Close all open forms except the main menu in C#

Question

Trying to close all forms except for the main menu using

FormCollection formsList = Application.OpenForms;

with a foreach loop and saying,

Answer 1

Collection was modified; enumeration operation may not execute.

FormCollection formsList = Application.OpenForms;
            //for (int i = 0; i < formsList.Count; i++)
            foreach(Form  f in formsList )
            {
                if (f.Name != "Form1" || f.Name != "Home" || f.Name != "AdminHome")
                    f.Close();
            }
            this.Close();