SPDX license and pragma version in Blockchain / Solidity

Introduction

SPDX license and pragma version help make your smart contract clear and safe. SPDX shows the license type, and pragma tells the compiler which Solidity version to use.

When you create a new Solidity smart contract to share or publish.

When you want to make sure your contract works with a specific Solidity compiler version.

When you want to clearly state the license for your contract code to others.

When you want to avoid errors from using incompatible compiler versions.

When you prepare your contract for deployment on blockchain networks.

Syntax

Blockchain / Solidity

// SPDX-License-Identifier: <license>
pragma solidity ^<version>;

The SPDX license line must be the very first line in your Solidity file.

The pragma line tells the compiler which Solidity version(s) your code supports.

Examples

This example uses the MIT license and supports Solidity version 0.8.0 and above, but less than 0.9.0.

Blockchain / Solidity

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

This example uses the GPL-3.0 license and requires exactly Solidity version 0.7.6.

Blockchain / Solidity

// SPDX-License-Identifier: GPL-3.0
pragma solidity 0.7.6;

This example has no license (unlicensed) and supports Solidity versions from 0.6.0 up to but not including 0.9.0.

Blockchain / Solidity

// SPDX-License-Identifier: UNLICENSED
pragma solidity >=0.6.0 <0.9.0;

Sample Program

This simple contract uses the MIT license and Solidity version 0.8.0 or higher. It stores a greeting message.

Blockchain / Solidity

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract HelloWorld {
    string public greet = "Hello, blockchain!";
}

Important Notes

Always put the SPDX license identifier as the very first line to avoid compiler warnings.

Use the caret (^) in pragma to allow compatible newer versions but avoid breaking changes.

Choosing the right license helps others know how they can use your code.

Summary

SPDX license line states the code license clearly.

Pragma version controls which Solidity compiler versions can compile your code.

Both help keep your smart contract safe, clear, and compatible.

Practice

1.

What is the purpose of the SPDX-License-Identifier line in a Solidity smart contract?

easy

A. To specify the license type of the contract's source code

B. To set the Solidity compiler version

C. To declare the contract's owner

D. To define the contract's storage variables

Solution

1. Step 1: Understand SPDX-License-Identifier usage

   The SPDX-License-Identifier line is a comment that clearly states the license under which the source code is released.

2. Step 2: Differentiate from pragma version

   Unlike pragma which sets compiler version, SPDX is about legal licensing information.

3. Final Answer:

   To specify the license type of the contract's source code -> Option A

4. Quick Check:

   SPDX-License-Identifier = License type [OK]

Hint: SPDX line always shows license info, not compiler version [OK]

Common Mistakes:

• Confusing SPDX line with pragma version
• Thinking SPDX sets compiler behavior
• Ignoring SPDX line as just a comment

2.

Which of the following is the correct way to specify the Solidity compiler version to accept any version from 0.8.0 up to but not including 0.9.0?

pragma solidity ?;

easy

A. pragma solidity ^0.8.19;

B. pragma solidity 0.8.0;

C. pragma solidity >=0.8.0 <0.9.0;

D. pragma solidity >0.8.0 <=0.9.0;

Solution

1. Step 1: Understand version range syntax

   The syntax >=0.8.0 <0.9.0 means any version from 0.8.0 up to but not including 0.9.0.

2. Step 2: Compare with caret (^) operator

   The ^0.8.19 syntax allows >=0.8.19 <0.9.0, excluding earlier 0.8.x versions like 0.8.0. The explicit range >=0.8.0 <0.9.0 matches exactly.

3. Final Answer:

   pragma solidity >=0.8.0 <0.9.0; -> Option C

4. Quick Check:

   Version range syntax = >=0.8.0 <0.9.0 [OK]

Hint: Use >= and < to specify exact version range [OK]

Common Mistakes:

• Using exact version without range
• Misplacing comparison operators
• Confusing caret (^) with range syntax

3.

What will happen if you compile this Solidity code snippet?

// SPDX-License-Identifier: MIT
pragma solidity ^0.7.0;
contract Test {
  uint public x = 5;
}

Assuming you use Solidity compiler version 0.8.10.

medium

A. Compilation fails due to pragma version mismatch

B. Compilation fails due to SPDX license missing

C. Compilation succeeds but with a warning

D. Compilation succeeds without errors

Solution

1. Step 1: Check pragma version compatibility

   The pragma ^0.7.0 means compiler versions from 0.7.0 up to but not including 0.8.0 are allowed.

2. Step 2: Compare with compiler version used

   Compiler version 0.8.10 is outside the allowed range, so compilation will fail due to version mismatch.

3. Final Answer:

   Compilation fails due to pragma version mismatch -> Option A

4. Quick Check:

   Pragma ^0.7.0 excludes 0.8.10 = mismatch [OK]

Hint: Compiler version must fit pragma range exactly [OK]

Common Mistakes:

• Ignoring pragma version mismatch
• Thinking SPDX license affects compilation
• Assuming caret (^) allows all newer versions

4.

Identify the error in this Solidity contract header:

// SPDX-License-Identifier MIT
pragma solidity ^0.8.0;
contract Sample {}

medium

A. SPDX license identifier must be in quotes

B. Incorrect pragma version syntax

C. Contract name must be lowercase

D. Missing colon ':' after SPDX-License-Identifier

Solution

1. Step 1: Check SPDX license line format

   The SPDX license line must be in the format // SPDX-License-Identifier: LicenseName with a colon after the identifier.

2. Step 2: Verify other parts

   The pragma syntax and contract name are correct; SPDX license identifier does not require quotes.

3. Final Answer:

   Missing colon ':' after SPDX-License-Identifier -> Option D

4. Quick Check:

   SPDX line requires colon ':' [OK]

Hint: SPDX line always has colon after identifier [OK]

Common Mistakes:

• Omitting colon after SPDX-License-Identifier
• Adding quotes around license name
• Thinking contract names must be lowercase

5.

You want to write a Solidity contract that is compatible with any compiler version from 0.6.0 up to but not including 0.9.0, and you want to specify the MIT license. Which of the following headers is correct?

hard

A. // SPDX-License-Identifier: MIT
pragma solidity 0.6.0;

B. // SPDX-License-Identifier: MIT
pragma solidity >=0.6.0 <0.9.0;

C. // SPDX-License-Identifier MIT
pragma solidity ^0.6.0;

D. // SPDX-License-Identifier: MIT
pragma solidity >=0.6.0 <=0.9.0;

Solution

1. Step 1: Check SPDX license line correctness

   The SPDX line must have a colon after the identifier and specify the license, so // SPDX-License-Identifier: MIT is correct.

2. Step 2: Verify pragma version range

   The pragma >=0.6.0 <0.9.0 correctly specifies compatibility from 0.6.0 up to but not including 0.9.0. Using <=0.9.0 would include 0.9.0, which is not desired.

3. Final Answer:

   // SPDX-License-Identifier: MIT pragma solidity >=0.6.0 <0.9.0; -> Option B

4. Quick Check:

   SPDX with colon + correct pragma range = // SPDX-License-Identifier: MIT pragma solidity >=0.6.0 <0.9.0; [OK]

Hint: Use colon in SPDX and < for upper pragma bound [OK]

Common Mistakes:

• Omitting colon in SPDX line
• Using exact version instead of range
• Including upper bound version with <= instead of <