Smart Contract Interaction

Smart Contract Interaction

Nethereum uses typed C# classes (DTOs) to represent every part of a smart contract — deployment bytecode, function calls, events, and return values. This gives you compile-time safety: if a parameter name or type is wrong, you catch it before sending a transaction, not after.

NuGet: Nethereum.Web3

dotnet add package Nethereum.Web3

Contract DTO Definitions

Every interaction starts with defining C# classes that map to the Solidity contract. These can be hand-written or auto-generated with Nethereum.Generator.Console.

Deployment Message

A deployment message represents the contract constructor. It inherits from ContractDeploymentMessage and holds the compiled bytecode plus any constructor parameters:

public class StandardTokenDeployment : ContractDeploymentMessage
{
    public static string BYTECODE = "0x60606040...";
    public StandardTokenDeployment() : base(BYTECODE) { }

    [Parameter("uint256", "totalSupply")]
    public BigInteger TotalSupply { get; set; }
}

Function Messages

Each contract function maps to a class inheriting from FunctionMessage. The [Function] attribute takes the Solidity function name and return type:

[Function("balanceOf", "uint256")]
public class BalanceOfFunction : FunctionMessage
{
    [Parameter("address", "_owner", 1)]
    public string Owner { get; set; }
}

[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; }
}

Event DTOs

Events implement IEventDTO. The fourth argument in [Parameter] marks indexed parameters, which enable server-side filtering:

[Event("Transfer")]
public class TransferEventDTO : IEventDTO
{
    [Parameter("address", "_from", 1, true)]
    public string From { get; set; }

    [Parameter("address", "_to", 2, true)]
    public string To { get; set; }

    [Parameter("uint256", "_value", 3, false)]
    public BigInteger Value { get; set; }
}

Function Output DTOs

For functions returning complex values, define an output DTO implementing IFunctionOutputDTO:

[FunctionOutput]
public class BalanceOfOutputDTO : IFunctionOutputDTO
{
    [Parameter("uint256", "balance", 1)]
    public BigInteger Balance { get; set; }
}

Deploy

The deployment handler encodes constructor parameters into the bytecode, estimates gas, and waits for the receipt. The returned ContractAddress is where your contract lives:

var deploymentMessage = new StandardTokenDeployment { TotalSupply = 100000 };
var deploymentHandler = web3.Eth.GetContractDeploymentHandler<StandardTokenDeployment>();
var receipt = await deploymentHandler.SendRequestAndWaitForReceiptAsync(deploymentMessage);
var contractAddress = receipt.ContractAddress;

Gas, gas price, and nonce are estimated automatically. Set them on the message only when you need explicit control.

Query (Read-Only)

Use GetContractQueryHandler<T> to call view or pure functions. These are free — no gas, no transaction:

var balanceHandler = web3.Eth.GetContractQueryHandler<BalanceOfFunction>();

// Single return value
var balance = await balanceHandler.QueryAsync<BigInteger>(contractAddress,
    new BalanceOfFunction { Owner = ownerAddress });

// Deserialize to output DTO (for multiple return values)
var output = await balanceHandler.QueryDeserializingToObjectAsync<BalanceOfOutputDTO>(
    balanceOfMessage, contractAddress);

Transact (State-Changing)

Use GetContractTransactionHandler<T> for functions that modify state. This sends a real transaction and costs gas:

var transferHandler = web3.Eth.GetContractTransactionHandler<TransferFunction>();
var transfer = new TransferFunction { To = receiverAddress, TokenAmount = 100 };
var receipt = await transferHandler.SendRequestAndWaitForReceiptAsync(contractAddress, transfer);

Decode Events from Receipt

After a transaction, extract events from the receipt logs. This decodes all matching events — if the transaction triggered multiple transfers, you get them all:

var transferEvents = receipt.DecodeAllEvents<TransferEventDTO>();
var from = transferEvents[0].Event.From;
var to = transferEvents[0].Event.To;
var value = transferEvents[0].Event.Value;

Historical State Query

All query methods accept BlockParameter to read contract state at a past block. The node must have archive state for the requested block:

var historicalBalance = await balanceHandler.QueryDeserializingToObjectAsync<BalanceOfOutputDTO>(
    balanceOfMessage, contractAddress,
    new BlockParameter(deployReceipt.BlockNumber));

Gas Estimation

Nethereum auto-estimates gas. For manual control — useful when you want to display costs or set a hard limit:

var estimate = await transferHandler.EstimateGasAsync(contractAddress, transfer);
transfer.Gas = estimate.Value;
transfer.GasPrice = Web3.Convert.ToWei(25, UnitConversion.EthUnit.Gwei);

Payable Functions

If a Solidity function is payable, set AmountToSend to include ETH with the call:

transfer.AmountToSend = Web3.Convert.ToWei(1); // 1 ETH

Nonce Control

Nethereum manages nonces automatically, including an in-memory counter for rapid submissions. Only set manually for specific ordering needs:

transfer.Nonce = 42;

Offline Signing

Sign without broadcasting — useful for cold wallets or deferred submission. You must set Nonce, Gas, and GasPrice since there's no node to query:

transfer.Nonce = 2;
transfer.Gas = 60000;
transfer.GasPrice = Web3.Convert.ToWei(25, UnitConversion.EthUnit.Gwei);
var signedTx = await transferHandler.SignTransactionAsync(contractAddress, transfer);

Non-Type-Safe (ABI JSON)

When you have a raw ABI string and don't need typed classes. This approach is less safe — parameter mismatches fail at runtime, not compile time:

var contract = web3.Eth.GetContract(abi, contractAddress);
var balanceFunction = contract.GetFunction("balanceOf");
var transferFunction = contract.GetFunction("transfer");

var balance = await balanceFunction.CallAsync<BigInteger>(ownerAddress);
var gas = await transferFunction.EstimateGasAsync(senderAddress, null, null, toAddress, amount);
var txReceipt = await transferFunction.SendTransactionAndWaitForReceiptAsync(
    senderAddress, gas, null, null, toAddress, amount);

When to Use Which Handler

For standard tokens (ERC-20, ERC-721, ERC-1155), use the built-in services (web3.Eth.ERC20, web3.Eth.ERC721) instead of defining custom DTOs.

For full documentation, see: https://docs.nethereum.com/docs/smart-contracts/guide-smart-contract-interaction