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How to Resolve the “Warning: SPDX license identifier not provided in source file” Warning in Solidity?

How to Resolve the _Warning_ SPDX license identifier not provided in source file_ Warning in Solidity

Before identifying the solution for Warning: SPDX license identifier not provided in source file warning, let’s explore what is the role of SPDX license identifier in Solidity.

Purpose of SPDX license identifiers in Solidity contracts

  1. The purpose of SPDX (Software Package Data Exchange) license identifiers in Solidity contracts is to provide clear and standardized information about the licensing terms under which the code is distributed.
  2. SPDX helps define the rights and permissions associated with using, modifying, and sharing the code. This practice enhances transparency and facilitates collaboration within the open-source community.

Consequences of not including SPDX license identifier

If you do not include an SPDX license identifier in your Solidity contract, it may lead to ambiguity regarding the licensing terms. And you may see the following warning: Warning: SPDX license identifier not provided in source file.

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Coding example

To address the warning, you can add an SPDX license identifier comment to your contract.

// No SPDX-License-Identifier included
pragma solidity ^0.8.0;

contract ExampleContract {
    uint256 public data;

    constructor(uint256 _data) {
        data = _data;
    }
}

Explanation

  1. First, we added the comment for the SPDX identifier. It is the best practice to add this at the start of the file.
  2. Next, we defined the Solidity version, which is the common practice in any Solidity coding file.
  3. At last, we added the example contract.

To resolve the warning, developers should add an SPDX license identifier comment to their contract. Let’s highlight two scenarios.

Non-open source code

For non-open source code, developers can use the following SPDX identifier:

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.7.5;

contract TestContract {
    // Some logic
}

Open source code

For open-source code, developers should choose an appropriate open-source license. An example with the MIT license is provided

// SPDX-License-Identifier: MIT
pragma solidity ^0.7.5;

contract TestContract {
    // Some logic
}

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Conclusion

Adding the SPDX comment helps in providing clarity about the licensing terms for your code. Although SPDX license identifiers may not affect the functionality or deployment of your contract, adding them is considered good practice, especially when sharing or publishing your code.

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