How does metamask choose the msg.sender?

Question

When coding an application that looks to be metamask compatible how are multiple account handled by meta mask ? how can i make sure that metamask / my app are in sync account-wise ?

does metamask set the web3.eth.defaultAccount ?

Answer 1

According to Metamask's docs (https://github.com/MetaMask/faq/blob/master/DEVELOPERS.md):

When a user selects an account in MetaMask, that account silently becomes the web3.eth.accounts[0] in your JS context, the only member of the web3.eth.accounts array.

So, by default, anytime you check for web3.eth.accounts[0] it will refer to the currently selected account in Metamask.

Answer 2

The user chooses web3.eth.defaultAccount, the option is not available to your web application. Although, you can query web3.eth.accounts.