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## `setBlockhash`

### Signature

```solidity
function setBlockhash(uint256 blockNumber, bytes32 blockHash) external;
```

### Description

Sets the hash returned by the `BLOCKHASH` opcode for the given block number.

The block number must not be greater than the current `block.number`, and `blockhash` only serves blocks in the window `[block.number - 256, block.number)`. On chains with [EIP-2935](https://eips.ethereum.org/EIPS/eip-2935) (Prague and later), the history storage contract is updated as well.

This is useful for testing contracts that derive randomness or commitments from block hashes, which are otherwise zero for locally mined blocks.

### Parameters

| Parameter     | Type      | Description                              |
|---------------|-----------|------------------------------------------|
| `blockNumber` | `uint256` | The block number to set the hash for     |
| `blockHash`   | `bytes32` | The hash to return for that block        |

### Examples

```solidity [test/SetBlockhash.t.sol]
function testSetBlockhash() public {
    vm.roll(100);

    bytes32 hash = keccak256("fake block");
    vm.setBlockhash(99, hash);

    assertEq(blockhash(99), hash);
}
```

### Related Cheatcodes

* [`roll`](/reference/cheatcodes/roll) - Set `block.number`
* [`prevrandao`](/reference/cheatcodes/prevrandao) - Set `block.prevrandao`