Collection was modified; enumeration operation may not execute.

Question

foreach ( Form frm in Application .OpenForms) { frm.Close(); } the error occur above code Collection was modified; enumeration operation may not execute.

Vulpes · Answer

In most cases 'foreach' won't allow you to modify the collection which messes up how it works internally. When you're deleting multiple elements from a collection the best thing to do is to iterate through the collection backwards which doesn't mess up the indexing of the elements still to be examined: for(int i = Application.OpenForms.Count - 1; i >=0 ; i--) { Application.OpenForms[i].Close(); } However if you're simply wanting to close all open forms and the application itself doing it this way is unnecessary. All you need to do is close the startup form (normally the one at index 0 in the collection) and all the others close automatically and the application ends.