# Solidity File Structure

## Structure of a Solidity file

### SPDX License Identifier

All solidity contracts should have License declaration in the first line.

```solidity
// SPDX-License-Identifier: MIT
```

List of licenses from SPDX repository: <https://spdx.org/licenses/>

### Pragmas

`pragma` is the keyword used to declare the compiler version of Solidity. `pragma` only applies to the current local file so you must add `pragma` to all files in the project directory.

```solidity
// SPDX-License-Identifier: MIT
pragma solidity 0.8.20;
```

You can use the `^` sign or the comparison operators `<`, `<=`, `>`, `>=` in conjunction with the compiler declaration.

```solidity
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20; // use compiler version 0.8.20 and above
```

```solidity
// SPDX-License-Identifier: MIT
pragma solidity >=0.4.22 <0.9.0; // use compiler version bigger or equal to 0.4.22 and less than 0.9.0
```

### Import file from other sources

#### Import the whole file

```solidity
import "FileName"
```

#### Import all and assign alias

```solidity
import * as symbolName from "FileName";
```

#### Name import

`Name import` means you will specify the name of the import object from another file. The reason you should use this option is that it makes your code clearer.

```solidity
import {ContractOne as alias, ContractTwo} from "FileName";
```

### Comment

To comment, you can use `//` and `/* */`

```solidity
// comment for 1 line.

/*
Comment for
multiple lines
*/
```

There is also [NatSpec](https://docs.soliditylang.org/en/v0.8.24/natspec-format.html#natspec) comment with `///` or `/** **/`

```solidity
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.16 <0.9.0;

/// @author The Project Team
/// @title A simple storage example
contract SimpleStorage {
    uint storedData;

    /// Store `x`.
    /// @param x the new value to store
    /// @dev stores the number in the state variable `storedData`
    function set(uint x) public {
        storedData = x;
    }

    /// Return the stored value.
    /// @dev retrieves the value of the state variable `storedData`
    /// @return the stored value
    function get() public view returns (uint) {
        return storedData;
    }
}
```


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