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## `getChainId`

### Signature

```solidity
function getChainId() external view returns (uint256 blockChainId);
```

### Description

Gets the current `block.chainid` of the currently selected environment.

Prefer this over reading `block.chainid` directly when using [`selectFork`](/reference/cheatcodes/select-fork) or [`createSelectFork`](/reference/cheatcodes/create-select-fork): the compiler may assume `block.chainid` is constant within a transaction and cache it, returning a stale value after a fork switch. See [issue #6180](https://github.com/foundry-rs/foundry/issues/6180).

### Returns

| Type      | Description                            |
|-----------|----------------------------------------|
| `uint256` | The chain ID of the active environment |

### Examples

```solidity [test/GetChainId.t.sol]
function testFork_getChainId() public {
    vm.createSelectFork("mainnet");
    assertEq(vm.getChainId(), 1);

    vm.createSelectFork("optimism");
    assertEq(vm.getChainId(), 10);
}
```

### Related Cheatcodes

* [`chainId`](/reference/cheatcodes/chain-id) - Set `block.chainid`
* [`selectFork`](/reference/cheatcodes/select-fork) - Switch to a fork
* [`getBlockTimestamp`](/reference/cheatcodes/get-block-timestamp) - Same pattern for `block.timestamp`
* [`getBlockNumber`](/reference/cheatcodes/get-block-number) - Same pattern for `block.number`