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Close all open forms except the main menu in c#
FormCollection formsList = Application.OpenForms;

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

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();
}

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

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

Form[] forms = Application.OpenForms.Cast<Form>().ToArray();
foreach (Form thisForm in forms)
{
    if (thisForm.Name != "Menu") thisForm.Close();
}