokenA"",""type"":""address""},{""internalType"":""address"",""name"":""tokenB"",""type"":""address""},{""internalType"":""uint256"",""name"":""amountADesired"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountBDesired"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountAMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountBMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""addLiquidity"",""outputs"":[{""internalType"":""uint256"",""name"":""amountA"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountB"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""token"",""type"":""address""},{""internalType"":""uint256"",""name"":""amountTokenDesired"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountTokenMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETHMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""addLiquidityETH"",""outputs"":[{""internalType"":""uint256"",""name"":""amountToken"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETH"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""}],""stateMutability"":""payable"",""type"":""function""},{""inputs"":[],""name"":""factory"",""outputs"":[{""internalType"":""address"",""name"":"""",""type"":""address""}],""stateMutability"":""view"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountOut"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""reserveIn"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""reserveOut"",""type"":""uint256""}],""name"":""getAmountIn"",""outputs"":[{""internalType"":""uint256"",""name"":""amountIn"",""type"":""uint256""}],""stateMutability"":""pure"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountIn"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""reserveIn"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""reserveOut"",""type"":""uint256""}],""name"":""getAmountOut"",""outputs"":[{""internalType"":""uint256"",""name"":""amountOut"",""type"":""uint256""}],""stateMutability"":""pure"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountOut"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""}],""name"":""getAmountsIn"",""outputs"":[{""internalType"":""uint256[]"",""name"":""amounts"",""type"":""uint256[]""}],""stateMutability"":""view"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountIn"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""}],""name"":""getAmountsOut"",""outputs"":[{""internalType"":""uint256[]"",""name"":""amounts"",""type"":""uint256[]""}],""stateMutability"":""view"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountA"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""reserveA"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""reserveB"",""type"":""uint256""}],""name"":""quote"",""outputs"":[{""internalType"":""uint256"",""name"":""amountB"",""type"":""uint256""}],""stateMutability"":""pure"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""tokenA"",""type"":""address""},{""internalType"":""address"",""name"":""tokenB"",""type"":""address""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountAMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountBMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""removeLiquidity"",""outputs"":[{""internalType"":""uint256"",""name"":""amountA"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountB"",""type"":""uint256""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""token"",""type"":""address""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountTokenMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETHMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""removeLiquidityETH"",""outputs"":[{""internalType"":""uint256"",""name"":""amountToken"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETH"",""type"":""uint256""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""token"",""type"":""address""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountTokenMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETHMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""removeLiquidityETHSupportingFeeOnTransferTokens"",""outputs"":[{""internalType"":""uint256"",""name"":""amountETH"",""type"":""uint256""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""token"",""type"":""address""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountTokenMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETHMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""},{""internalType"":""bool"",""name"":""approveMax"",""type"":""bool""},{""internalType"":""uint8"",""name"":""v"",""type"":""uint8""},{""internalType"":""bytes32"",""name"":""r"",""type"":""bytes32""},{""internalType"":""bytes32"",""name"":""s"",""type"":""bytes32""}],""name"":""removeLiquidityETHWithPermit"",""outputs"":[{""internalType"":""uint256"",""name"":""amountToken"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETH"",""type"":""uint256""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""token"",""type"":""address""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountTokenMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETHMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""},{""internalType"":""bool"",""name"":""approveMax"",""type"":""bool""},{""internalType"":""uint8"",""name"":""v"",""type"":""uint8""},{""internalType"":""bytes32"",""name"":""r"",""type"":""bytes32""},{""internalType"":""bytes32"",""name"":""s"",""type"":""bytes32""}],""name"":""removeLiquidityETHWithPermitSupportingFeeOnTransferTokens"",""outputs"":[{""internalType"":""uint256"",""name"":""amountETH"",""type"":""uint256""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""tokenA"",""type"":""address""},{""internalType"":""address"",""name"":""tokenB"",""type"":""address""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountAMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountBMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""},{""internalType"":""bool"",""name"":""approveMax"",""type"":""bool""},{""internalType"":""uint8"",""name"":""v"",""type"":""uint8""},{""internalType"":""bytes32"",""name"":""r"",""type"":""bytes32""},{""internalType"":""bytes32"",""name"":""s"",""type"":""bytes32""}],""name"":""removeLiquidityWithPermit"",""outputs"":[{""internalType"":""uint256"",""name"":""amountA"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountB"",""type"":""uint256""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountOut"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""swapETHForExactTokens"",""outputs"":[{""internalType"":""uint256[]"",""name"":""amounts"",""type"":""uint256[]""}],""stateMutability"":""payable"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountOutMin"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""swapExactETHForTokens"",""outputs"":[{""internalType"":""uint256[]"",""name"":""amounts"",""type"":""uint256[]""}],""stateMutability"":""payable"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountOutMin"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""swapExactETHForTokensSupportingFeeOnTransferTokens"",""outputs"":[],""stateMutability"":""payable"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountIn"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountOutMin"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""swapExactTokensForETH"",""outputs"":[{""internalType"":""uint256[]"",""name"":""amounts"",""type"":""uint256[]""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountIn"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountOutMin"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""swapExactTokensForETHSupportingFeeOnTransferTokens"",""outputs"":[],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountIn"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountOutMin"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""swapExactTokensForTokens"",""outputs"":[{""internalType"":""uint256[]"",""name"":""amounts"",""type"":""uint256[]""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountIn"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountOutMin"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""swapExactTokensForTokensSupportingFeeOnTransferTokens"",""outputs"":[],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountOut"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountInMax"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""swapTokensForExactETH"",""outputs"":[{""internalType"":""uint256[]"",""name"":""amounts"",""type"":""uint256[]""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountOut"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountInMax"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""swapTokensForExactTokens"",""outputs"":[{""internalType"":""uint256[]"",""name"":""amounts"",""type"":""uint256[]""}],""stateMutability"":""nonpayable"",""type"":""function""},{""stateMutability"":""payable"",""type"":""receive""}]";
    
            var yourWalletPrivateKey = "YOUR_WALLET_KEY_FOR_BSC_TESTNET"; // Only this needs to be changed
    
            Account account = new Account(yourWalletPrivateKey, binanceChainId);
            Web3 web3 = new Web3(account, binanceTestnetNode);
            Contract smartContract = web3.Eth.GetContract(abi, pancakeSwapRouterAddressTestnet);
    
            Function swapExactTokensForTokensFunction = smartContract.GetFunction("swapExactTokensForTokens");
    
            var amountIn = Web3.Convert.ToWei(amount, UnitConversion.EthUnit.Ether);
            var amountOutMin = Web3.Convert.ToWei(0.05, UnitConversion.EthUnit.Ether);
            var path = new[] { tBNBToken, CAKEToken };
            object[] parametersForSwap = new object[4] { amountIn, amountOutMin, path, account.Address };
    
            BigInteger wei = Web3.Convert.ToWei(amount);
            HexBigInteger value = new HexBigInteger(wei);
    
            // Fails here
            HexBigInteger estimatedGas = await swapExactTokensForTokensFunction.EstimateGasAsync(account.Address, null, value, parametersForSwap);

            // My goal is to succeed in this transaction, so it would not be in 'Fail' state
            TransactionReceipt transactionReceiptForSwap = await swapExactTokensForTokensFunction.SendTransactionAndWaitForReceiptAsync(account.Address, estimatedGas, value, null, parametersForSwap);
        }
    }

ANY help is much appreciated.

Answer 1

Jesus christ, I finally managed to make it work, HUGE thanks to @tqdpham96 for pointing out to root cause... I believe I wouldn't be able to find this somewhere else as there is absolutely NO information, and no one is talking that you should APPROVE coin for swap before doing actual swap.

So, basically, in details - before doing any swap, you make one-time approval for coin to swap from, you have to call approve() function on that coin. For example, if you're willing to swap from WBNB to CAKE then you have to approve WBNB, so that PancakeSwap would be able (had permission) to withdraw your WBNB in order to do swap for CAKE. And vice-verse, if you're willing to do swap from CAKE to WBNB, then you have to do everything the same - approve for CAKE, so that PancakeSwap had permission to withdraw your CAKE and deposit WBNB.

Here is FULLY working code on PancakeSwapV2Router for C#, which does approve() for initial coin (WBNB), and then does swap WBNB to CAKE, just provide private key and you are ready to go:

using Nethereum.Contracts;
using Nethereum.Hex.HexTypes;
using Nethereum.RPC.Eth.DTOs;
using Nethereum.Web3;
using Nethereum.Web3.Accounts;

using System;
using System.Numerics;
using System.Threading.Tasks;

class PancakeSwapExample
{
    // ready-to-run code
    static async Task Main(string[] args)
    {
        var yourWalletPrivateKey = "YOUR_WALLET_PRIVATE_KEY"; // Only this needs to be changed

        var pancakeSwapRouterV2AddressMainnet = "0x10ED43C718714eb63d5aA57B78B54704E256024E";
        var wbnbAddress = "0xbb4CdB9CBd36B01bD1cBaEBF2De08d9173bc095c";
        var cakeAddress = "0x0E09FaBB73Bd3Ade0a17ECC321fD13a19e81cE82";
        var binanceTestnetNode = "https://bsc-dataseed.binance.org/";
        var binanceChainId = 56;
        var amount = 0.0002M; // amount in coins to swap

        var pancakeSwapRouterV2Abi =
            @"[{""inputs"":[{""internalType"":""address"",""name"":""_factory"",""type"":""address""},{""internalType"":""address"",""name"":""_WETH"",""type"":""address""}],""stateMutability"":""nonpayable"",""type"":""constructor""},{""inputs"":[],""name"":""WETH"",""outputs"":[{""internalType"":""address"",""name"":"""",""type"":""address""}],""stateMutability"":""view"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""tokenA"",""type"":""address""},{""internalType"":""address"",""name"":""tokenB"",""type"":""address""},{""internalType"":""uint256"",""name"":""amountADesired"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountBDesired"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountAMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountBMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""addLiquidity"",""outputs"":[{""internalType"":""uint256"",""name"":""amountA"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountB"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""token"",""type"":""address""},{""internalType"":""uint256"",""name"":""amountTokenDesired"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountTokenMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETHMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""addLiquidityETH"",""outputs"":[{""internalType"":""uint256"",""name"":""amountToken"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETH"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""}],""stateMutability"":""payable"",""type"":""function""},{""inputs"":[],""name"":""factory"",""outputs"":[{""internalType"":""address"",""name"":"""",""type"":""address""}],""stateMutability"":""view"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountOut"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""reserveIn"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""reserveOut"",""type"":""uint256""}],""name"":""getAmountIn"",""outputs"":[{""internalType"":""uint256"",""name"":""amountIn"",""type"":""uint256""}],""stateMutability"":""pure"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountIn"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""reserveIn"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""reserveOut"",""type"":""uint256""}],""name"":""getAmountOut"",""outputs"":[{""internalType"":""uint256"",""name"":""amountOut"",""type"":""uint256""}],""stateMutability"":""pure"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountOut"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""}],""name"":""getAmountsIn"",""outputs"":[{""internalType"":""uint256[]"",""name"":""amounts"",""type"":""uint256[]""}],""stateMutability"":""view"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountIn"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""}],""name"":""getAmountsOut"",""outputs"":[{""internalType"":""uint256[]"",""name"":""amounts"",""type"":""uint256[]""}],""stateMutability"":""view"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountA"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""reserveA"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""reserveB"",""type"":""uint256""}],""name"":""quote"",""outputs"":[{""internalType"":""uint256"",""name"":""amountB"",""type"":""uint256""}],""stateMutability"":""pure"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""tokenA"",""type"":""address""},{""internalType"":""address"",""name"":""tokenB"",""type"":""address""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountAMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountBMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""removeLiquidity"",""outputs"":[{""internalType"":""uint256"",""name"":""amountA"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountB"",""type"":""uint256""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""token"",""type"":""address""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountTokenMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETHMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""removeLiquidityETH"",""outputs"":[{""internalType"":""uint256"",""name"":""amountToken"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETH"",""type"":""uint256""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""token"",""type"":""address""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountTokenMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETHMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""removeLiquidityETHSupportingFeeOnTransferTokens"",""outputs"":[{""internalType"":""uint256"",""name"":""amountETH"",""type"":""uint256""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""token"",""type"":""address""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountTokenMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETHMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""},{""internalType"":""bool"",""name"":""approveMax"",""type"":""bool""},{""internalType"":""uint8"",""name"":""v"",""type"":""uint8""},{""internalType"":""bytes32"",""name"":""r"",""type"":""bytes32""},{""internalType"":""bytes32"",""name"":""s"",""type"":""bytes32""}],""name"":""removeLiquidityETHWithPermit"",""outputs"":[{""internalType"":""uint256"",""name"":""amountToken"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETH"",""type"":""uint256""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""token"",""type"":""address""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountTokenMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountETHMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""},{""internalType"":""bool"",""name"":""approveMax"",""type"":""bool""},{""internalType"":""uint8"",""name"":""v"",""type"":""uint8""},{""internalType"":""bytes32"",""name"":""r"",""type"":""bytes32""},{""internalType"":""bytes32"",""name"":""s"",""type"":""bytes32""}],""name"":""removeLiquidityETHWithPermitSupportingFeeOnTransferTokens"",""outputs"":[{""internalType"":""uint256"",""name"":""amountETH"",""type"":""uint256""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""address"",""name"":""tokenA"",""type"":""address""},{""internalType"":""address"",""name"":""tokenB"",""type"":""address""},{""internalType"":""uint256"",""name"":""liquidity"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountAMin"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountBMin"",""type"":""uint256""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""},{""internalType"":""bool"",""name"":""approveMax"",""type"":""bool""},{""internalType"":""uint8"",""name"":""v"",""type"":""uint8""},{""internalType"":""bytes32"",""name"":""r"",""type"":""bytes32""},{""internalType"":""bytes32"",""name"":""s"",""type"":""bytes32""}],""name"":""removeLiquidityWithPermit"",""outputs"":[{""internalType"":""uint256"",""name"":""amountA"",""type"":""uint256""},{""internalType"":""uint256"",""name"":""amountB"",""type"":""uint256""}],""stateMutability"":""nonpayable"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountOut"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""},{""internalType"":""address"",""name"":""to"",""type"":""address""},{""internalType"":""uint256"",""name"":""deadline"",""type"":""uint256""}],""name"":""swapETHForExactTokens"",""outputs"":[{""internalType"":""uint256[]"",""name"":""amounts"",""type"":""uint256[]""}],""stateMutability"":""payable"",""type"":""function""},{""inputs"":[{""internalType"":""uint256"",""name"":""amountOutMin"",""type"":""uint256""},{""internalType"":""address[]"",""name"":""path"",""type"":""address[]""},{""internalType"":""address"",""name"":""to"",""type"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        var wbnbAbi =
            @"[{""constant"":true,""inputs"":[],""name"":""name"",""outputs"":[{""name"":"""",""type"":""string""}],""payable"":false,""stateMutability"":""view"",""type"":""function""},{""constant"":false,""inputs"":[{""name"":""guy"",""type"":""address""},{""name"":""wad"",""type"":""uint256""}],""name"":""approve"",""outputs"":[{""name"":"""",""type"":""bool""}],""payable"":false,""stateMutability"":""nonpayable"",""type"":""function""},{""constant"":true,""inputs"":[],""name"":""totalSupply"",""outputs"":[{""name"":"""",""type"":""uint256""}],""payable"":false,""stateMutability"":""view"",""type"":""function""},{""constant"":false,""inputs"":[{""name"":""src"",""type"":""address""},{""name"":""dst"",""type"":""address""},{""name"":""wad"",""type"":""uint256""}],""name"":""transferFrom"",""outputs"":[{""name"":"""",""type"":""bool""}],""payable"":false,""stateMutability"":""nonpayable"",""type"":""function""},{""constant"":false,""inputs"":[{""name"":""wad"",""type"":""uint256""}],""name"":""withdraw"",""outputs"":[],""payable"":false,""stateMutability"":""nonpayable"",""type"":""function""},{""constant"":true,""inputs"":[],""name"":""decimals"",""outputs"":[{""name"":"""",""type"":""uint8""}],""payable"":false,""stateMutability"":""view"",""type"":""function""},{""constant"":true,""inputs"":[{""name"":"""",""type"":""address""}],""name"":""balanceOf"",""outputs"":[{""name"":"""",""type"":""uint256""}],""payable"":false,""stateMutability"":""view"",""type"":""function""},{""constant"":true,""inputs"":[],""name"":""symbol"",""outputs"":[{""name"":"""",""type"":""string""}],""payable"":false,""stateMutability"":""view"",""type"":""function""},{""constant"":false,""inputs"":[{""name"":""dst"",""type"":""address""},{""name"":""wad"",""type"":""uint256""}],""name"":""transfer"",""outputs"":[{""name"":"""",""type"":""bool""}],""payable"":false,""stateMutability"":""nonpayable"",""type"":""function""},{""constant"":false,""inputs"":[],""name"":""deposit"",""outputs"":[],""payable"":true,""stateMutability"":""payable"",""type"":""function""},{""constant"":true,""inputs"":[{""name"":"""",""type"":""address""},{""name"":"""",""type"":""address""}],""name"":""allowance"",""outputs"":[{""name"":"""",""type"":""uint256""}],""payable"":false,""stateMutability"":""view"",""type"":""function""},{""payable"":true,""stateMutability"":""payable"",""type"":""fallback""},{""anonymous"":false,""inputs"":[{""indexed"":true,""name"":""src"",""type"":""address""},{""indexed"":true,""name"":""guy"",""type"":""address""},{""indexed"":false,""name"":""wad"",""type"":""uint256""}],""name"":""Approval"",""type"":""event""},{""anonymous"":false,""inputs"":[{""indexed"":true,""name"":""src"",""type"":""address""},{""indexed"":true,""name"":""dst"",""type"":""address""},{""indexed"":false,""name"":""wad"",""type"":""uint256""}],""name"":""Transfer"",""type"":""event""},{""anonymous"":false,""inputs"":[{""indexed"":true,""name"":""dst"",""type"":""address""},{""indexed"":false,""name"":""wad"",""type"":""uint256""}],""name"":""Deposit"",""type"":""event""},{""anonymous"":false,""inputs"":[{""indexed"":true,""name"":""src"",""type"":""address""},{""indexed"":false,""name"":""wad"",""type"":""uint256""}],""name"":""Withdrawal"",""type"":""event""}]";

        Account account = new Account(yourWalletPrivateKey, binanceChainId);
        Web3 web3 = new Web3(account, binanceTestnetNode);
        var yourWalletAddress = account.Address;

        // Step 1: We need to approve PancakeSwap router for wbnb

        // Setup function input
        Contract wbnb = web3.Eth.GetContract(wbnbAbi, wbnbAddress);
        BigInteger valueToApprove = Web3.Convert.ToWei(1000000); // Spender limit (set any)
        object[] routerApproveFunctionInput = new object[2] { pancakeSwapRouterV2AddressMainnet, valueToApprove };

        // Setup transaction 
        Function wbnbApprove = wbnb.GetFunction("approve");
        var wbnbApproveTransaction = wbnbApprove.CreateTransactionInput(yourWalletAddress, routerApproveFunctionInput);
        wbnbApproveTransaction.Gas = await wbnbApprove.EstimateGasAsync(yourWalletAddress, null, null, routerApproveFunctionInput);
        wbnbApproveTransaction.Nonce = await web3.Eth.Transactions.GetTransactionCount.SendRequestAsync(yourWalletAddress);
        wbnbApproveTransaction.GasPrice = await web3.Eth.GasPrice.SendRequestAsync();

        // Send transaction and wait for approve
        TransactionReceipt wbnbApproveResult = await wbnbApprove.SendTransactionAndWaitForReceiptAsync(wbnbApproveTransaction, functionInput: routerApproveFunctionInput);
        if (wbnbApproveResult.Status != new HexBigInteger(1))
            throw new Exception("Approve failed");

        // Step 2: Now we can swap

        // Setup function input
        var amountIn = Web3.Convert.ToWei(amount);
        var amountOutMin = Web3.Convert.ToWei(0);
        var wbnbToCake = new string[] { wbnbAddress, cakeAddress };
        var to = account.Address;
        var deadline = (uint)DateTimeOffset.UtcNow.AddSeconds(40).ToUnixTimeSeconds();
        object[] swapFunctionInput = new object[5] { amountIn, amountOutMin, wbnbToCake, to, deadline };

        // Create transaction swapExactTokensForTokens
        Contract pancakeRouter = web3.Eth.GetContract(pancakeSwapRouterV2Abi, pancakeSwapRouterV2AddressMainnet);
        Function swapFunction = pancakeRouter.GetFunction("swapExactTokensForTokens");
        var routerTransaction = swapFunction.CreateTransactionInput(yourWalletAddress, swapFunctionInput);
        routerTransaction.Gas = await swapFunction.EstimateGasAsync(yourWalletAddress, null, null, swapFunctionInput);
        routerTransaction.Nonce = await web3.Eth.Transactions.GetTransactionCount.SendRequestAsync(yourWalletAddress);
        routerTransaction.GasPrice = await web3.Eth.GasPrice.SendRequestAsync();

        // Do swap
        TransactionReceipt transactionReceiptForSwap = await swapFunction.SendTransactionAndWaitForReceiptAsync(routerTransaction, functionInput: swapFunctionInput);
        if (transactionReceiptForSwap.Status != new HexBigInteger(1))
            throw new Exception("Failed swap");
    }
}

Please note, that code runs against Mainnet, not Testnet. I wish there was more information for C# developers on web3 for things like that... Hope this gonna save time to someone.

Answer 2

Your code is correct so far. I got the same error when starting to work with web3. The error comes from the allowance for pancakeswap to use your token. That means, you need to "Approve" for "pancakeswap router" to use your token to make a swap.

Thus, you can fix the code by the following 2 approaches.

#1. Use Approve function directly in blockchain (i.e., bscscan). Let say you want to swap USDT to USDC. Basically, you just need to go to USDT contract function (https://bscscan.com/token/0x55d398326f99059ff775485246999027b3197955#writeContract). Then, connect your wallet and approve the spender (the pancakeswap router address) with a really big amount of allowance. Then, your code will be working well.

#2. Approve directly in the code. Connect to the USDT smart contract then call approve function. Here is a piece of Python code to approve (you can translate it to your programming language)

def approve(sender_address, privateKey):
    txn = tokenContract.functions.approve(utils.pancakeAddress, 1000000000000000000000000000).buildTransaction({
        'from': sender_address,
        'gasPrice': web3.toWei('7', 'gwei'),
        'nonce': web3.eth.get_transaction_count(sender_address),
    })
    executeTxn(txn, privateKey)

acc = web3.toChecksumAddress('')
priv = ''
# just need to approve 1 time
approve(acc, priv)

Note that, you just need to approve only one time with a big allowance. Then, next time you do not need to approve again. Hope it helps.