How to hide multiple forms with one click in Visual Studio?

Try following code:

public static void closeAllOpenedForms()
{
    FormCollection FM = Application.OpenForms;
    if (FM.Count > 1)
    {
        for (int i = (FM.Count); i > 1; i--)
        {
            Form sForm = Application.OpenForms[i - 1];
            sForm.Close();
        }
    }
}

You can check/clone/test this code on https://github.com/JomaStackOverflowAnswers/HideForms

If you need to reuse the opened form you can hide/show as neeeded. But if you create new forms everytime is recommended to close the forms(not hide).

FormMain

namespace HideForms
{
    public partial class FormMain : Form
    {
        public FormMain()
        {
            InitializeComponent();
        }

        private void buttonOpen_Click(object sender, EventArgs e)
        {
            Employee employee = new Employee("123");
            FormViewEmployee formViewEmployee = new FormViewEmployee(employee);
            FormViewNewUpdates formViewNewUpdates = new FormViewNewUpdates(employee, this, new List<Form>{ formViewEmployee });
            formViewEmployee.Show();
            formViewNewUpdates.Show();
            Hide();
        }
    }
}

FormViewEmployee

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace HideForms
{
    public partial class FormViewEmployee : Form
    {
        private readonly Employee employee;

        public FormViewEmployee(Employee employee)
        {
            InitializeComponent();
            this.employee = employee;
        }
    }
}

FormViewNewUpdates

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace HideForms
{
    public partial class FormViewNewUpdates : Form
    {
        private readonly Employee employee;
        private readonly Form parent;
        private readonly List<Form> formsToClose;

        public FormViewNewUpdates(Employee employee, Form parent, List<Form> formsToClose)
        {
            InitializeComponent();
            this.employee = employee;
            this.parent = parent;
            this.formsToClose = formsToClose;
            
        }

        private void buttonClose_Click(object sender, EventArgs e)
        {
            foreach(var form in formsToClose)
            {
                form.Close();//form.Hide(); //Hide in case you need to reuse the form.
            }
            Close();
            parent.Show();// Show the parent form.
        }

        private void FormViewNewUpdates_FormClosing(object sender, FormClosingEventArgs e)
        {
            foreach (var form in formsToClose)
            {
                form.Close();
            }
            parent.Show();
        }
    }
}

Output
FormMain When click in open button, it opens FormViewEmployee, FormViewNewUpdates instances.

FormViewEmployee, FormViewNewUpdates instances
When click in Close All button it close both instances and show FormMain instance.
2

References

Control.Hide
Form.Close