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Writing Test Cases

In this section, we will go over the different ways we can write test cases for our stateful precompile.

Adding Config Tests​

Precompile generation tool generates skeletons for unit tests as well. Generated config tests will be under ./precompile/contracts/helloworld/config_test.go for Subnet-EVM and ./helloworld/config_test.go for Precompile-EVM. There are mainly two functions we need to test: Verify and Equal. Verify checks if the precompile is configured correctly. Equal checks if the precompile is equal to another precompile. Generated Verify tests contain a valid case. You can add more invalid cases depending on your implementation. Equal tests generate some invalid cases to test different timestamps, types, and AllowList cases. You can check each config_test.go files for other precompiles under the Subnet-EVM's ./precompile/contracts directory for more examples.

Adding Contract Tests​

The tool also generates contract tests to make sure our precompile is working correctly. Generated tests include cases to test allow list capabilities, gas costs, and calling functions in read-only mode. You can check other contract_test.go files in the /precompile/contracts. Hello World contract tests will be under ./precompile/contracts/helloworld/contract_test.go for Subnet-EVM and ./helloworld/contract_test.go for Precompile-EVM. We will also add more test to cover functionalities of sayHello() and setGreeting(). Contract tests are defined in a standard structure that each test can customize to their needs. The test structure is as follows:

// PrecompileTest is a test case for a precompile
type PrecompileTest struct {
// Caller is the address of the precompile caller
Caller common.Address
// Input the raw input bytes to the precompile
Input []byte
// InputFn is a function that returns the raw input bytes to the precompile
// If specified, Input will be ignored.
InputFn func(t *testing.T) []byte
// SuppliedGas is the amount of gas supplied to the precompile
SuppliedGas uint64
// ReadOnly is whether the precompile should be called in read only
// mode. If true, the precompile should not modify the state.
ReadOnly bool
// Config is the config to use for the precompile
// It should be the same precompile config that is used in the
// precompile's configurator.
// If nil, Configure will not be called.
Config precompileconfig.Config
// BeforeHook is called before the precompile is called.
BeforeHook func(t *testing.T, state contract.StateDB)
// AfterHook is called after the precompile is called.
AfterHook func(t *testing.T, state contract.StateDB)
// ExpectedRes is the expected raw byte result returned by the precompile
ExpectedRes []byte
// ExpectedErr is the expected error returned by the precompile
ExpectedErr string
// BlockNumber is the block number to use for the precompile's block context
BlockNumber int64

Each test can populate the fields of the PrecompileTest struct to customize the test. This test uses an AllowList helper function allowlist.RunPrecompileWithAllowListTests(t, Module, state.NewTestStateDB, tests) which can run all specified tests plus AllowList test suites. If you don't plan to use AllowList, you can directly run them as follows:

	for name, test := range tests {
t.Run(name, func(t *testing.T) {
test.Run(t, module, newStateDB(t))

Adding VM Tests (Optional)​

This is only applicable for direct Subnet-EVM forks as test files are not directly exported in Golang. If you use Precompile-EVM you can skip this step.

VM tests are tests that run the precompile by calling it through the Subnet-EVM. These are the most comprehensive tests that we can run. If your precompile modifies how the Subnet-EVM works, for example changing blockchain rules, you should add a VM test. For example, you can take a look at the TestRewardManagerPrecompileSetRewardAddress function in here. For this Hello World example, we don't modify any Subnet-EVM rules, so we don't need to add any VM tests.

Adding Solidity Test Contracts​

Let's add our test contract to ./contracts/contracts. This smart contract lets us interact with our precompile! We cast the HelloWorld precompile address to the IHelloWorldinterface. In doing so, helloWorld is now a contract of type IHelloWorld and when we call any functions on that contract, we will be redirected to the HelloWorld precompile address. The below code snippet can be copied and pasted into a new file called ExampleHelloWorld.sol:

//SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
import "./IHelloWorld.sol";

// ExampleHelloWorld shows how the HelloWorld precompile can be used in a smart contract.
contract ExampleHelloWorld {
address constant HELLO_WORLD_ADDRESS =
IHelloWorld helloWorld = IHelloWorld(HELLO_WORLD_ADDRESS);

function sayHello() public view returns (string memory) {
return helloWorld.sayHello();

function setGreeting(string calldata greeting) public {

Hello World Precompile is a different contract than ExampleHelloWorld and has a different address. Since the precompile uses AllowList for a permissioned access, any call to the precompile including from ExampleHelloWorld will be denied unless the caller is added to the AllowList.

Please note that this contract is simply a wrapper and is calling the precompile functions. The reason why we add another example smart contract is to have a simpler stateless tests.

For the test contract we write our test in ./contracts/test/ExampleHelloWorldTest.sol.

//SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
import "../ExampleHelloWorld.sol";
import "../interfaces/IHelloWorld.sol";
import "./AllowListTest.sol";

contract ExampleHelloWorldTest is AllowListTest {
IHelloWorld helloWorld = IHelloWorld(HELLO_WORLD_ADDRESS);

function step_getDefaultHelloWorld() public {
ExampleHelloWorld example = new ExampleHelloWorld();
address exampleAddress = address(example);
assertRole(helloWorld.readAllowList(exampleAddress), AllowList.Role.None);
assertEq(example.sayHello(), "Hello World!");

function step_doesNotSetGreetingBeforeEnabled() public {
ExampleHelloWorld example = new ExampleHelloWorld();
address exampleAddress = address(example);
assertRole(helloWorld.readAllowList(exampleAddress), AllowList.Role.None);
try example.setGreeting("testing") {
assertTrue(false, "setGreeting should fail");
} catch {}

function step_setAndGetGreeting() public {
ExampleHelloWorld example = new ExampleHelloWorld();
address exampleAddress = address(example);
assertRole(helloWorld.readAllowList(exampleAddress), AllowList.Role.None);
string memory greeting = "testgreeting";
assertEq(example.sayHello(), greeting);

Adding DS-Test Case​

We can now trigger this test contract via hardhat tests. The test script uses Subnet-EVM's test framework test in ./contracts/test. You can find more information about the test framework here. We also can test the events emitted by the precompile. The test script looks like this:

The test script looks like this:

// (c) 2019-2022, Ava Labs, Inc. All rights reserved.
// See the file LICENSE for licensing terms.

import { expect } from "chai";
import { SignerWithAddress } from "@nomiclabs/hardhat-ethers/signers";
import { Contract } from "ethers";
import { ethers } from "hardhat";
import { test } from "./utils";

// make sure this is always an admin for hello world precompile
const ADMIN_ADDRESS = "0x8db97C7cEcE249c2b98bDC0226Cc4C2A57BF52FC";
const HELLO_WORLD_ADDRESS = "0x0300000000000000000000000000000000000000";

describe("ExampleHelloWorldTest", function () {

beforeEach("Setup DS-Test contract", async function () {
const signer = await ethers.getSigner(ADMIN_ADDRESS);
const helloWorldPromise = ethers.getContractAt(

return ethers
.getContractFactory("ExampleHelloWorldTest", { signer })
.then((factory) => factory.deploy())
.then((contract) => {
this.testContract = contract;
return contract.deployed().then(() => contract);
.then(() => Promise.all([helloWorldPromise]))
.then(([helloWorld]) => helloWorld.setAdmin(this.testContract.address))
.then((tx) => tx.wait());

test("should gets default hello world", ["step_getDefaultHelloWorld"]);

"should not set greeting before enabled",

"should set and get greeting with enabled account",

describe("IHelloWorld events", function () {
let owner: SignerWithAddress;
let contract: Contract;
let defaultGreeting = "Hello, World!";
before(async function () {
owner = await ethers.getSigner(ADMIN_ADDRESS);
contract = await ethers.getContractAt(

// reset greeting
let tx = await contract.setGreeting(defaultGreeting);
await tx.wait();

it("should emit GreetingChanged event", async function () {
let newGreeting = "helloprecompile";
await expect(contract.setGreeting(newGreeting))
.to.emit(contract, "GreetingChanged")
// old greeting
// new greeting

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