Using Native ETH

This article discusses how the Vault can wrap ETH to WETH. While this concept applies to the native assets on other EVM compatible chains (ex. MATIC on Polygon), this article will refer to native assets as ETH and their wrapped counterparts as WETH.
Overview
While native ETH is not directly compatible with Balancer, the Vault can automatically wrap/unwrap ETH to/from WETH when performing typical operations.
Sentinel Value
In the documentation and smart contract comments, you may find references to the Sentinel Value when dealing with Native ETH. This is a placeholder address to denote that the asset you're dealing with is not an ERC20 token, but is instead Native ETH.
When dealing with Native ETH, the Sentinel Value you'll provide is the zero address0x0000000000000000000000000000000000000000
Swaps
Single Swaps
If you wish to send or receive native ETH in a single swap, simply provide the sentinel address for assetIn or assetOut respectively.
Batch Swaps
If you wish to send or receive native ETH in a batch swap, provide the sentinel address in the assets array and refer to it by its index in your swap steps as you would any other token. For Batch Swaps, you must sort the sentinel address numerically; do not sort it in the array as if it is the WETH address.

Note: it is possible to send ETH and WETH in the same swap by providing both of the corresponding asset indices as inputs.
Join/Exit
When joining or exiting a pool, you have to construct a JoinPoolRequest or ExitPoolRequest struct. Within this struct, you have the assets array:
1
address[] assets
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As you'll find in the documentation for Joins and Exits, this array must be sorted numerically; there is a caveat here though. If you wish to join with or exit to Native ETH, you need to order the array as if you're dealing with WETH. Note that it is not possible to combine ETH and WETH in the same join/exit; any excess ETH will be sent back to the caller (not the sender, which is important for relayers).
Correctly Ordered Example with WETH
Token
Address
DAI
0x6b175474e89094c44da98b954eedeac495271d0f
BAL
0xba100000625a3754423978a60c9317c58a424e3D
WETH
0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2
Correctly Ordered Example with Native ETH
Token
Address
DAI
0x6b175474e89094c44da98b954eedeac495271d0f
BAL
0xba100000625a3754423978a60c9317c58a424e3D
ETH
0x0000000000000000000000000000000000000000

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