My project in Visual Studio is Read Only. What did I do?

In my case it was a ReSharper 9.0/9.1 related issue. After clearing the ReSharper cache, I was able to edit files in my solution again.

Clearing the ReSharper cache can be done here: ReSharper -> Options -> Environment -> General and click: Clear Caches and restart your visual studio.

(The image is contained within the first of the linked JetBrains articles).

JetBrains instructions:

Ultimate Guide to Speeding Up ReSharper (and Visual Studio) - Performance degradation after ReSharper upgrade

ReSharper Online Documentation

A couple of possibilities spring to mind:

Have you added the project to source control?

Have you marked the project folder (and all it's sub folders) as read only?

Of course, someone else may have done either of these things if they have had access to your machine.

One Possible stupid thing.

Your project is still running and you forgot that. Stop debugging (CTRL + F5)

How do I know. Guess..

Restarting my computer worked for me. My project became read only in between compiles - suddenly I couldn't save changes to files, Visual Studio started presenting me with 'save as' dialog boxes, and SVN couldn't commit or clean the project because "sqlite could not write to readonly database." Checking the file properties on the files in Windows Explorer showed that none of them were marked read-only. I was able to make a backup copy of the project, then I restarted and it was fine.