Check only one ToolStripMenuItem

I have another way which works:

Each item is going to ToolStripMenuItem_CheckStateChanged(object sender, EventArgs e) and every item has its own tag 1, 2, 3, 4, 5. One item is checked on start and has tag = 1.

    int selecteditem = 1;
    bool atwork = false;
    private void dzienToolStripMenuItem_CheckStateChanged(object sender, EventArgs e)
    {
        if (atwork) return;
        else atwork = true;

        selecteditem = Convert.ToInt32(((ToolStripMenuItem)sender).Tag);

        foreach (ToolStripMenuItem it in sometooltipdropdown.DropDownItems)
        {
            if (Convert.ToInt32(it.Tag) != selecteditem)
            {
                it.Checked = false;
            }                
        }

        atwork = false;
    }

So easy...

Implement their method as described below:

    private void subietm1ToolStripMenuItem_Click(object sender, EventArgs e)
    {
        UncheckOtherToolStripMenuItems((ToolStripMenuItem)sender);
    }

    public void UncheckOtherToolStripMenuItems(ToolStripMenuItem selectedMenuItem)
    {
        selectedMenuItem.Checked = true;

        // Select the other MenuItens from the ParentMenu(OwnerItens) and unchecked this,
        // The current Linq Expression verify if the item is a real ToolStripMenuItem
        // and if the item is a another ToolStripMenuItem to uncheck this.
        foreach (var ltoolStripMenuItem in (from object 
                                                item in selectedMenuItem.Owner.Items 
                                            let ltoolStripMenuItem = item as ToolStripMenuItem 
                                            where ltoolStripMenuItem != null 
                                            where !item.Equals(selectedMenuItem) 
                                            select ltoolStripMenuItem))
            (ltoolStripMenuItem).Checked = false;

        // This line is optional, for show the mainMenu after click
        selectedMenuItem.Owner.Show();
    }

One detail is that you can implement the same method for all click menuItens, for this add same call for method UncheckOtherToolStripMenuItems((ToolStripMenuItem)sender); into the Event click for each ToolstripMenuItem, see this example to the another two ToolstripMenuItens:

    private void subietm2ToolStripMenuItem_Click(object sender, EventArgs e)
    {
        UncheckOtherToolStripMenuItems((ToolStripMenuItem)sender);
    }

    private void subietm3ToolStripMenuItem_Click(object sender, EventArgs e)
    {
        UncheckOtherToolStripMenuItems((ToolStripMenuItem)sender);
    }

I just set all the items in my menu with the event of item_Click so if one is clicked then it will just run the code below. Dont need an event for each button that way.

    private void item_Click(object sender, EventArgs e)
    {
        // Set the current clicked item to item
        ToolStripMenuItem item = sender as ToolStripMenuItem;
        // Loop through all items in the subMenu and uncheck them but do check the clicked item
        foreach (ToolStripMenuItem tempItemp in (ToolStripMenuItem)item.OwnerItem.DropDownItems)
        {
            if (tempItemp == item)
                tempItemp.Checked = true;
            else
                tempItemp.Checked = false;
        }
    }

If you want to add several items to your list during runtime and have them connected in the way above you can run the below code.

    private void subItemsMenus(ToolStripMenuItem parentItem, string[] listItems)
    {
        // Clear tool strip items first
        parentItem.DropDownItems.Clear();
        // Add items that are in the list
        foreach (string subMenuItem in listItems)
        {
            ToolStripMenuItem item = new ToolStripMenuItem();
            //Name that will appear on the menu
            item.Text = subMenuItem; 
            //Put in the Name property whatever necessary to retrieve your data on click event
            item.Name = subMenuItem;
            //On-Click event
            item.Click += new EventHandler(item_Click);
            //Add the submenu to the parent menu
            parentItem.DropDownItems.Add(item);
        }