Question

Trying to close all forms except for the main menu using

FormCollection formsList = Application.OpenForms;

with a foreach loop and saying,

if (thisForm.Name != "Menu") thisForm.Close();

Which works ok, it skips the menu, and closes the first, but it then errors:

Collection was modified; enumeration operation may not execute

and stops. I have tried a few places, and they all say that this foreach loop is the way to do it, and it is especially annoying as I am not updating my forms list after closing the forms, which I thought might work. The only thing I could think of was to start at the back and work forward using a while.

Answer 1

If you use foreach to enumerate through a collection, it can’t be modified (items added or removed) during the iteration. Try copying references to the forms to another collection, and then remove them by iterating through that collection.

In situations like this, you can use a list or a simple array, such as:

List<Form> openForms = new List<Form>();

foreach (Form f in Application.OpenForms)
    openForms.Add(f);

foreach (Form f in openForms)
{
    if (f.Name != "Menu")
        f.Close();
}

Or you can use a for loop:

for (int i = Application.OpenForms.Count - 1; i >= 0; i--)
{
    if (Application.OpenForms[i].Name != "Menu")
        Application.OpenForms[i].Close();
}

Or, my new and current favorite, you can use the Reverse() method:

foreach (Form f in Application.OpenForms.Reverse())
{
    if (f.Name != "Menu")
        f.Close();
}