What is that little padlock symbol right near every visual studio tab?

Question

hello,

since some days I have on the right side of any visual studio tab a little padlock symbol. What is it? I thought it was something like source-control, but my solution is not checked in source-control.

Thank you for your answer in advance!

EDIT: Thank you for your answers so far, BUT I can edit the files. When I am in debug mode it has also this padlock sign, but when I am holding my mouse over the tab I can read "Read-Only". Not in debug-mode (normal edit mode, nothing running), there is also the padlock symbol. But when I hold my mouse over the tab there is no readonly text.

Is there an other definition of the padlock sign? Or how can I see the padlock sign while editing my normal source code (and no debugging running)?

Answer 1

It indicates that the file is read-only.

Answer 2

That means that the file is not editable by Visual Studio; usually it comes up when you ask VS to navigate to a definition that exists only in metadata. It will display what data it can, but show a padlock to indicate that you can't edit it.