HARDHAT Immutable Error

[furyydev]

Immutable variable error in Hardhat — am I assigning it wrong?

I declared an immutable variable in Solidity, but Hardhat throws TypeError: Cannot write to immutable variable when I assign it inside a function. I thought immutables can be set later.

Am I misunderstanding how immutables are supposed to be assigned?

[gauravsonii]

🧠 Why the Error Happens

An immutable variable in Solidity is not stored in contract storage.
It’s encoded directly into the contract’s deployed bytecode.
Since bytecode is written only during deployment, the assignment must happen in the constructor.
Trying to set it later (inside a function) breaks the rule, so Hardhat throws:

TypeError: Cannot write to immutable variable.

🛠 Correct Usage Example

`// SPDX-License-Identifier: MIT
pragma solidity ^0.8.19;

contract FeeManager {
uint256 public immutable OWNER_FEE;

constructor(uint256 _fee) {
    OWNER_FEE = _fee; // ✔ allowed only here
}

function getFee() external view returns (uint256) {
    return OWNER_FEE;
}

}`

💡 Why It Must Be Set in Constructor

Bytecode is fixed at deploy time
Immutable values are embedded into bytecode
Functions execute after deploy, when bytecode cannot be modified

So assigning inside a normal function is the same as trying to rewrite deployed bytecode — which is impossible.

[furyydev]

Ohh that's a great approach for solving this problem