🧾 OnChainInvoice – Simple Invoice Management System in Solidity

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📌 Description

OnChainInvoice is a minimal and gas-efficient Solidity smart contract that enables users to:

• Create invoices with amount, client, and description
• Accept payments securely via ETH
• Cancel invoices (by issuer or admin)
• Withdraw collected funds safely

Built with Foundry, the project showcases clean separation of logic, CEI pattern, and robust access control — with full 100% test coverage (lines, statements, functions, branches) and fuzzing included.

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📁 Structure

├── src/
│   └── OnChainInvoice.sol         # Main contract
│   └── RejectETH.sol              # Mock contract to force transfer failure
├── test/
│   └── OnChainInvoiceTest.t.sol   # Full test suite using Foundry

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🧱 Contract Overview

Enum: Status

enum Status { Pending, Paid, Cancelled }

Struct: Invoice

struct Invoice {
    address issuer;
    address client;
    uint256 amount;
    string description;
    Status status;
}

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🔐 Modifiers

• onlyClient(invoiceId) – Only the client can pay
• onlyIssuerOrAdmin(invoiceId) – Only issuer or admin can cancel

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🚀 Functions

✅ Create Invoice

function createInvoice(address client, uint256 amount, string calldata description)

Creates a new invoice assigned to a client.

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💸 Pay Invoice

function payInvoice(uint256 invoiceId) external payable

Allows the invoice's client to pay the exact amount in ETH.

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❌ Cancel Invoice

function cancelInvoice(uint256 invoiceId) external

Only issuer or admin can cancel invoices in Pending state.

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💰 Withdraw Funds

function withdrawEther(uint256 amount) external

Allows issuers to withdraw collected ETH using the CEI pattern. Handles edge cases like transfer failure using call.

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⚠️ Rejection Handlers

receive() external payable {
    revert("Use payInvoice");
}

fallback() external payable {
    revert("Invalid function");
}

Prevents accidental ETH transfers or undefined calls.

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🧪 Testing

• Built with Foundry
• Covers all paths, including:
  • Valid/invalid payments
  • Cancellation logic (issuer/admin)
  • Transfer failure simulation using RejectETH contract
  • Fallback and receive handling
  • Fuzz testing of core logic (e.g. invoice creation) using random inputs
• ✅ 100% line, function, and branch coverage

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📄 License

Licensed under the GNU Lesser General Public License v3.0 – see the LICENSE file.

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🙋‍♂️ Author & Contributions

Open to contributions, PRs, and improvements. This project serves as a showcase of best practices in minimal on-chain billing systems.