Normally, the read-only attribute is set on add-ins files to provide an appropriate source control through the Perforce client.

If you are using Perforce client

Do not attempt to make the file writeable or to edit in memory - check-it out first in a regular way through the Perforce client. There's no need to close the file in Visual Studio Express if it's already open - it will become editable as soon as you check it out in the Perforce client.

If you are not using Perforce client and you are the only person editing VB

It make sense to remove the read-only attribute on all VB files as soon as you download them. If you didn't do so, you can make the file writeable when prompted.

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Add-ins and daily build