Creating a Contract object for an already deployed contract is rather straightforward. 
Contract contract = new Contract(contractAddress, abi);
While it is not strictly required, it is highly recommended to provide the contract ABI as a string when creating a contract object. Failure to do so will make it so that you cannot fully take advantage of our ABI encoding and decoding. If you do chose to go down this route, you will need to provide the entire function signature (function name + parameter types in brackets - e.g. transfer(address,uint256) for the ERC20 transfer method) when calling a function or querying the contract and you will only ever receive a string as a response to queries.

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Calling Smart Contract Functions

To call a smart contract, you’ll use the CallFunction method to create a CallContractFunction object which will determine the appropriate gasPrice, gasLimit, nonce, and data to include in a newly assembled EthTransaction when provided with a client and a ContractCall object to the Create async Task An example of calling a smart contract would look like: 
Contract erc20Contract = new Contract(contractAddress, contractAbi); // We'll use the well-known ERC20 contract as our example case
TransactionReceipt receipt = await erc20Contract.CallFunction("transfer", toAddress, amountAsBigInteger).SendTransactionMethodAndWaitForReceipt(wallet, client);
Note: if you don’t want to wait for the receipt, you can use SendTransactionMethod instead. Alternatively, if you want to simply create the EthTransaction and send it at a later time, you can use the CallContractFunction object from CallFunction directly. 
Contract erc20Contract = new Contract(contractAddress, contractAbi); // We'll use the well-known ERC20 contract as our example case
EthTransaction transaction = await erc20Contract.CallFunction("transfer", toAddress, amountAsBigInteger).Create(client, new ContractCall(wallet.GetAddress()));
TransactionReceipt receipt = await wallet.SendTransactionAndWaitForReceipt(client, transaction);

// or 
CallContractFunction transactionCreator = erc20Contract.CallFunction("transfer", toAddress, amountAsBigInteger);
EthTransaction transaction = await transactionCreator.Create(client, new ContractCall(wallet.GetAddress()));
TransactionReceipt receipt = await wallet.SendTransactionAndWaitForReceipt(client, transaction);

// or 
CallContractFunction transactionCreator = erc20Contract.CallFunction("transfer", toAddress, amountAsBigInteger);
TransactionReceipt receipt = await transactionCreator.SendTransactionMethodAndWaitForReceipt(wallet, client);
You’ll notice that the CallFunction method accepts an arbitrary number of arguments. You’ll want to provide the arguments in the order they are provided in the ABI/function signature.

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Understanding Data Type Mappings

When interacting with smart contracts, it is important to understand how EVM datatypes are mapped to C# datatypes in the SequenceEthereum library. If you were to, for example, provide a string where the ABI expects an Integer, you will receive an exception, even if that string could be converted into an integer.

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Address

In C# you can either use a string type or create an instance of Address. Ensure that your string is a hexadecimal string starting with 0x and a fixed length of 42 characters. 
Address address = new Address("0x123...");

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Integers

For integer types such as int256, uint8, or uint256 you use the BigInteger type from System.Numerics. 
// Simple number
BigInteger number = new BigInteger(10000);

// From hex
string hexString = "0x0000000...01";
BigInteger number = hexString.HexStringToBigInteger();

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Bytes

To define byte data types from Solidity in C#, you have the options to create a FixedByte for types such as byte16 or byte32. When your contract requires bytes you can convert any value into a byte[] of any length. If your data is represented as a hex string in C#, make sure to use our HexStringToByteArray() function to convert the hex value back to the original byte array. For byte arrays such as byte32[], you simply create a FixedByte[] in C#. 
// byte16 or byte32
new FixedByte(16, new byte[] {});

// bytes
string someString = "abc0123456789";
byte[] bytes = someString.ToByteArray();

// signature
string signature = "0x0ab123...";
byte[] bytes = signature.HexStringToByteArray();

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Structs

You use Tuples to call an on-chain function with a struct. Here is an example Solidity struct and how to define it using Sequence’s Unity SDK and pass it as an arg in a Contract.CallFunction function. Solidity Struct 
struct ExampleStruct {
    address wallet;
    uint256 amount;
    byte32 data;
}
C# Equivalent 
Address wallet = new Address("0x...");
BigInteger amount = new BigInteger(10000);
FixedByte data = new FixedByte(32, byteArrayData);
var arg = new Tuple<Address, BigInteger, FixedByte>(wallet, amount, data);

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Other Types

For traditional data types in Solidity like string or bool, you can use the same data types in C#.

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Querying Contracts

To query a smart contract (read data from it), you’ll use the SendQuery<T> method to query the contract and return the result as type T (if possible). An example of querying a smart contract would look like: 
Contract erc20Contract = new Contract(contractAddress, contractAbi); // We'll use the well-known ERC20 contract as our example case
BigIntegar balance = await erc20Contract.SendQuery<BigIntegar>(client, "balanceOf", address);
Alternatively, if you wish to simply construct the query and send it at a later time, you can use QueryContract<T> to create a delegate. 
Contract erc20Contract = new Contract(contractAddress, contractAbi); // We'll use the well-known ERC20 contract as our example case
QueryContractMessageSender<BigIntegar> balanceQuery = erc20Contract.QueryContract<BigIntegar>("balanceOf", address);
BigIntegar balance = await balanceQuery(client);
// or
BigIntegar balance = await balanceQuery.SendQuery(client);

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Deploying Contracts

If you want to deploy a contract, you can use the ContractDeployer 
ContractDeploymentResult deploymentResult = await ContractDeployer.Deploy(client, wallet, contractBytecodeAsString);
string newlyDeployedContractAddress = deploymentResult.Receipt.contractAddress;