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Decode Function Calls from Transaction Input

When you retrieve a transaction from the blockchain (as in Query Blocks), its Input field contains the raw ABI-encoded function call. This guide shows how to check if a transaction calls a specific function and decode its parameters into typed C# objects.

dotnet add package Nethereum.Web3
dotnet add package Nethereum.Contracts

Define the Function Message​

The first 4 bytes of a transaction's input data identify which function is being called. This selector is the Keccak-256 hash of the function signature (e.g., transfer(address,uint256) produces a9059cbb). The remaining bytes are the ABI-encoded parameters.

To decode that data, define a C# class that mirrors the Solidity function signature. The [Function] and [Parameter] attributes tell Nethereum how to map the ABI-encoded bytes back to typed properties:

using Nethereum.ABI.FunctionEncoding.Attributes;
using Nethereum.Contracts;
using System.Numerics;

[Function("transfer", "bool")]
public class TransferFunction : FunctionMessage
{
    [Parameter("address", "_to", 1)]
    public string To { get; set; }

    [Parameter("uint256", "_value", 2)]
    public BigInteger TokenAmount { get; set; }
}

Check and Decode the Transaction​

IsTransactionForFunctionMessage<T>() compares the first 4 bytes of the transaction input against the function selector derived from TransferFunction. If the selector matches, DecodeTransaction ABI-decodes the remaining bytes into the typed message. If the selector does not match, the transaction is for a different function and is skipped.

using Nethereum.Web3;
using Nethereum.Contracts.Extensions;

var web3 = new Web3("https://mainnet.infura.io/v3/YOUR-PROJECT-ID");

var txn = await web3.Eth.Transactions.GetTransactionByHash
    .SendRequestAsync(
        "0x0404a0517a7443db1787b5461b9d5fc18546809419c0cc6a736599b60677ed71");

if (txn.IsTransactionForFunctionMessage<TransferFunction>())
{
    var transfer = new TransferFunction().DecodeTransaction(txn);
    Console.WriteLine("To: " + transfer.To);
    Console.WriteLine("Amount: " + Web3.Convert.FromWei(transfer.TokenAmount));
}

Web3.Convert.FromWei converts a raw BigInteger value in wei to its ether-unit decimal representation. See Unit Conversion for details.

Next Steps​