If you want true thread safety, then the OpenForms property will need to be wrapped in a lock(Application.OpenForms) statement. Any other places in your application that open or close a form will likewise need to lock the same object.

public static void CloseAllForms()
{
//Create a Collection to Store all Opened Forms.
List formsList = new List();
//All all opened forms into a Collection.
foreach (Form frm in Application.OpenForms)
{
//Execulde the Current Form.
if (frm.Name == "Form1″)
continue;
else
formsList.Add(frm);
}
//Now Close the forms
foreach (Form frm in formsList)
{
frm.Close();
}
}

FormCollection forms = Application.OpenForms;
for( int i = 0; i < forms.Count; i++ ) {
forms[i].Close();
}

public static Form[] OpenForms
{
get
{
Form[] forms = null;
int count = Application.OpenForms.Count;
forms = new Form[count];
if (count > 0)
{
int index = 0;
foreach (Form form in Application.OpenForms)
{
forms[index++] = form;
}
}
return forms;
}
}